BSc Agriculture (Honours) 1995
Efficacy of Reduced Dosage Rates of Post-emergence Applied Atrazine to Control Broad Leaf Weeds in Maize - Eremencia Honamombe
Supervisor: Mr A.B. Mashingaidze
Two field experiments were done to determine the efficacy of reduced dosage of post-emergence atrazine to control broad leaf weeds in a maize field at the University of Zimbabwe (UZ) and Department of Crop Science. The number of weeds in each of the plots was counted before herbicide application. Reduced rates of atrazine (562.5, 742.5, 1507.5, 1687.5 and 2250 g ha-1 (active ingredient)) were applied at the two to three-leaf stage of weed growth. Three weeks after herbicide application the number of surviving weeds was counted and recorded. Weed dry weight and maize grain yield per plot were determined. Percent control of the resident weed populations was calculated. At UZ farm, the 75% rate had as high percent weed control, as did the full label rate for all weed species. Amaranthus hybridus and Nicandra physalodes were the most susceptible species and were completely controlled even by the lowest rate (25%) at the UZ farm, while the same happened with Galinsoga parviflora, Bidens pilosa and N. physalodes with the lowest rate (33%) at the UZ Department of Crop Science. Grass weed species (Setaria spp and E. indica) were more tolerant to reduced atrazine dosages than broadleaf weeds. There was no significant difference (P>0.05) in percent weed control at the UZ Department of Crop Science for the 100%, 67% and 33% rates for Commelina benghalensis and the grass species. Reduced rates of atrazine can be used to achieve equitable weed control as the full label rates without loss of maize yield. Results agree with work done by other authors.
A Study to Assess the Potential for Domestication of Selected Indigenous Vegetable Species and their Nutritional Contribution to the Diet of the People of Zimbabwe - Goldwater Lukuta
Supervisor: Dr A.D. Turner
Experiments were conducted to determine the effect of growing season on fresh leaf and seed yield of four indigenous vegetable species, namely Cleome gynandra, Cleome monophylla, Corchorus olitorius and Corchorus tridens. The harvested leaves were analyzed in the fresh form and preserved form for content of ascorbic acid, Beta-carotene, iron, calcium, phosphorus and protein. For preservation, leaves of Cleome species were plunged in boiling water for three minutes and then dried under shade. Corchorus species leaves were dried under shade without blanching. Another experiment was carried out to determine the effect of seed age and also the effect of seed soaking (Corchorus spp. only) on germination of the indigenous vegetable species.
In all species higher yields of fresh leaves were recorded in summer than in winter. For the Corchorus species, there were significant differences in nutritional content of summer and winter grown crops, with nutrient levels decreasing from summer to winter. Unprocessed leaf samples had a higher nutritional content than processed leaves in all four species. Pre-soaking the Corchorus seeds in hot water before planting them increased the germination percentage. Germination percentage in all species increased with an increase in seed age, with seeds sown one week after harvesting recording the lowest germination percentages. Higher seed yields occurred in summer than in winter.
Supervisor: Dr A.D. Turner
Fresh market tomatoes were dried using a chimney-type solar drier and an open air-drying system. Half of the tomatoes were salted before drying in both systems.
Comparisons were made between drier-dried tomatoes and open air-dried tomatoes with respect to total drying time, rate of drying, final moisture content and quality of the dried products. Similar comparisons were also made between tomatoes which were salt-treated and those that were not salted prior to drying.
Results indicated that drying using a chimney-type solar drier could effectively protect the tomatoes from direct sun radiation, dust, debris, insects and rain. Drier-dried products also had higher levels of ascorbic acid. All the samples managed to store for 16 weeks.
Supervisor: Dr P. Tongoona and Dr K. Pixley
Sixteen genotypes of maize were grown and artificially infested with the maize streak virus (MSV), using reared leafhoppers (Cicadulina mbila) that had acquired the virus from streak infested plants, to determine the level of MSV resistance in each genotype and to determine the relationship between plant height, yield and the MSV disease score. Most cultivars except Pool 16, R201, Pool 16 SR and Maka SR had MSV disease scores of greater than 3 showing that they were relatively susceptible to the disease. There was a negative relationship between MSV disease score and plant height. Uninfected plants were higher yielding than their infected counterparts. However, correlation analysis showed that there was no significant relationship between MSV scores, yield and flowering dates. The MSV disease results in plant height and yield reduction in susceptible genotypes. The screening technique is useful for identification of MSV resistance vis-à-vis ratings of entries.
Key words: Maize streak virus (MSV), disease resistance, Zea mays.
Supervisor: Mr A.B. Mashingaidze
The effect of salinity on the growth and nutrient uptake by tobacco seedlings of cultivar Kutsaga Mammoth 10 (KM 10) is described. The sodium chloride-fed plants showed that salinity induced a general decrease in all the fluxes involved in partitioning of nitrogen and the various ions.
The number of tobacco seedlings at two weeks after sowing (W.A.S) decreased from 523 per m2 at 62 micro-siemens per centimetre (µS/cm) to 386 per square metre at 4000 µS/cm and at 5 W.A.S. the corresponding decrease was from 538 to 114 per m2. At conductivity levels of 1000 µS/cm and above, the number of tobacco seedlings were lower at 5 W.A.S. that at 2 W.A.S., signifying the death of some seedlings. At the time of pulling, the number of transplantable seedlings decreased from 132 per m2 to nil as salinity was raised from 62 to 4000 µS/cm.
The effect of source of topdress nitrogen was minimal. While, as expected, chloride content increased from 1.53% to 5.2% as salinity was raised from 62 to 4000 µS/cm, nitrogen and phosphorus did not differ much with treatment. Starch was highest (3.35%) at the highest salinity but differed marginally among the other five salinity levels. Salinity had minimal effect on magnesium and potassium content of the seedlings.
The Effects of Different Levels of Pruning in the Production of Cherry Tomatoes (Lycopersicon esculentum var. cerasiforme L.) - Simangaliso Mangena
Supervisor: Dr A.D. Turner
The cherry tomato (Lycopersicon esculentum var. cerasiforme) cultivar Sweetie, which is indeterminate in growth habit, was pruned to different levels in a trial conducted at the Horticultural Research Centre in Marondera during the 1994 to 1995 summer season. The unpruned plants had a higher yield both in fruit numbers and mass compared to the pruned plants. On the pruned treatments, the yield increased with an increase in the number of stems per plant: plants with three stems had a higher yield both in mass and fruit numbers than plants with one or two stems. The unpruned plants produced the highest number of unmarketable fruits. However, in the pruned plants, unmarketable fruit numbers increased with an increase in the level of pruning. The treatment pruned to one stem had the highest level of early blight (Alternaria solani).
Response to Simulated Drought Conditions of Tall and Short Maize (Zea mays) Near-isogenic Lines - Raymond Chifamba
Supervisor: Dr P. Tongoona
The responses for yield, anthesis-silking, harvest index and wilt rating of four tall and four short maize (Zea mays) near-isogenic lines were studied at 100, 50, 25 and 10 per cent soil AWC (available water content) in the greenhouse. In short lines, grain yield showed significant positive correlations with harvest index (r = 0.9760) and height (r = 0.3556). In tall lines, grain yield was found to have significant positive correlations with harvest index (r = 0.9616) and height (r = 0.5210). A significant negative correlation was found between the overall grain yield and anthesis-silking interval (ASI) for both tall and short lines, with correlation coefficients of -0.8565 and -0.9054 respectively. There was no significant correlation between overall grain yield and wilt rating for both tall and short lines. The tall genotypes significantly yielded better than the dwarf lines at 100% and 50% soil AWC. At 25% and 10% soil AWC, the short plants performed better than their tall counterparts but this was not statistically significant. At 100% soil AWC, there was no significant differences in the overall mean ASI between the tall and short genotypes. At 50%, 25% and 10% soil AWC, the tall lines had significantly higher overall mean ASI than those of short counterparts. There were significant differences between the overall mean wilt ratings and overall mean height of tall and short genotypes at each moisture level. The effects of decreasing moisture levels were more pronounced in tall plants than in short plants.
Trials of in vitro Establishment of Sweet Potatoes (Ipomoea batatas) - Emmanuel T. Nhema
Supervisor: Dr A.D. Turner
Trials were conducted to establish sweet potatoes (Ipomoea batatas), cultivar Brondal, in vitro on different media, which had been tested by other investigators in different environments. The objective was to determine suitable media for the environment to be used for micropropagation in Zimbabwe with the locally grown cultivars of sweet potato. The importance of using in vitro propagation is to produce virus-free plants. Shoot tips were used on five different media, the fifth being a control of Murashige and Skoog salts (M&S). The other four media had variations in the concentrations of plant growth regulators (PGR) and differences in the PGR constituencies.
Experimental results indicated that sweet potato explants established much more quickly and produced healthier plants in the medium which had 1 mg/l indole acetic acid (IAA) + 1 mg/l kinetin, and the medium with 1 mg/l naphthyleneacetic acid (NAA) + 1 mg/l kinetin. Explant establishment was noticeably slower in the change in the size and form of explants in the liquid medium with 1 mg/l IAA and the control with straight M&S salts. The other three media all started callus formation after about 11 days of culturing.
Susceptibility of Groundnut Germplasm to Cercospora arachidicola and its Phytotoxins - Moses Chourombo
Supervisor: Dr D.L. Cole
Cercospora arachidicola Hori attacks only groundnut (Arachis hypogaea, L.) and causes distinct necrotic spots on leaves and stems. These spots, which are the symptoms of early leaf spot (ELS), can also be induced by a phytotoxin produced by C. arachidicola, the orange toxin, which is at present being characterized.
Cultivars having genetic resistance to both the pathogen and phytotoxins produced by C. arachidicola are likely to be the only feasible ways of reducing yield losses. Ninety-one groundnut cultivars and breeding lines differing in resistance to C. arachidicola were tested for resistance to purified orange toxin. The breeding material comprised of 83 lines considered resistant and eight susceptible, which included one local susceptible cultivar, Plover. Resistance to the toxin, which was measured as necrotic areas on toxin-treated leaves, was proportional to the concentration of the orange toxin applied. Germplasm lines ICG 10510, ICGV-SM 91002, ICG 7005, ICG 11593 and ICG 11281 were least sensitive to both concentrations of the toxin used.
Response to natural infection under field conditions was measured in terms of number of spots per pot. Resistant cultivars exhibited longer latent period, reduced lesion area and less defoliation than susceptible cultivars, which were ICGV-SM 90008, ICGV-SM 90006, CG 7, ICG 10920, ICG 7897 and Plover.
Evaluation of the germplasm lines for their response to the local strain of C. arachidicola using the detached-leaf technique was problematic and experiments with plugs of inoculum on attached leaves indicated that low inoculum levels in the detached-leaf technique may have been the reason for no symptoms.
The Germination and Emergence Behaviour of Mexican Clover (Richardia scabra L.) and its Competition with Groundnut (Arachis hypogaea L.) - Llyn Munjoma
Three experiments were conducted to determine the effect of seed age on the germination of Richardia scabra seeds, seedling emergence and the competitive ability of R. scabra with two short season cultivars of groundnuts (Arachis hypogaea), Falcon and Plover.
The freshly harvested seeds (one-year old) had the highest germination (58.50%) followed by the four-year old seeds. Seeds that were nine years old did not germinate. The highest emergence (24.24%) occurred from the one-year old seeds followed by the four-year old seed. The nine-year old seeds failed to emerge at all. This shows that germination and emergence do not occur as long as the seeds are old and have lost viability. This suggests that secondary dormancy does not exist in R. scabra seeds.
In the crop-weed competition, the weed had suppressive effects on the groundnut growth in both cultivars. Greater yield reductions were observed with Plover except in the mixture with one R. scabra and two Plover plants which was reduced by 9.49%, and 10.60% in the mixture with Falcon. Groundnut grain yield of both Falcon and Plover decreased with an increase in weed number. Shelling percentage decreased with an increase in weed number in both cultivars but the percentages were higher with Plover. Plover pure-stand had 71.52%, one-weed mixture 66.49%, two-weeds mixture 55.00% and the three-weeds mixture 52.72% shelling. Shelling percentages in Falcon were 66.58, 57.89, 55.90 and 47.15 with respect to the stated mixtures in Plover.
The Prevalence of Sprouting in Fifteen Wheat (Triticum aestivum) Genotypes Grown in 1994 Season and its Influence on Seed Emergence - Tegwe Soko
The prevalence of sprouting and its influence on seed emergence were studied on 15 wheat genotypes grown at six different sites, four of which were harvested after rain.
The Falling Number test showed that there were significant differences between genotypes harvested before rain and those harvested after rain, at the 5% level. Genotypes harvested after rains had low Falling Numbers at sites 2, 5 and 6 while variation in counts were significantly different between genotypes harvested before and after rains. A significant positive correlation (0.365) between visual sprouted-grain counts and emergence while a negative correlation between emergence and Falling Number (-0.421) were found. A negative correlation was also found between Falling Number and visual sprouted-grain counts (-0.826). Genotypes from sites which were harvested after rain had higher rates of emergence as compared to genotypes from sites harvested before rain.
Two populations of maize were studied for their response to selection for drought tolerance by: 1) determining the relationship between yield and anthesis-silking interval (ASI) of lines from cycle 2 (C2) and cycle 3 (C3) of recurrent selection of ZM601 (DR) and lines from Tuxpeno Sequia (a CIMMYT population which has undergone six cycles of recurrent selections); 2) comparing ASI, grain yield, and other physiological traits and to confirm the utility of various secondary traits; and 3) confirming the use of secondary traits as selection criteria when breeding for drought tolerance. The secondary traits were: leaf rolling, leaf erectness and tassel size. Under stressed conditions Tuxpeno Sequia produced a significantly higher yield than ZM601. Yield was negatively correlated with ASI (r = -0.08), but not significantly so (P>0.05), for Tuxpeno Sequia and (r = -0.22) for ZM601, meaning yield increased as ASI decreased. Mean ASI was significantly shorter for Tuxpeno Sequia (0.63 d) under moisture stress than for ZM601 (2.23 d) whereas under adequate moisture conditions ASI did not differ for the two populations. Linear correlation analysis indicated significant correlations with yield for both populations (P<0.05) for the secondary traits (r = -0.11 to -0.18). The results seem to indicate that Tuxpeno Sequia performs better than ZM601 under moisture stressed conditions. The fact that the correlation coefficient of yield with ASI (r = -0.08) was not significant (P>0.05) for Tuxpeno Sequia is an indication that the population has been more for this character than ZM601 (r = -0.22), (P<0.05).
Studies on Macadamia Insect Pests in Chipinge District, Zimbabwe - Vhukile Kutywayo
A survey aimed at establishing the pest spectrum, extent of damage caused by the insect pests and its pesticides used by the growers was conducted in twelve macadamia nut orchards in Chipinge from December 1994 to March 1995. Characteristics such as altitude, variety, age of plantation and whether the crop was intercropped or not were also noted during the survey. The most common insecticides used by farmers were: thiodan (endosulphan) used by 41.67% of farmers, Parathion (folidol), monocrotophos and lambda cyhalothrin (karate) were all used by 25% of the farmers and dimethoate (rogor), carbaryl (sevin), Cypermethrin (cypermethrin) and Fenvelerate (agrithrin) were each used by 8.3% of the farmers.
The most prevalent pests of macadamia nuts were found to be Cryptophlebia leucotreta (false codling moth), Nezara viridula (stink bug) and Bathycoelia sp. (two spotted bug). A wide range of other insect pests were also recorded mainly in the orders Coleoptera, Hemiptera and Lepidoptera. Except for the above-mentioned three species, the others were not regarded to be of major importance.
Parasites and predators were also sampled from the orchards. Parasitism of C. leucotreta eggs by Trissolcus maro and Trichogrammaidae cryptophlebiae was found to be high. Mesocomys sp. and Ooencyrtus sp., which are egg parasites of Bathycoelia sp., were also sampled. Assessment of damage to nuts showed that, on average, 39.8% of harvested nuts are damaged by C. leucotreta. Monitoring of C. leucotreta adults with pheromone traps showed that population density was highest in April.
Supervisor: Mr S. Masimbe
A comparative assessment of three root stocks: Rough lemon, "Swingle" citrumelo and "Troyer" citrange was carried out at Dodhill Nurseries in the Mashonaland West Province of Zimbabwe for eight months and pot plants were used. The scion cultivar used was "Valencia" delta. Sixty plants of each rootstock were used and they were laid out in a randomised block design with 20 plants of each rootstock per plot. There were three blocks and three plots. Results were based on leaf nutrient levels in both the rootstock and scion, general rate of increase of scion length, increase in the number of leaves and the number of successful buds.
There were no significant differences in the leaf nutrient status of the rootstocks but the scion budded on rough lemon had higher levels of Ca, K, Cu, and Zn. The scions on this rootstock also had a more vigorous growth recording the highest number of successful buds, highest (16 compared to 9 and 8 for "Swingle" and "Troyer" respectively), number of leaves and leaf area. "Valencia" delta on rough lemon also had the tallest scions.
The Biology of Purslane (Portulaca oleracea L.) and its Competition with Rape (Brassica napus L.) in Zimbabwe - John Derera
Supervisor: Dr O.A. Chivinge
The biology of purslane (Portulaca oleracea L.) was studied by examining the effect of seed age on germination, effect of stem node number and desiccation of stem cuttings on the regeneration and survival of purslane in the glasshouse and laboratory. In the competition experiment the rape plants were grown with zero to five purslane plants in the glasshouse for 14 weeks. The highest seed germination (73.4%) occurred in the 0.3-year old seeds, while the lowest (0.2%) occurred in the 19.7-year old seeds in the laboratory. The rate of germination and emergence of seedlings followed the same trend. The number of leaves, branches, seeds, stem diameter and dry weight per plant increased significantly (P<0.05) with the increasing number of nodes per stem. There was a significant (P<0.05) desiccation effect on survival of purslane with least (29%) number of sprouted stems in the one-day desiccated stems and the highest (67%) in the five-day desiccated stems. A significant (P<0.05) reduction in rape yield occurred which increased with purslane density. The highest fresh weight reduction (18.95%) occurred where rape was grown with five purslane plants. This study should be repeated in the field for recommendations to be made to farmers.
Survey of Sunflower (Helianthus annuus L.) Production Constraints and Comparative Performance with Maize (Zea mays L.) and Sorghum (Sorghum bicolor L.) in Matibi II Communal Area of Zimbabwe - Walter Chivasa
Supervisor: Dr C. Chiduza
The study was carried out to investigate production constraints and comparative performance with maize and sorghum in the semi-arid area of Matibi II communal area of Zimbabwe. It was postulated that farmers could derive more stable income by adopting sunflower as a cash crop and economic yield of sunflower will be greater than that of maize and sorghum in Matibi II communal area. The research project focused on the communal area farmers of Matibi II. Sunflower, maize and sorghum were compared for economic yield and income stability under farmer managed conditions. Data collection was through a programme of surveys carried out in the area alongside farmer implemented and farmer managed on-farm trials, which were monitored up to harvesting. Gross income for each crop variety per hectare was obtained by multiplying seed yield per hectare by local and GMB prices. Net income was obtained by subtracting total variable costs from gross income. Net incomes were subjected to analysis of variance.
Overall, sorghum performed better than both sunflower and maize (P<0.05) using local prices. On a per site basis, sunflower performed better than maize at four out of eight sites (P<0.05) using the GMB prices. However, maize outperformed sunflower at four out of eight sites (P<0.05) using local prices. Sunflower was outperformed by sorghum at six out of eight sites (P<0.05) using local prices. There was no significant difference at four sites between sunflower and sorghum using GMB prices (P>0.05). Using GMB prices sunflower outperformed sorghum at two out of eight sites, and equally sorghum outperformed sunflower at two sites (P<0.05). There was not enough evidence to accept the hypotheses under investigation, hence the hypotheses were rejected at 5 per cent level of significance.
Survey of Sorghum Production in Matibi II, Comparative Performance of Traditional and Improved Varieties - Leonard Munamati
Supervisor: Dr C. Chiduza
An informal and formal survey of sorghum production was conducted in Matibi II in Chiredzi during the 1994/95 cropping season. A monitoring exercise on sorghum agronomy was carried out on farmers' fields during the same season. Farmer-managed on-farm experiments were conducted to determine the performance of traditional and improved sorghum varieties. Both informal and formal surveys indicated that there is a food deficit in Matibi II. Farmers depend on inappropriate sorghum varieties, which give them low yields under the moisture stress conditions that are prevalent in the area. The surveys also indicated some other major productions constraints.
The monitoring process revealed that farmers use high plant populations in their fields. High seeding rates and the consequent low yields were part of the normal farming system of Matibi II residents. Late sowing resulted in lower yields among the traditional compared to the improved varieties.
On-farm experiments also indicated that ZWSH-1 performs far much better than SV-2 and Chitichi. Mean yields for ZWSH-1, SV-2, and Chitichi were 2291 kg/ha, 1575 kg/ha and 1284 kg/ha respectively. This showed that improved varieties perform better than traditional varieties in drought prone areas. Also, the hybrid cultivar of ZWSH-1 produced higher yields compared to the open pollinated cultivar SV-2.
Supervisors: Mr A.B. Mashingaidze and Mr C. Nyakanda
Maize hybrid R201 was inter-row intercropped and intra-row intercropped with the short-season groundnut cultivar Falcon at different maize and groundnut spatial arrangements over the 1994-1995 growing season at the University of Zimbabwe farm. Planting groundnut in alternate rows with maize rows or in the same row with maize did not affect maize grain and biomass yields nor did it affect groundnut kernel and biomass yields. However, sole crops gave higher yields when compared to the intercrops. Both intra-row and inter-row intercropping maize with groundnuts were more advantageous than sole cropping when evaluated through the Land Equivalent Ratio. Weed spectrum and biomass were, however, not significantly different across treatments. Intra-row cropping will facilitate planting of the two crops simultaneously and allow use of cultivators to weed between the rows, operations which are not possible with inter-row intercropping.
Growth and Yield of F1 Maize Hybrids Compared with Saved F2 Hybrid Seed and with Open Pollinated Varieties at High and Low Levels of N Fertilizer - Charity Chikwati
Supervisors: Dr I.K. Mariga and Dr S.R. Waddington
A field experiment was carried out with F1 and F2 generations of two 3-way hybrids, R201 and SC601, and two open-pollinated varieties (OPVs), Kalahari Early Pearl (KEP) and ZM607, to determine their differences in growth habit and grain yield. This was assessed at high and low levels of nitrogen fertilizer.
The growth pattern was significantly affected by fertilizer management level (P<0.001). The F2’s had a lower leaf area index and dry matter production than their corresponding F1s during the early stages of development. In the later stages of development the F2’s caught up with the F1’s. Open-pollinated varieties were intermediate in leaf area index and dry matter production. F2’s were delayed in their development compared to F1’s especially at the low level of fertilizer management.
Grain yields differed by genotype (P<0.001) but there were no genotype by fertilizer management level interaction effects. The F1 hybrids out-yielded their corresponding F2‘s by 1606 kg/ha (P<0.001) and the OPVs also out-performed the F2’s by 1609 kg/ha. There was no grain yield difference between F1’s and OPVs. OPVs yield performance was similar at low and high level of fertilizer management.
Effect of Bruchid Damage on Dry Bean (Phaseolus vulgaris) Seed Quality - Soil Shumba
Supervisor: Dr I.K. Mariga
Bruchid damage by Zabrotes subfasciatus (Boh) and Acanthoscelides obtectus (Say) to untreated dry bean seed is common and results in both quantitative and qualitative losses. Grain infestation occurs in the field towards grain maturity and damage continues in the storage. Use of damage grain as seed is common for grain legumes in the smallholder sector and results in poor stands and reduction in yield.
Dry bean seed weight loss due to bruchid damage on two bean cultivars was determined in the laboratory. A field trial with the same bean cultivars was conducted at UZ and Domboshava to evaluate the effect of one-holed, two-holed and three-holed seed on dry bean seed viability and yield performance. It was laid out as a 2 x 4 factorial in a randomized complete block design with three replications at each site.
The results showed that seed weight loss increases as the number of holes per seed increases. Seed weight losses up to 32 % were recorded for the three-holed beans. Plant stand and grain yield per hectare were significantly reduced when damaged seeds were used for planting. Two-holed seed and three-holed seed achieved about one half and one third of the yield from whole seed achieved respectively. The resultant plant stand due to bruchid damage effect did not affect the numbers of pods per plant and seeds per pod.
BSc Agriculture (Honours) 1996
Effect of Open-pit Sunken Planting and Circle Weeding on Growth, Survival and Establishment of Pigeonpea (Cajanus cajan L.) - Elson W. Zhakata
Supervisors: Mr A.B. Mashingaidze and Mr C. Nyakanda
Several problems of tree establishment and survival in agroforestry have been faced by farmers and moisture availability seems to be one of the major hindrances. An experiment was carried out to assess and evaluate whether open-pit sunken planting and circle weeding would result in better establishment, growth and survival of pigeonpea (Cajanus cajan). Measurements were taken of height, basal circumference and branching and gravimetric soil moisture content of the multipurpose tree. The results indicated that under conditions of high rainfall, there was no advantage in planting in pits as far as vigour was concerned, but in fact problems of waterlogging could be experienced. Results also showed that circle weeding was a better alternative to complete weeding since labour was cheaper and weeds could later act as a mulch when they dried.
A field experiment was conducted in summer to determine the effect of maize residue at 0 t/ha, 5 t/ha, 10 t/ha and 15 t/ha and the herbicide dimethenamid and alachlor applied as pre-emergence treatments on the control of five weed species, namely Nicandra physalodes, Richardia scabra, Galinsoga parviflora, Chenopodium album and Amaranthus hybridus in a soyabean crop. Rainfall was abundant at the time of herbicide application. Two different assessments, weed counts and percent control, were carried out at three weeks after planting (WAP) and 6 WAP. At three weeks after herbicide treatment, emergence of all weeds had not been significantly suppressed by maize residues at all levels of mulch. Both herbicides significantly (P<0.01) reduced emergence of all weeds except C. album and A. hybridus but their effect was not influenced by maize residues at all levels although a gradual decline in weed emergence as mulch increased was noticeable. For all the weeds added together there was a significant (P<0.01) mulch herbicide interaction. Dimethenamid was more effective than alachlor when no mulch was applied but had similar levels of control to alachlor on weed emergence at all levels of mulch. Visually assessed percent control of weeds 6 WAP showed that herbicide efficacy was reduced by the presence of maize residues. There was a consistently significant decline in percent control as mulch level was increased for all weeds except C. album. C. album had the lowest percent control and its control was not influenced by the presence of maize residues.
Effect of Weed Density (Galinsoga parviflora) on Tomato (Lycopersicon esculentum) at Different Fertilisation Rates - Marshall Shumba
Supervisors: Dr O.A. Chivinge and Mr A.B. Mashingaidze
An experiment was done in a greenhouse to investigate the effect of increasing the density of Galinsoga parviflora in pots carrying one tomato plant with different fertilizations of 56 g/pot goat manure, 34 g/pot compound S and the control (no fertilization). Densities of G. parviflora of 2, 4, 6, 8, 10 weeds/pot were used. Weed density had a significant effect on shoot dry weight (P<0.01) and on fruit dry weight (P<0.05) of tomatoes. There was no significant (P>0.05) effect on the number of fruits, number of flowers and number of branches. Fertilization had a highly significant effect (P<0.01) on shoot dry weight and number of branches. A significant effect (P<0.05) was also shown on fruit dry weight and number of fruits. There was no significant effect (P>0.05) of fertilization on the number of flowers. The weed actually proved a serious competitor with tomato at different fertilities and its competitiveness was independent of the weed density.
The Effect of Undersowing Eucalyptus (Eucalyptus camaldulensis L.) with Annual Crops on Tree Growth and Crop Performance - Fred Zinanga
Supervisors: Dr O.A. Chivinge and Mr C. Nyakanda
Maize (Zea mays L.) hybrid R201 and cowpea (Vigna unguiculata L.) variety CP875 were separately inter-row intercropped with River Red gum-tree (Eucalyptus camaldulensis L.) during the 1995/96 growing season at Domboshava Training Centre. Planting E. camaldulensis with cowpeas and maize had no marked effect on the yields of the two crops. Sole, clean-weeded E. camaldulensis had significantly thicker circumference than the Eucalyptus in other treatments, but was comparable to Eucalyptus and annual crop mixtures in terms of branching habit. E. camaldulensis intercropped with maize showed a slightly higher performance (branching and circumference) compared with that intercropped with cowpeas. This was probably related to the artificial fertiliser added to the maize/E. camaldulensis intercrop since no fertiliser was added to cowpea/E. camaldulensis intercrop. Management factors like clean weeding in Eucalyptus markedly increased tree performance as observed through parameters of stem circumference and branching compared to those of the unweeded situation. Neither tree branching pattern, stem circumference nor soil moisture were significantly affected by intercropping with annual crops. Undersowing annual crops to E. camaldulensis will facilitate maximum utilization of land and light in the early years of E. camaldulensis woodlot establishment. This approach will also allow better tree growth when compared to the unweeded situation that is common on smallholder farms.
Detection of Ipomoea batatas Virus Infection in the Nursery-grown Planting Stock of Two Sweet Potato Cultivars 'Brondal' and 'Pamhayi' at the Horticultural Research Centre in Marondera - Joana Makuvise
Supervisors: Dr A.D. Turner
An experiment was conducted at the Horticultural Research Centre in Marondera in an attempt to determine the presence or absence of sweet potato (Ipomoea batatas) virus infection in the nursery-grown planting stock of the sweet potato cultivars 'Brondal' and 'Pamhayi'. The methods of detection used were mechanical sap inoculation and approach, apical and side veneer grafting of the sweet potato material onto the indicator species Ipomoea setosa.
The development of symptoms was monitored on the foliage of the indicator plants over a period of seven weeks. No virus infection was detected in the indicator plants over this entire period.
Resistance Evaluation and Minimum Sample Size Determination for Maize Stalk Borer (Busseola fusca) in Different Maize (Zea mays) Genotypes - Tsungai Bwerazuva
Supervisors: Dr P. Tongoona and Dr D.C. Jewell
A field study was conducted to evaluate resistance to maize stalk borer and minimum sample size requirement for leaf damage rating among selected maize genotypes. Leaf damage ratings showed the susceptible entry DMRESR-w to have the highest mean rating (on a scale of 1 to 9) of 7.5, while the hybrid entries which were crosses between a susceptible and a resistant source showed no significant differences between the different mean leaf damage ratings. The hybrid entries gave progeny with an average leaf rating of 6.3 (on a scale of 1 to 9) showing intermediate resistance to maize stalk borer. On a yield reduction basis, CML123/DMRESR-w showed the greatest yield reduction of 46% compared to the thionex-protected entry. CML139/DMRESR-w showed the least yield reduction of 10% in comparison to the thionex-protected entry. Thus, resistance should not be evaluated on a leaf damage basis only but should also include measurements of yield reduction as some material may have poor resistance to leaf damage but still show little yield reduction. Calculation of minimum sample size showed that DMRESR-w/CML123 required the largest sample size (49 plants), while CML67/DMRESR-w had the lowest sample requirement of 31 plants. Differences in sample sizes and yield between reciprocal crosses suggest the possible presence of maternal effects.
A Comparative Evaluation of the Fertilizer Value of Compound and Non-composted Castor (Ricinus communis L.) and Jatropha curcas L. Presscakes on the Yield of Tomato (Lycopersicum esculentum Mill.) - J.Tichawona Tasosa
Supervisor: Dr. C. Chiduza
The fertilizer values of (Jatropha curcas L.) and castor bean (Ricinus communis L.) presscakes were evaluated on the yield of tomato (Lycopersicum esculentum Mill.). Chemical analysis of the composted presscakes showed the following macronutrient composition. Uncomposted castor N 3.75 percent, P 0.6 percent; K 0.4 percent; composted castor N 1.07 percent; P 0.044 percent; K 0.86 percent; uncomposted jatropha N 3.43 percent; P 0.80 percent; K 0.57 percent and composted jatropha N 0.78 percent; P 0.40 percent and K 0.74 percent. In a field experiment composted and uncomposted jatropha and castor presscakes were applied at 6 t/ha each to Rodhead tomato seedlings. These were compared to 1000kg/ha compound L (50 kg N, 80 kg P, 80 kg K, 80 kg S and 2.5 kg B) as the control. There was no significant difference in the total fresh fruit yield due to treatment. The average plant heights after eight weeks ranged from 57.9 cm for the uncomposted castor to 71.7 cm for the composted jatropha presscakes. In a potted greenhouse experiment composted jatropha and castor presscakes were applied at 6, 12, 18 and 24 t/ha to Rodhead tomato seedlings. The total above ground dry matter (TDM) yields were highly significant. TDM increased with increased application rates. TDM yields ranged from 1.226 g for the zero fertilizer application (control) to 33.275 g for 24-t/ha jatropha application.
In an in vitro assessment to determine whether plain castor cake has phytotoxic elements at certain concentrations, a significant difference in the total yield fresh mass (TFM) of Moneymaker tomato was found when the castor concentrations were increased. TFM yields ranged from 0.567 g when no castor was applied to 0.056 g where 50 g of castor were applied.
Increasing the Supply of Rooted Peach (Prunus parsica) 'Nemagard' Rootstocks through the Use of Softwood Cuttings - Thompson Mukahlera
Supervisor: Mr S. Masimbe
The production of peaches in the country is low compared to the potential of the farming areas.
A comparison of rooting success between hardwood and softwood cuttings was carried out at Nyanga Experimental Station in Nyanga. A total of 250 cuttings each of softwood and hardwood cuttings were used. Three dates of collection and plantings were used in order to establish the optimum dates.
Results significantly show that Nemagard softwood cuttings root better than the hardwood cuttings. With the use of softwood cuttings, the supply of rooted Nemagard cutting is increased by almost four times.
Effects of Temperature, Seed Burial and Fertiliser on the Germination and Growth of Corn Spurry (Spergula arvenis L.) and its Competition with Wheat (Triticum aestivum L.) - Fortune Sangweni
Supervisor: Dr O.A. Chivinge
Four experiments were conducted to determine the effects of temperature on the germination of corn spurry (Spergula arvenis L.) seeds, seedling emergence, the effects of fertiliser levels on growth and competition between corn spurry and wheat (Triticum aestivum L.). Seeds germinated at all temperatures. The highest germination was obtained at 250C (80.37%) followed by 200C (66.13%), 300C (48%) and 150C (27.5%). Emergence only occurred at the depth of 0 cm (on the surface) and this is attributed to the small seed size of S. arvensis. Fertiliser was applied at five different levels.
The different fertiliser levels significantly (P<0.05) affected the root, shoot and seed weight of the weed. Shoot and seed dry weight were highest at the full fertiliser application level. Root dry weight was highest in the pots where no fertiliser was applied. The rooting depth of the weed was, however, not significantly affected (P>0.05) by the different fertiliser levels.
In the crop-weed competition study, the rooting depth, root dry weight, leaf number, seed number, seed weight and shoot dry weight of the wheat plants were not significantly affected (P<0.05) by the presence of the weed. S. arvenis had significant effects on the wheat height and tiller number. On the other hand, the presence of the wheat had no effects on rooting depth, root dry weight and seed weight of the weed. Any differences obtained are attributed to competition within the species. The shoot dry weight was, however, significantly affected by the wheat.
Effect of Simulated Tillage on the Efficacy of Reduced Herbicide Dosages to Control Weeds in the Greenhouse - Carlton Makwasha
Supervisor: Mr A.B. Mashingaidze
The effect of simulated tillage on the efficacy of reduced herbicide dosages was investigated in greenhouse trials conducted at the University of Zimbabwe, Crop Science Department in 1996. Atrazine and nicosulfuron were applied to Portulaca oleracea, Richardia scabra, Eleusine indica and Setaria verticillata at 0%, 12.5%, 25%, 50%, 75% and 100% of the recommended rates. The weeds were grown in 80 x 380 x 500 mm trays containing sandy loam soil. Five days before herbicide application at 3 to 4 leaf stage, half the trays were tilled with a garden rake to simulate cultivation. The trays were then kept close to field capacity. Lower dosages of herbicides i.e. 12.5% through to 75% rates were found to be significantly different from the control, when combined with cultivation. Atrazine performed better than nicosulfuron in percent weed control and reduction of dry weight. The only exception was on dry weight of S. verticillata, where nicosulfuron reduced weight of this weed more than atrazine. For both herbicides the 50% and 75% herbicide rates were found to be similar to the full rate. The combined effect of tillage and reduced rates was found to be significant at lower rate of herbicides and the effect of tillage was nullified at higher rates of the herbicides.
The Effect of Basal Fertiliser Level and Placement Methods on Growth, Yield and Dry Matter Partitioning in Cabbage - Philipa Marume
The efficiency and effect of basal fertiliser placement methods in growth, yield and dry matter partitioning of the common cabbage (Brassica oleracea var. capitata) cv. Hercules was evaluated on a red clay soil at the Department of Crop Science (University of Zimbabwe) from July 1996 to October 1996. Three rates of fertiliser (250, 500 and 1000 kg/ha) compound S (6:17:6 + 0.04% B) were applied using three placement methods. Spot placement on surface immediately after transplanting, hole placement just after transplanting and banding immediately after transplanting. There were significant differences in leaf area due to placement method. Hole placement resulted in the highest leaf area followed by spot and lastly banding. Placement methods were significant for the stem and leaf dry weight. Rate and placement methods had significant effects on head dry weight. The head weight for hole placement was 70.14 g and for 1000 kg fertilizer/ha it was 69.34 g. Farmers are recommended to use 1000 kg fertilizer/ha and hole placement on clay soils.
A Study of the Relationship between Seed Size and Growth Parameters of Selected Weed Species and Maize (Zea mays L.) Using Growth Analysis Techniques - Themba Sibanda
Supervisor: Mr A.B. Mashingaidze
The relationships between seed size and growth parameters of two weed species, Amaranthus hybridus L. (pig weed) and R. cochinchinensis L. (Shamva grass) and one crop, Zea mays L. (maize) were studied using growth analysis in greenhouse pot experiments. The main aim of this study was to find how weeds grow, allocate their biomass to various plant parts and how they manage to out compete large seeded crops such as maize under field conditions. Weeds in this study exhibited greater overall resource use and production efficiency than the crop and weeds also attained greater leafiness than the crop over the four-week experiment. Weeds exhibited high relative growth rates (RGR) and leaf area expansion rates (LAER) and had large leaf area ratios (LAR), specific leaf areas (SLA) and leaf weight ratios (LWR) than the crop which only had higher stem weight ratio (SWR) than the weeds. When different species were compared, small seeded A. hybridus, had the highest values of RGR, LAER, LAR, SLA and LWR and had the lowest SWR. Simple linear regressions between seed size and growth parameters indicated that LWR, SLA, LAR, LAER, RGR and NAR (net assimilation rate) were negatively related with species seed weight with only SWR exhibiting a positive relationship. At 7 DAE (days after emergence), seedling dry weight showed a positive relationship with species seed weight, but at 14 DAE the relationship was negative in all three species. The smallest seeded species A. hybridus appeared to be the best competitor as it had the highest RGR and dry weight when the experiment was terminated at 28 DAE.
Effect of Maize (Zea mays L.) and Pumpkin (Curcubita moschata (Duch. ex Lam.) Duch. ex Poir) Intra-row Intercropping on Yield and Weed Pressure - Amon Mwashaireni
Supervisor: Mr A.B. Mashingaidze and Mr C. Nyakanda
An experiment to assess the effects of intra-row intercropping of maize and pumpkin was conducted at the University of Zimbabwe Farm during the 1995/96 season. Maize hybrid R201 was intra-row intercropped with a pumpkin landrace, Nzunzu. Intra-row intercropping with pumpkins did not significantly reduce maize grain yield and maize stover biomass when compared with the sole-maize stands. However, maize and pumpkin intra-row intercropping led to poor pumpkin spread and subsequently resulted in absence of pumpkin fruit yield. The sole-pumpkins spread better than pumpkins in the intercrop situation. As a result, the pumpkins in the sole situation yielded significant amount of fruits. Weed biomasses, unlike weed numbers, were significantly higher in the pumpkin sole-crops compared to intercrops. Therefore, pumpkin intra-row intercropped in maize was effective in suppressing weed growth but did not have any effect on weed germination. Besides weed growth suppression, maize and pumpkin intra-row intercropping will allow farmers to benefit from the pumpkin leaf (useful as a vegetable) without losing out on maize yield.
Evaluation of Maize (Zea mays L.) Experimental Varieties for Nitrogen Use Efficiency - Tanaka Gumunyu
Supervisor: Dr P. Tongoona and Mr A.R. Maclaurin
Due to high cost involved in the manufacture and distribution of nitrogenous fertilizers, it has become important to develop maize (Zea mays L.) varieties that are superior in utilization of available soil nitrogen (N). This study was conducted to evaluate maize experimental varieties from CIMMYT at high and low N levels, using a selection index which included grain yield, ear leaf chlorophyll content, ear leaf area under low N only, anthesis-silking interval (ASI) and plant height. Significant interactions between maize varieties and N levels were observed for ear leaf chlorophyll content and grain yield. Significant differences among N levels were observed for grain yield, ear leaf chlorophyll content, ear leaf area, plant height and ASI. The experimental varieties showed significant differences in grain yield, ear leaf chlorophyll concentration, plant height and ASI under high or low N levels. Low N levels resulted in reduced grain yield, reduced ear leaf chlorophyll content, reduced plant height and increased ASI. Correlation analysis showed grain yield to be highly increased with chlorophyll content, ear leaf area and plant height under low N.
Evaluation of Four CIMMYT Maize (Zea mays L.) Varieties after Cycles of Recurrent Selection for Changes in Stalk Strength Using the Penetrometer - Nelson Munyaka
Supervisor: Dr P. Tongoona and Dr K. Pixley
Four CIMMYT maize (Zea mays L.) genotypes which had each undergone four cycles of recurrent selection for other agronomic characteristics like yield were evaluated using the penetrometer method for changes in stalk strength. The maize varieties used were ZM601, ZM605, ZM607 and ZM609. Penetrometer measurements were done at the internode below the ear and the first well-developed internode above ground. Root lodging counts in the field were negatively correlated with root force (r = -0.35; P<0.05). Plant height was highly significantly correlated with ear height (r = 0.81; P<0.001). There was little change in rind puncture resistance over the four cycles of recurrent selection. No definite trend in changes in stalk strength was observed in the cycles of selection. Recurrent selection for other agronomic traits such as nitrogen use efficiency, high yield and disease resistance did not affect the stalk quality significantly.
The Effect of Seed Size, Simulated Moisture Stress and Planting Depth on Germination and Establishment of Dwarf Maize (Zea mays) Hybrids - Dadirai Chihande
Supervisor: Mr A.B. Mashingaidze
Three experiments were carried out on the germination of five maize hybrids. The hybrids were three dwarf hybrids EM42*6L57, 10LK2*12Dr and 32Y*12Dr, and two conventional hybrids R201 and SC701, which were all obtained from the African Centre for Fertiliser Development. The objectives of these experiments were to determine the effect of seed size and planting depth on germination and emergence of the maize hybrids, and to determine how seed size influences germination under simulated moisture stress conditions. The experiments were carried out in a greenhouse, incubator and growth room at a maximum temperature of 250C. Seed size did not influence the percent emergence of the hybrids and there was no significant difference in the percent emergence among the five hybrids. However, all the dwarf hybrids had a higher rate of water uptake than the conventional hybrids, and within the dwarf hybrids the small seed size had a higher rate of water uptake than the large seed size lots. Under moisture stress conditions the dwarf hybrids had a higher percent germination when compared with the conventional hybrids. The advantages of the dwarf hybrids was attributed to the characteristic small seed size which tend to have a high surface area to volume ratio. It is also probable that differences between the hybrids were due to genetic factors or variation in the composition of the maize seeds.
The Effect of Periodic Harvesting of the Heading Cabbage (Brassica oleracea var. capitata) Leaves Prior to Heading on Head Yield, Quality and the Economics of Cabbage Production - Kwadzanai Mushore
Supervisor: Mr S. Masimbe
An experiment using the cabbage cultivar "Sugarloaf" (Cape Spitzkool) was conducted at the University of Zimbabwe's Department of Crop Science in 1996. The objective was to study the effects of outer cabbage leaf (wrapper) harvesting (trimming) on head yield, quality and the economics of cabbage production. Trimming of wrapper leaves is common practice among smallholder farmers who claim to get more revenue from cabbage leaf sales during crop growth, and at harvest from head sales as opposed to the conventional method of harvesting which only takes place after head maturity. The factors tested in the experiment were (i) commencement of trimming (seven and eight weeks after transplanting (WAT)), (ii) trimming intensity (number of leaves removed per plant per harvest (one or two leaves), and (iii) termination of trimming (10 and 11 WAT). Results obtained showed that low level trimming (one or two leaves) at the early cabbage head formation stage has no effect on head development. There were no significant differences in fresh head weight; fresh head density; and cabbage head dry mass for all the treatments. This was attributed to the high leaf area index, high mutual shading at the time of commencement of harvesting, and removal of ageing leaves. Also, low trimming intensity at early head formation is profitable as compared to the conventional method that only realizes revenue from head sales.
The Effect of Different Basal Fertilizers and Seed Generations on Maize Growth and Yield - Berita Musara
Supervisor: Dr I.K. Mariga
Two fertiliser types, ammonium nitrate (AN) (70 kg/ha) and compound D (300 kg/ha) were used as basal fertiliser dressings on the F1 and the F2 generation of maize variety SC601 and on the F1 generation of maize variety R201 to determine their effect on growth and grain yield.
This work was conducted on a sandy soil at Domboshava Training Centre near Harare. The growth pattern in terms of leaf area index and dry matter production were significantly affected by type of basal fertiliser dressing. The F2 generation of SC601 and those treatments with ammonium nitrate had a lower leaf area index, and dry matter production than the F1's during the early stages of growth. However, these treatment differences disappeared after four weeks.
Genotype (F1 and F2 of SC601) had no effect on grain yield. However, grain yield was also significantly (P<0.05) affected by the type of basal fertiliser used, with F1 treatments which had compound D giving higher yields than the treatments which had AN. There was no interaction effect of basal fertiliser and maize generation.
Inorganic fertilizers are expensive and recommendations to resource poor farmers as a blanket application are not often profitable. For farmers that can afford fertiliser, there is urgent need to increase the profitability of this input by using better-targeted recommendations. Basically, if a farmer has more money then he/she can use compound D as well as AN, otherwise it is better to put AN as a basal and topdressing than not apply any fertiliser.
Supervisors: Dr C. Chiduza and Mrs B. Maasdorp
The physiological manipulation of sucrose concentration and yield in cane plants was investigated at the Zimbabwe Sugar Association Experiment station in the south east lowveld of the country at an altitude of 412 m above sea level. Sucrose yield is generally low under unlimited water conditions. The study was therefore conducted to determine whether simulated drought conditions and drying-offs by cessation of irrigation of sugarcane could be manipulated in order to increase sucrose yield in sugar cane (Saccharum officinarum L.). The trials were conducted over one season (1995/1996).
A standard cane variety NCO 376 was used. This is because the variety forms 90 per cent of the area under cane. The effects of varied Et/Eo crop factors and three moisture depletion levels (drying-offs) on sucrose concentration, sucrose yield and cane yield were investigated on rich deep paragneiss soils. A laboratory polarimeter was used to determine the sucrose content in sugarcane plants after the treatments.
Irrigating the crop at factor 0.85 and drying-off at 60 per cent of total available moisture had a significant effect on both sucrose yield and concentration (P<0.05). However, reduced amount of water to the plant (factor 0.70) and excessive drying-off had marked negative effects on both cane yield, sucrose concentration and sucrose yield. There was no significant negative agronomic effects recorded when the crop was irrigated at factor 0.85 and 1.00 (standard factor).
Drought induced conditions at crop factor 0.85 and drying-off to 60 per cent of total available moisture increased the sucrose content of sugarcane. This may improve the efficiency and economic use of the available limited water resource. In addition the efficiency of sucrose manufacture and concentration was improved. The concept of deficit irrigation can be taken advantage of in an attempt to improve sucrose yield, sucrose concentration and water saving.
Maize/Bean Intercropping Planting Pattern and Density Studies - Abisai Mafa
Supervisor: Dr I.K. Mariga
An intercropping experiment was done in the 1995/96 rainy season to study the effects of two maize (Zea mays) densities and two dry bean (Phaseolus vulgaris) row arrangements on maize and bean yield, yield components, and phenological development. A factorial arrangement was used with two maize densities (i.e. 90 cm x 30 cm and 90 cm x 45 cm) x two row arrangements of two bean varieties to give a total of four sole-crop and eight intercrop treatments. The trial was planted at two sites, Domboshava (Natural Region IIa) and Chinyika (Natural Region IIb), with Chinyika receiving one dry bean variety. Growth and yield data were collected and analyzed statistically to see if treatment means differed significantly (P=0.05). Grain yield of both components were significantly decreased with intercropping, but the overall total yield was higher in intercrop than in sole-crops. Although grain yield of beans was severely depressed in intercrops, maize yield compensated for the loss giving a system's advantage of up to 30 %. Maize grain yield also decreased with increase in number of rows of beans. The bean variety Natal Sugar depressed maize yield more than Carioca. Planting pattern and density had no effect on phenological development.
BSc Agriculture (Honours) 1997
Investigating the Effect of the Environment on Rind Puncture Resistance of Maize (Zea mays L.) as Measured by a Rind Penetrometer - Sakile Nsingo
Supervisors: Dr P. Tongoona and Dr K. Pixley
Lodging in maize (Zea mays L.) remains a factor of great concern for both breeders and agronomists alike, despite the numerous efforts by plant breeders to breed for resistance to it. A major limitation in the progress of breeding for lodging resistance could be the selection technique commonly used in breeding for lodging resistance, namely actual lodging counts. Though simple to measure, its major limitation as a selection trait is its high interaction with the environment. A potentially useful method of selecting for lodging resistance, which is also simple, is selecting for high rind puncture resistance as measured by a stem penetrometer. In this study the effect of environmental differences associated with different sites on rind puncture resistance was investigated. Two CIMMYT (International Maize and Wheat Improvement Centre) experimental populations EPOP and TCLEARLY were used in the trial. These were grown over different environments, which varied in soil type, climate and rainfall, namely Harare Block J (a low nitrogen block in Harare), Harare Block 4 (an adequately fertilised block in Harare), Glendale and Matopos. Rind puncture resistance was measured at the first fully developed internode above the ground and also at the internode below the ear, within three weeks after tassel emergence in all the sites except in Block 4 in Harare, where rind puncture resistance was measured again during late grain fill. Rind puncture resistance was altered by environmental differences associated with different sites. There was also a significant genotype by entry interaction for the character. Rind puncture resistance measured at ear level was however more stable across sites than that measured at ground level. Site by entry interaction was absent for rind puncture resistance measured at ear level in the trial for EPOP, and less in the trial for TCLEARLY as compared to that measured at ear level. The different entries in the populations did not vary significantly between each other in percentage lodging, probably due to the absence of enough lodging pressure to challenge the inherent lodging resistance of the entries. The only variation between entries for lodging resistance was observed in Glendale but this was due to an outbreak of grey leaf spot disease in the area. In Glendale stem lodging was not correlated to rind puncture resistance and this was due to the effect of grey leaf spot. Rind puncture resistance was also positively correlated to plant height, stem circumference and the number of days to flowering. Grain yield was not correlated to rind puncture resistance measured at ground level in TCLEARLY, but was positively correlated to rind puncture resistance measured at ground level in EPOP. It was however negatively correlated to rind puncture resistance measured at ear level in both trials.
Some Effects of Scarification, Stratification and Pre-soaking on the Germination of Cycad Seeds (Cycas revoluta and Encephalartos altensteinii) - Fungai Muposhi
Supervisor: Dr R.M. Madakadze
Scarification, stratification and pre-soaking (to gibberellic acid and polyethylene glycol) treatments were carried out to improve the germination of seeds of two cycad species. For Cycas revoluta, mechanical scarification or six-month after-ripened seed reduced the time lag to germination by about six weeks compared to untreated seeds. For the species Encephaalartos altensteinii, all treatments had no effect on germination, possibly due to lack of enough after-ripening period of the seed (only one month). C. revoluta seed growers are therefore encouraged to nick the testa before sowing seeds provided the seeds have after-ripened for six months. However, careful disease control measures should be taken on the unenclosed seed.
Analysis of Different Maize Hybrids for Resistance to Grey Leaf Spot Disease - Cain Manzira
Supervisor: Dr Z.A. Chiteka
Grey leaf sport disease, a new disease in Zimbabwe, can cause up to 50% yield loss if fungicides are not used to control the disease. The objectives of this project were to quantify the levels of resistance to grey leaf spot on different maize hybrids, to find the effectiveness of spraying on the severity of grey leaf spot and to find the best scoring date in screening for resistance. Eleven commercially available maize hybrids were planted at the Agricultural Research Trust Farm in the 1996/97 season in two trials with and without fungicide spray to control grey leaf spot. Scores of the severity of the disease were taken on three different days, 90, 120 and 150 after planting in the unsprayed trials. In the sprayed trials Benomyl was applied at 750 g/ha. Grain yield was measured in both trials. The varieties SC709 and SC625 were tolerant to grey leaf spot giving yields of 9.93 and 8.53 t/ha respectively. These varieties also had the lowest scores in the unsprayed trials. Spraying significantly increased the yields of all the varieties. There were significant negative correlations between yield and disease scores at days 120 and 150 with r-values of 0.601 and 0.634 respectively.
Maize (Zea mays L.) Growth and Weed Dynamics Under Two-year Old Eucalyptus (Eucalyptus camaldulensis L.) Stands - Penniless Ngwenya
Supervisors: Mr C. Nyakanda and Dr O.A. Chivinge
Two-year old River Red gum tree (Eucalyptus camaldulensis L.) was undersown with maize (Zea mays L.) hybrid R201 during the 1996/97 growing season at Domboshava Training Centre. Undersowing E. camaldulensis with maize significantly (P>0.05) reduced weed numbers but not weed biomass. There was no competition for soil moisture between E. camaldulensis and maize. Lack of weeding in the year significantly reduced eucalyptus growth (P>0.05). Undersowing of maize to E. camaldulensis also reduced eucalyptus growth in the second year although this reduction was to a lesser extent when compared to that caused by lack of weeding in the first year. Furthermore, in maize grain and stover yields were not reduced by undersowing maize to two-year old eucalyptus. This suggests that farmers can successfully produce maize under Eucalyptus in the second year, as in the first year; and that Eucalyptus undersown with annual crops will benefit from the weeding and other management activities intended for the annual crop.
Effect of the Bio-control Agent Trichoderma harzianum (Rifai) on the Fungus Thanatephorus cucumeris (Frank Donk) Cause of Rhizoctonia Leaf Spot on Tobacco - Freddie Marume
Supervisors: Mrs J. Sibiya and Dr D.L. Cole
The fungus Trichoderma harzianum isolate T77 was tested for bio-control activity against the fungal pathogen Thanatephorus cucumeris the cause of Rhizoctonia leaf spot (RLS) on tobacco plants. In addition, the best time of applying the bio-control agent was also studied.
Macroscopic observations of the antagonist (T. harzianum) and pathogen (T. cucumeris) in dual culture on Potato Dextrose Agar, showed that the growth of two isolates of T. cucumeris (R552 and R557) was inhibited before contact with the antagonist. This suggests involvement of an antimicrobial substance in the antagonism. The phenomenon is referred to as antibiosis. T77 grew over the pathogen and on subculturing, only T77 was revived showing an antagonism by hyperparasitism. In both single and dual cultures T77 had a faster growth rate that both pathogen isolates, indicating that the antagonist might also competitively inhibit the pathogen.
On tobacco plants T77 was effective in reducing Rhizoctonia leaf spot disease caused by T. cucumeris (P<0.05). The number of dead seedlings due to T. cucumeris were fewer in T77 treated trays than in untreated trays for both isolates R551 and R557 (P<0.01). There was greater mortality in plants inoculated with isolate R551 than R557. Some seedlings also died of soreshin (Rhizoctonia solani teleomorph T. cucumeris). There was no significant difference in applying the bio-control agent prior to appearance of symptoms or after appearance of first disease symptoms.
Development and Non-Preference of Maize Weevil on "Stackburnt" Maize - Learners Mondo
Supervisor: Prof. D.P. Giga
A comparative study of preference and development of the maize weevil, Sitophilus zeamais, on stackburnt and normal maize was conducted in the entomology laboratory of the Department Crop Science. Populations of S. zeamais were collected from maturing maize in the field from the Department of Crop Science, freshly harvested maize from these plots, from the infested wheat in storage and from cultures maintained in the entomology laboratory of the Crop Science Department. Both the normal and stackburnt maize were collected from the Mvurwi Grain Marketing Board Depot in 1995.
There were significant differences in the preference, number of progeny emergences and mean developmental periods of the maize weevil on the two types of maize. The weevil showed higher preference for normal maize than stackburn maize. The weevil had a longer mean development period on stackburn maize than on non-stckburnt maize. Higher number of insects emerged from normal maize than from the damaged maize. This showed that normal maize support the insect life better than the stackburnt maize.
Key words: Sitophiulus zeamais, normal maize, stackburnt maize, development period, preference.
Effect of Seed Size and Planting Depth on Germination/Emergence, Plant Performance (Vigour) and Yield of Maize (Zea mays L.) - Marciline Fusire
Supervisors: Dr P. Tongoona and Dr J.D. Jewel
The project was designed to determine the optimum planting depth desirable for each seed size in a single environment in Zimbabwe. An experiment was carried out at the International Centre for Maize and Wheat Improvement (CIMMYT). Five 5 cm and 10 cm depths were used for eight different seed grades classified according to a CIMMYT classification and grading system. Total percentage of germination, plant height at the fifth and tenth leaf stage, and yield were used to evaluate the effect of maize seed size, plant performance and yield as influenced by planting depth. The Minitab statistical package was used for variance analysis. The 5 cm depth produced the tallest pants and the highest percentage of germination especially for mixed, thick and flat seeds. The mixed (largest) seed had the tallest height at both fifth and tenth leaf stage as influenced by the initial seed size. The large seed also had the highest yield due to higher vigour or plant performance as compared to the smaller seeds. Even though the latter had the largest count, it had the least seed weight resulting in the lowest tonnage. It was also concluded that a planting depth of 5 cm was ideal for early plant establishment/germination and better yields in environments with adequate moisture.
Survival of Pseudomonas syringae pathovar tabaci in Soil and Evaluation of Media for Isolation and Enumeration of the Bacterium from the Soil - Tadious Nyamayevu
Supervisor: Mrs J. Sibiya
A study was conducted to determine the condition for survival of Pseudomonas syringae pathovar tabaci (P.s. pv. tabaci) in soil and to evaluate media for isolation and enumeration of the bacterium from soil. P.s. pv. tabaci inoculum was prepared from wildfire-infected lead discs of tobacco cultivar Kutsaga Mammoth 10 (KM10).
The bacterium was made to stream out of the leaf discs and then streaked on King's medium B (KB). Inoculum was standardised to 104 cfu (colony forming units) and used to inoculate sterilized soil, (wet soil, dry soil and alternating wet-dry-wet soil). The inoculated soil was incubated at set temperatures of 100C, 250C and 300C. Population of the bacterium in soil after a given time (1 hr, 24 hrs, 72 hrs and 120 hrs) was determined by the plate dilution method. Two media, P-1 medium and Soil extract-peptone agar were tested for selectivity for the isolation and enumeration of Pseudomonas species in relation to KB.
The Effect of Fertiliser and Manure on Yield and Residual Nitrogen of Parsley, Petroselinum crispum - Onisimo Khumalo
Supervisor: Dr R.M. Madakadze
Parsley was planted in a greenhouse to test the effects of different types of manure and nitrogen fertilizers on its yield and residual nitrogen in the leaves. Potassium nitrate performed better than ammonium nitrate on the dry weight yield of parsley. The time interval between successive topdresings had a significant effect on the yield of parsley. The shortest time interval between the topdressings gave the highest yield. The green gold plus applications had no significant effect on the yield of parsley. The type of manure used significantly affected the yield of parsley, with poultry manure giving the highest yield followed by horse manure and then ruminant manure. Ammonium nitrate had the highest percentage nitrogen in parsley leaves compared to potassium nitrate. These results will be useful in giving recommendations for parsley production in Zimbabwe.
Response of Local Maize Hybrids to Sugarcane Mosaic Virus Infection - Ciphas Mlambo
Supervisor: Mrs J. Sibiya
The response of nine maize hybrids to infection by sugarcane mosaic virus strain MDB (SCMV-MDB) was tested in a field trial. The maize hybrids were mechanically inoculated with SCMV-MDB at the three- to four-leaf stage. Plants without symptoms were reinoculated at 6, 11, 16 and 28 days after the first inoculation. A disease index system suggested by Khun and Smith (1977) to evaluate maize for resistance under greenhouse conditions was used in the field. The progress of symptom development was assessed at 6, 11, 16 and 28 days post-inoculation using the severity rating method.
Both the severity rating and index methods indicated three varieties to be tolerant while the other six varieties were shown to be susceptible. In the susceptible hybrids, 10-32% of the plants had developed symptoms by six days after the first inoculation.
Analysis of plant heights before, during and after tasselling showed that growth of two of the tolerant varieties was not significantly affected. Grain yield of all the tolerant cultivars, including one of the susceptible ones, was not significantly reduced.
Evaluation of the Effect of Incidence and Time of Infection by Sugarcane Mosaic Virus Strain B (SCMV-MDB) on the Growth and Yield of Maize - Charles Karavina
Supervisor: Mrs J. Sibiya
In a field trial, the influence of time of infection and virus incidence on grain yield, plant height, and predisposing the plant to other diseases was investigated on one maize cultivar (SC501). Maize plants were mechanically inoculated with SCMV-MDB at different leaf stages and virus incidence. The time taken for symptoms to appear was noted and a description of the symptoms observed.
Symptoms appeared much earlier in early-infected plants than in late-infected plants. The earlier the plants were infected with SCMV-MDB, the greater was the yield and height reductions. An infection at the third leaf stage reduced grain yield by 25.5% and plant height by 11.6%. Virus incidence of 75 and 100% also resulted in significant yield reductions of 19.6 and 23.5% respectively. The virus infected plants were also infected by other diseases like cob rots, northern leaf blight and grey leaf spot but this was not influenced by the time of infection or virus incidence.
Moisture content and temperature effected the survival of the Pseudomonas syringae pv. tabaci in soil. P.s. pv. tabaci survived well in wet soil as compared to dry and alternating wet-dry-wet soils. In the three soil moisture regimes, maximum populations were reached after 72 hours at 250C. The bacterium multiplied slowly at 100C and 300C. It was concluded therefore that optimum temperature for P.s. pv. tabaci growth in all the soil moisture regimes is 250C. King’s Medium B (KB) had the highest mean bacterium colony forming units per gram of soil (cfu/g) making it the best medium of the three media used. It enhanced fluorescein production more than P-1 medium and soil extract-peptone agar. P-1 medium was better than Soil extract - peptone agar for the same purpose.
The Effect of Seed Size, Simulated Moisture Stress and Planting Depth on Germination and Establishment of Dwarf Maize (Zea mays) Hybrids - Misheck Chitokomere
Supervisor: Mr A.B. Mashingaidze
A study was carried out at the University of Zimbabwe to find out the effect of genotype and seed size on germination and establishment of maize seed. The study consisted of three experiments. The first experiment was designed to determine the effect of genotype (variety) and seed size on the amount of water taken up by maize seeds in the first 48 hours after immersion in water. The second experiment was designed to determine the effect of different levels of simulated moisture stress (0, -5, -10 and -12 bars), seed size and genotype on maize seed germination. The third experiment was designed to determine the effect of genotype, seed size and planting depth on emergence of maize seed in pots. Water uptake rate, as indicated by the change in mass per unit seed mass, significantly (P<0.01) increased with a decrease in seed size. Water uptake significantly (P<0.01) varied among maize varieties, but the differences were not consistent at 24 and 48 hours after immersion in water. Germination under simulated moisture stress significantly (P<0.01) decreased with increase in level of moisture stress. Large seeds had significantly lower germination than medium and small seeds under simulated moisture stress. Genotype had significant effect (P<0.01) on percent germination under simulated moisture stress. However, at eight days after seed immersion in osmotic solutions, there was a significant genotype and simulated moisture stress level interaction indicating that germination percentage of varieties differed according to simulated moisture stress level. Emergence decreased with planting depth and there were varietal differences in their ability to emerge from depth, but seed size had no effect. At 20 days after planting, there was a significant interaction (P<0.05) between variety and planting depth, showing that the ability of the different varieties to emerge differed with the depth at which they are planted.
There was a clear relationship between water uptake in the initial 48 hours and germination of varieties under simulated moisture stress.
Evaluation of Five Maize Cultivars under Four Different Fertility Levels in a Sandy Soil - Collins Musimwa
Supervisor: Dr P. Tongoona
Five Pannar cultivars, PAN 695, PAN 6243, PAN 473, PAN 6549 and PAN 6355 of maize were evaluated at four fertility levels ranging from stressful to optimum levels. Yield evaluation, which had been initially planned, was not done because of heavy leaching which caused stunting. A foliar analysis was done to determine the nitrogen and phosphorus which had been accumulated by the different cultivars. PAN 695 and PAN 473 showed highest accumulation of phosphorus and the rest did not show any significant differences. All the cultivars showed no significant differences in nitrogen accumulation. There was, however, a trend of increasing nitrogen composition (for all the cultivars) with increasing nitrogen supply. Plant height for the cultivars was also evaluated and PAN 6533 and PAN 695 were outstandingly responding quicker to increased fertility levels than the other cultivars. Plant height correlated positively with nitrogen nutrient composition.
Root morphology within a soil profile affects the extent of nutrient accumulation by a plant. The effects of fertilization on maize (Zea mays) root morphology and genotypic effects were investigated. Five maize cultivars (as above) were grown under four fertiliser rates in a pot experiment in a greenhouse. The fertility levels were proportional to those in the field experiment. The plants were grown in sand. Root and shoot samples were obtained from all the cultivars at the four-week stage. No significant varietal differences in root morphology were detected. However, root lengths, root-to-shoot ratios and root branching increased with decreasing fertility. The cultivars adjusted their root morphology at low nutrient supplies.
The Effect of Season Quality on the Time to Flowering and on Pollen Shed Synchronization with Silk Emergence of Different Maize Hybrids (Zea mays) - Hopwell G. Zheke
Supervisor: Dr Z.A. Chiteka
Eight maize hybrids, SC701, ZS206, SC601, 93MW5, SC501, R201, ZS225 and SC401 were grown for four consecutive season 1993/94, 1994/95, 1995/96 and 1996/97 at Rattray Arnold Research Station (RARS). The hybrids, SC701, ZS206, 93MW5, SC501 and R201 were stable in their days to mid-pollen shed (DMP) and days to mid-silk emergence (DMS). But ZS206, SC601 and ZS225 varied significantly (P<0.05) from season to season.
Six of the eight hybrids SC701, ZS206, SC601, ZS225, SC501 and 93MW5 were more stable in their synchronization of pollen shed with silk emergence than SC401 and R201. Thus, SC401 and R201 were the least stable of the eight hybrids.
Diallel Analysis of Quality Protein Maize Resistance to the Maize Weevil (Sitophilus zeamais Motsch) – Vivian Kazembe
Supervisor: Professor D.P. Giga
The maize weevil is a serious pest of stored maize grain worldwide and causes significant losses, particularly in small farmers' stores. The long-term solution for reduction in loss is maize varieties that are resistant to storage pests, especially the maize weevils. CIMMYT-Zimbabwe is currently testing some Quality Protein Maize hybrids from Mexico for adaptation in Zimbabwe. As part of this test the Quality Protein Maize hybrids were evaluated for resistance to weevils in no-choice and free-choice tests. The measures of susceptibility in the no-choice test were as follows:
(i) the number of dead weevils 10 days after infestation out of 32 weevils originally infested in each sample;
(ii) the number of F1 progeny weevils that had emerged 46 days after infestation; and
(iii) the weight loss of the grain 46 days after infestation.
The other measure of resistance was weight of grain remaining after 56 days of exposure to weevils in free-choice environment. Percent parent mortality ranged from 62 to zero. This was generally regarded as unsatisfactory and therefore gene action was not determined. Among the Quality Protein Maize hybrids, the mean number of F1 progeny weevils ranged from 3.53 for hybrid P5\P2 to 106.70 for hybrid P6\P4. The lowest mean weight loss in the no-choice test was 0.2% for P3\P2 while P7\P5 lost 8% of its original weight. In the free-choice test weight loss ranged from 5.1% for P3\P2 to 13.7% for P6\P4. For all the parameters of susceptibility measured differences between genotypes existed and in most cases very large. The no-choice test was used to measure the level of the antibiosis mechanism of resistance while the free-choice test was used to measure the non-preference mechanism of resistance. Specific combining ability was found important for non-preference. A method of recurrent selection for specific combining ability was suggested as the breeding strategy to improve antibiosis among the Quality Protein Maize hybrid lines studied. To concentrate for non-preference, a method of reciprocal recurrent selection was suggested.
Studies of Zinc Nutrition in 'Low-Chill' Apples - Mercy Chivinge
Supervisor: Mr S. Masimbe
An experiment on zinc nutrition in 'low-chill' apple (Malus domestica Borkh) cultivar "Anna" was done to determine the effects of apple rootstocks MM106, MM111 and "Anna" seedling on leaf zinc, nitrogen, phosphorus, calcium and iron and on the canopy volume in Marondera in 1997. The trees were planted in 1993 in a randomised complete block design with a split plot arrangement of treatments. The main plot was the tree/rootstock and the subplots were leaf types: leaves showing zinc deficiency symptoms ("little leaf") and symptomless ("normal") leaves. "Anna" seedling had a significantly (P<0.05) larger canopy volume (7.750 m3) than MM106 and MM111 (4.120 m3), and it also had significantly (P<0.05) higher leaf iron levels (138 ppm) than MM106 and MM111 (118 ppm). Symptomless leaves had significantly (P<0.05) higher leaf zinc and calcium levels than symptomatic leaves had significantly (P<0.05) higher leaf nitrogen and iron that symptomless leaves. There were no significant (P>0.05) differences in phosphorus levels between symptomatic and symptomless leaves. Latent deficiency of zinc was observed in "normal" leaves (17.111 ppm) when adequate levels should be between 35 and 50 ppm. Farmers therefore need to increase their zinc supply from 4 g/tree at planting the "low-chill" apple cultivar "Anna". Using standard phosphorus to zinc rates, the zinc deficiency was shown to be phosphorus induced. The iron levels in the symptomatic and symptomless leaves were as much as + 100 ppm when the recommended levels should be + 50 ppm. There is, therefore, need to reduce iron levels.
Effect of Different Rooting Media and Rooting Hormone on the Propagation of Abelia grandiflora, Spirae cantoniensis and Plumbago capensis - Kudakwashe Jerry Dengu
Supervisor: Dr R. Madakadze
The effects of four rooting media (vermiculite, pine bark, sand and soil) and the presence and absence of a rooting hormone (indole butyric acid, IBA) on the propagation of ornamental shrubs (Abelia grandiflora "tricolor", Spirae cantoniensis and Plumbago capensis) using stem cuttings were studied. Medium type used was found to have an effect on the number of cuttings that survived to develop roots in A. grandiflora and P. capensis. Cutting survival was greatest in soil and in sand for A. grandiflora and P. capensis respectively. For A. grandiflora cutting survival was lowest in vermiculite and for P. capensis it was lowest in pine bark. All cuttings of P. capensis treated with rooting hormone, except those in sand, did not survive past six weeks. Of the untreated cuttings only those in pine bark completely perished before six weeks. All shrubs had the greatest mean shoot numbers in sand. The presence of rooting hormone improved shoot numbers in sand and vermiculite for P. capensis.
In all three shrubs, there were no significant differences in the rooting percentages in the four medium types. Rooting hormone treatments were found to have no effect on rooting of the three shrubs. It is suggested that the best medium for the propagation of the shrubs is a mixture of sand, soil and a lighter medium than the two.
Supervisor: Dr O.A. Chivinge
The effect of residual atrazine on the weed seed bank was evaluated in the herbicide and fertiliser interaction trials conducted in Chinyika Resettlement Area (CRA) from 1993/4 season to 1996/7 season. The herbicide was either full cover sprayed or banded 45-cm in the crop row. Hand weeding at two and five weeks after crop emergence was used as a control. Weed counts were done from soil samples collected at maize harvest, in each season. The main weed species were Galinsoga parviflora L. (gallant soldier), Eleusine indica (L.) Gaertn. (rapoko grass), Richardia scabra L. (Mexican clover), Amaranthus hybridus L. (pigweed), Leucas martinicensis R.Br. (bobbin weed), Tagetes minuta L. (Mexican marigold), Setaria homonyma (Steud.) Chiov. (fan-leaf bristle grass) and Cyperus rotundus L. (yellow nutgrass). Atrazine was persistent enough to keep the total number of viable weed seeds in the soil low. There was a general decline in the weed seed bank over the four-year period for both the resistant and susceptible weed species. The numbers of weed species had declined by the fourth year.
Grain Yield, Light Interception and Canopy Development in Intercrop Systems of Maize and Sorghum with Cowpea in Natural Region Five - George Madzokere
An intercropping experiment is described for the 1996/97 growing season, at Save Valley Experiment Station. The experiment was a row intercrop in which maize at 11 000 and 22 000 plants/ha and sorghum at 33 000 and 66 000 plants/ha were intercropped with a single or double rows of cowpeas spaced at 0.75 m x 0.1 m and 0.385 m x 0.1 m between the cereal component respectively. Combined grain yields of maize + cowpea and sorghum + cowpea were more than the yield of sole components and therefore the total productivity of the system was well above that of pure stands. Maize at 22 000 plants/ha intercropped with one row of cowpea and sorghum at 66 000 plants/ha intercropped with one row of cowpea proved to be the best intercropping systems in terms of grain yield and land equivalent ratio (LER). Leaf area index of maize and sorghum (LAI) was lower in intercropped systems than in pure stands. In terms of light interception, intercropping systems were more efficient in light utilization compared to sole crops. The most efficiency was realized in intercrop systems of cereals at high populations with two rows of cowpea during the first weeks due to early canopy cover but during the later weeks there is was no significant difference in light interception between the intercrop systems.
Introduction of Improved Fallows of Sesbania (Sesbania sesban L.) and Pigeonpea (Cajanus cajan L.) Under a Maize (Zea mays L.) Stand - Nyasha Tinofirei
Supervisor: Mr C. Nyakanda
Cajanus cajan (pigeonpea), Sesbania sesban (sesbania) and a mixture of the two tree species were sown under a maize stand in the first year of improved fallow establishment as a way of reducing fallow periods in view of land shortage in smallholder areas. Maize grain yield was not significantly affected by the presence of the trees. However, the trees affected maize stover weight. In turn the presence of maize did not affect pigeonpea growth, grain and pod weight neither was the growth of sesbania affected by the maize crop. Therefore, smallholder farmers can establish improved fallows under a maize stand in the first year of fallow establishment with their primary objective of maintaining a "full" maize yield being achieved.
Effect of Maize Canopy Architecture and Nitrogen Fertilizer Rates on Bean Yield in a Maize+Bean Intercrop - Justin Chipomho
Supervisors: Dr I.K. Mariga and Dr S.R. Waddington
An intercropping experiment was conducted in the 1996/97 rainy season to study the effect of maize (Zea mays) canopy architecture and nitrogen rate on yield in a maize+bean intercrop trial. Two maize hybrids, which differ morphologically, SC 501 with a planophile canopy architecture and PHB 3442 with an erectophile canopy architecture, were used. The maize hybrids were intercropped on the row with Umkuzi variety of bean. The experiment had a total of eight intercrop treatments and five sole crop treatments. A two by four factorial arrangement in a randomized complete block design with three replicates was used. The trial was conducted at Domboshava Training Centre in Natural Region IIa. Growth measurements were taken at six weeks after emergence during the bean flowering phase. Yield data was collected and analyzed statistically to see if treatment means differed significantly (P = 0.05). Maize yield, in the intercrop averaged with no significant difference between the two hybrids. Fertilizer level had an effect on maize with both hybrids responding well to increase in nitrogen level. Maize yield increased by 38 kg per single kilogram unit of fertilizer. Bean yield responded to applied nitrogen between zero and 30 kg N per ha, however there was no significant difference between the different nitrogen rates of 30, 60 and 90 kg N per ha. Maize variety had an effect on bean yield with PHB 3442 intercropped bean giving a higher yield than SC 501. Yield reduction of the bean crop with SC 501 was between 25 and 56% and with PHB 3442 between 13 and 30%.
Pumpkin (Cucurbita maxima L.) as a Live Mulch Under a Maize (Zea mays L.) Stand - Khayelihle W. Dube
Supervisors: Mr A.B. Mashingaidze and Mr C. Nyakanda
An experiment to assess the effectiveness of pumpkin as a live mulch under a maize stand was conducted at the University of Zimbabwe farm during the 1996-1997 season. Maize hybrid R201 was intra-row intercropped with pumpkin Flat White Boer variety. The pumpkin-maize intercrop weeded at three weeks only had a similar weed biomass as the sole-maize weeded at 3, 8, and 12 weeks, suggesting that the extra weedings at 8 and 12 weeks after planting were not necessary in the pumpkin-maize intercrop. The intercrop weeded at three weeks only was also able to replace two weedings (at 8 and 12 weeks after planting) as regards Nicandra phylosades counts. The degree of weed control in the sole-maize and intercrop situation was the same. This means that maize was the major suppressor of the weeds. However, statistically, most of the pumpkin fruit yields under the maize stands and from the sole-crop stands were similar, except for the sole-pumpkin yield weeded at three weeks only which had significantly the highest yields.
The sole-maize stand weeded three times at 3, 8 and 12 weeks and that weeded at three weeks only gave significantly higher yields compared to their correspondingly weeded intercrop yields. A late weeding at eight weeks only and not weeding the sole-maize stands at all resulted in similar yields compared to the correspondingly weeded intercrops.
Generally farmers can save on labour expended in weeding operations and gain significantly in terms of extra pumpkin fruit yields via intercropping at a slight compromise on the maize grain yield. However, the intercrops had land equivalent ratios higher than one (107-178%), showing a more efficient utilisation of land and consumable resources.
Dry Bean Variety Evaluation for Maize/Bean Intercropping - Jerita T. Chiyanike
Supervisors: Dr I.K. Mariga
An intercropping experiment was done in the 1996/97 rainy season to evaluate eight bean (Phaseolus vulgaris) varieties in maize (Zea mays L.) bean intercrop. Of the eight varieties, Natal Sugar was used as a control as it is the most widely grown in the smallholder sector. Results from national trials have shown that there are other varieties that yield higher than Natal Sugar in sole cropping. This trial was to evaluate some of these varieties in intercrops as this is a fast growing practice in the smallholder sector. The trial was conducted at two sites, Domboshava (Natural Region IIa) and Chinyika (Natural Region IIb). A split plot design was used, the maize and beans were planted in the same row (maize at 90 cm x 30 cm spacing and beans at 90 cm x 10 cm spacing). Yield data was collected and analyzed statistically to see if treatment means differed significantly (P£0.05) both the maize and bean yields in the intercrop were reduced. There were no significant differences in the maize yield from the different treatments but there were significant differences in the bean varieties. The dry varieties tested yielded higher than Natal Sugar in the intercrop with A286, MC5001 and 36/6/10 giving the highest yields.
A Survey on the Farmer Participation in On-farm Trials, and the Impact of On-farm Trials in Chinyika Resettlement Area of Zimbabwe – Sheila Mageza
Supervisor: Dr O.A. Chivinge
A survey was carried out in Chinyika Resettlement Area (CRA) in late February 1997 to determine farmer participation during on-farm trials and to assess the impact of the on-farm trials on farmers in the area. Farmer participation in this area has been observed to be very low, especially during such operations as planting, fertilizer top dressing and harvesting. Reasons for low participation were mainly due to researchers managing their trials and not encouraging farmers to participate and this made farmers think that they were incapable of running trials on their own without researchers. Another reason was that some of the operations were done when farmers also needed to attend to their own fields for example planting, where both farmers and researchers need to establish their early plantings within a few days whilst the soils have adequate moisture for germination. The other problem was that extension worker-researcher linkages were very weak and this also weakened extension-farmer linkages.
Extension workers' failure to participate was also partly due to inadequate finances to visit trials on a regular basis. In order to get the best out of on-farm trials there is need for farmers, researchers and extension personnel to work jointly as a team and they should be willing to learn from each other.
BSc Agriculture (Honours) 1999
Supervisors: Dr P. Tongoona and Dr M. Banzinger
Ten maize inbred lines obtained from the Centro International de Majoramiento de Maiz Trigo (CIMMYT) were evaluated in a greenhouse experiment for genotype responses to seedling drought stress. Inbred line CML202 was used in the first experiment to determine the most suitable parameter for measuring desired drought stress intensity. The level of drought stress at which 50 percent of inbred line CML202 seedlings would not recover when re-watered was used to screen the other nine genotypes in the second experiment. Parameters that were used of predicting drought stress included actual evapotranspiration (ET), growing degree-days (GDD), days after planting (DAP) and potential evapotranspiration (PET). Potential evapotranspiration seemed to be the most suitable of genotype response to drought stress. Fifty per cent plant survival was at 38 mm PET. Genotypes were provided with an initial irrigation to ensure germination and re-watered after signs of drought stress. Soil water potential and ET were determined by weighing each pot. Potential evapotranspiration was measured using evaporation pans. Among the ten inbred lines evaluated, inbred line Z180017 had the highest plant survival (58%) and the least leaf senescence (58%). Inbred line Z180028 had the least plant survival (8%) and highest leaf senescence (92%). The results indicated that genotype differences to the response to seedling drought stress existed in maize inbred lines.
The Effect of Grey Leaf Spot on some Commercial Maize Hybrids in Zimbabwe - Gorden Kimbini
Supervisor: Dr P. Tongoona
The objective of this project was to evaluate the effect of grey leaf spot on some locally available commercial maize hybrids on yield and to evaluate the effect of grey leaf spot on days to flowering of the commercial maize hybrids. Twenty-four commercial maize hybrids were planted at Cargill Research Station during the 1997/98 agricultural season, in two trials which included fungicide spraying and no fungicide spray to control the grey leaf spot. Three hybrids of maize, which are resistant to grey leaf spot, were used as checks. The resistant checks showed no yield reductions between sprayed and unsprayed treatments whilst susceptible entries showed significant reduction of yield in unsprayed treatments when compared to the respective sprayed treatments. Furthermore grey leaf spot also delayed days to flowering for those hybrids which succumbed to the disease.
The Residual Effects of Eucalyptus (Eucalyptus camaldulensis Dehn.) Stands on Weed Dynamics and Maize Yield - Gorden Mabuyaye
Supervisors: Mr C. Nyakanda and Dr O.A. Chivinge
A maize (Zea mays L.) hybrid SC601 was planted at Domboshava Training Centre in the 1997/98 season, on land that previously had two-year old Eucalyptus camaldulensis stands. The aim of the study was to explore the residual effects of eucalyptus stands on weed dynamics and maize yield. The Eucalyptus had residual effect on weeds, resulting in reduced biomass (P<0.05) and weed seedbank (P<0.05). There was, however, no effect on maize harvest indices and maize yields (stover and grain) (P>0.05). Significant interaction was observed between depth and treatments (P<0.05) on the content of soil mineral nitrogen. In treatments that previously had sole maize and those that had eucalyptus, soil nitrogen content decreased with depth (from 0-40 cm) and rose sharply at the 80 cm to 100 cm depth stratum. However, in the uncultivated land, soil mineral nitrogen content increased with depth (from 0-40 cm) and decreased sharply at the 80 cm to 100 cm depth stratum. Since the residual effects did not reduce the harvest indices and maize grain and stover yields, the results suggest that maize can be grown successfully on lands that previously had two-year old E. camaldulensis stands. The reduction in weed biomass and weed seedbank could be useful to farmers because they would save the labour for weeding and could obtain higher crop yields due to less weed competition for plant nutrients.
Assessment of the Effects of Maize Streak Virus (MSV) and Evaluation of MSV Resistance in Experimental and Commercial Hybrids - Farayi Murungu
Supervisors: Dr P. Tongoona and Mr Mutseyekwa
Sixty-seven topcross experimental hybrids, derived from crossing F5 inbreds and some testers, and nine commercial hybrids were naturally infested with the maize streak virus (MSV) disease. The main objective of this study was to select for F5 inbred lines that, performed well in topcrosses to be used to develop MSV resistant hybrids. The Cicadulina leafhoppers that infected the experimental material over-wintered in a wheat block. Wheat is an alternative host plant for the leafhoppers. The performance of the topcross experimental hybrids was measured against established varieties that are both resistant and susceptible to MSV. Measurements were also made to determine the relationship between plant heights, yield and MSV scores.
Maize streak virus infection resulted in reduced plant heights and lower yields, but the varieties differed in the tolerance to the disease. Entries 3 and 32 were very susceptible, while entries 42 and 74 were very tolerant of the disease. Correlation analysis showed that there was no significant relationship between MSV scores and plant heights and flowering dates in the topcrosses. Correlation analysis, however, showed that there was a significant positive relationship between MSV scores and yield. The F5 inbred lines used in topcrosses 42, 74, 75, 62, 14, 34, 40, 56, 16, 61, 66, 48, 35, and 43 must be advanced as they performed well to produce tolerant hybrids to MSV and high yield. The rest of the F5 lines, which did not perform well in topcrosses, should be discarded.
Influence of Heat Treatment on Postharvest Behaviour of Tomatoes - Washington Chiwato
Supervisor: Dr M.T. Masarirambi
Fruits, vegetables and root crops are much less hardy and are more quickly perishable, and if care is not taken in their postharvest handling, they soon lose their quality and value. Reduction in this wastage, particularly if it can be economically avoided, would be of great significance.
Tomato (Lycopersicon esculentum L. Mill.) fruit ripening is reversibly inhibited at high temperatures (300C and above). In this study, the effects of high temperatures in terms of red colour development, firmness, taste and susceptibility to chilling injury were investigated. Tomato fruit at turning stage were exposed to 200C, 250C, 300C or 350C and 95% relative humidity (RH) for 24, 48 or 72 hours. Some were transferred to air at 200C and others were placed in chamber at 50C, 90% RH, before transfer to air at 200C for ripening and assessment of chilling injury respectively. Tomatoes exposed to high temperatures for 24 hours showed little difference in colour development compared to those exposed to lower temperatures. Increasing the duration of high temperature treatment to 48 and 72 hours at 350C reversibly inhibited subsequent red colour development at 200C while prior exposure to 300C stimulated colour development. Also, tomato fruit exposed to higher temperatures (300C and 350C) were firmer than those exposed to lower temperatures (200C and 250C), although there was little differences in taste. Tomato fruit heated at high temperatures prior to storage at 50C did not develop chilling injury symptoms whilst those that were either heated at lower temperatures or were not heated prior to cold storage developed sever chilling injury (skin discoloration, uneven ripening and softening). These results suggest that postharvest heating of tomatoes result in development of better quality characteristics. These benefits occur as the residual effects of a non-chemical treatment, which does not need to be imposed continuously throughout storage to be effective.
Production of Tobacco (Nicotiana tabacum) Seedlings in Floats and South African Seedbeds Without Use of Methyl Bromide and the Effect of Monopotassium Phosphate (MKP) as a Chemical Hardening Agent - Leonard Bukuta
Supervisor: Dr C. Chiduza
Two trials were carried out at Kutsaga Research Station, Zimbabwe, to find a suitable and cost effective alternative to the conventional method of producing tobacco seedlings using methyl bromide as a soil fumigant, monopotassium phosphate (MKP) was also tested as a chemical hardening agent. One trial was a seedbed trial and the other trial was a field trial. The objective of the seedbed trial was to compare seedlings produced using either the speedling trays, the South African permanent beds or the conventional method and the effect of MKP on these seedlings as a chemical hardening agent. The objective of the field trial was to compare the performance of the differently produced tobacco seedlings and the effect of MKP on the seedlings after transplanting. Seedling lengths and diameter measurements at pulling showed no significant differences due to production method (P>0.05) and MKP application had no significant effect (P>0.05) on the type of seedling produced. Stalk heights and saleable yield measurements showed significant differences due to seedling production method (P<0.05) but there was no benefit from applying MKP as a hardening agent. In a Net Present Value (NPV) economic analysis, the South African method of raising tobacco seedlings was found to be a more suitable and more of a cost-effective substitute than the speedling tray method and therefore recommended on the basis of its economics.
The Evaluation of the Fungicide Eria 187.5SC for Grey Leaf Spot (Cercospora zeae-maydis) Control - Lombard K. Muzulu
Supervisor: Mrs J. Sibiya
Two trials were set up in the 1997/98 season at two sites, that is, one at Henderson Research Station along the Mazowe Road and another at Spring Farm in the Arcturus farming area. The objective was to establish the effectiveness of Eria l87.5 SC, a combination product of Difenoconazole (a triazole) and carbendazim (a benzimidazole), for grey leaf spot (GLS) control. Its performance was compared to the currently used fungicides, Benlate (Benomyl) and Bavistin (Carbendazim). The fungicides were used on the high yielding long season (ZS206) and the short season (SC401) GLS-susceptible maize hybrids to produce different levels of blighting and to determine their effects on grain yield. The GLS twin nozzle applicator was used to apply the fungicide treatments and to ensure adequate fungicide coverage of the leaves above the cobs. Two fungicide applications were done at an interval spacing of 17 and 25 days from the first application date for the Henderson and Spring Farm sites respectively. Disease symptoms were first observed at the Henderson trial at 78 days after planting and disease development was quite rapid. At Spring Farm, the first symptoms of the disease were observed at 55 days after planting. However, disease establishment was slow on the SC401 cultivar at Spring Farm although development after the first week steadily rose to a peak of 95% leaf blighting in the untreated control at 53 days after the first fungicide application.
No fungicide treatment regime managed to keep the plants free from blighting, although disease severity was greatly reduced. Fungicides were shown to slow down the development of the disease thereby lowering the final disease severity and infection rates resulting in considerable yield benefits. The treatments produced yield increases of 26.2% to 80.6%. Eria and Score, both containing a triazole group which has some growth promoting properties, had the highest yields. The good activity of Eria has shown it to be an ideal option for anti-resistance management. Fungicides can be applied successfully and effectively using the GLS twin nozzle applicator.
Effect of a Pumpkin Smother Crop (Curcubita maxima L.) in a Maize (Zea mays L.) Stand - Benhilda P. Chihota
Supervisor: Mr A.B. Mashingaidze
To assess the effectiveness of using a pumpkin smother crop in weed management in a maize stand, an experiment was conducted at the University of Zimbabwe (UZ) Farm during the 1997/98 growing season. Maize hybrid Pan 65 and the pumpkin cultivar Flat White Boer were grown in three cropping systems; sole maize, sole pumpkin and maize with a pumpkin smother crop. The crops received each of four weeding regimes, no weeding, weeding at 3 WAP (weeks after planting). Maize yields for the sole maize and the maize intercropped with the pumpkin did not significantly differ (P>0.05). Pumpkin leaf number was not affected by any of the two factors but pumpkin main vines were significantly longer (P<0.05) in the maize pumpkin intercrop than in the sole pumpkin crop. Weeding a maize stand with a smother crop (maize-pumpkin intercrop) once at 3 WAP resulted in the same weed biomass with weeding the intercrop twice at 3 and 5 WAP and thrice at 3, 5 and 8 WAP as well as the sole pumpkin crop and the sole maize stand three times at 3, 5 and 8 WAP. Farmers can therefore save on labour by introducing a pumpkin crop into a maize crop for weed management. Time saved can be used in other operations like granary preparations. The farmer also benefits from the pumpkin leaves that are served as a relish and fruits that are served as a side dish. Since the introduction of the pumpkin crop into the maize crop did not reduce the maize grain yield and stover weight farmers will have no opportunity cost by growing the maize-pumpkin intercrop. Practising maize-pumpkin intercropping to exploit the ability of the pumpkin crop to smother weeds will therefore be highly beneficial to the farmers in different aspects.
Heritability of Maize Streak Virus and Grey Leaf Spot Disease Resistance in Full-sib Families of Two Open-pollinated Maize Varieties ZM605 and ZM607 - Loretta T. Rukobo
Supervisors: Dr P. Tongoona and Dr K. Pixley
A study was carried out at Thornpark Estate to determine the levels of heritability for the resistance to grey leaf spot disease (GLS) and maize streak virus (MSV) in two populations of full-sibs derived from the open pollinated varieties ZM605 and ZM607. The estimates of heritability would then be used to determine whether selection for resistance in these populations would be worthwhile. The full-sibs were planted in a complete randomised block design. The variance component method was the main method used to calculate heritability estimates because of the unavailability of parental scores for MSV. Correlation and regression analysis of the mid-parent score and offspring scores were carried out for GLS. The disease scores were also correlated to grain yield and stem lodging counts. Heritability estimates for resistances in the two populations for the two diseases were different. Estimates for MSV were significantly low in both populations compared to GLS estimates and with regard to the two populations ZM607 had higher estimates with all the methods although some were not significantly higher. Heritability estimates obtained using the variance component method were 0.16 and 0.18 for MSV in ZM605 and ZM607 respectively and for GLS 0.43 for ZM605 and 0.46 for ZM607. The heritabilities for resistance to GLS estimated using correlations and regression analysis in ZM605 ranged from 0.40 to 0.46 and 0.21 to 0.32 respectively. The heritabilities were slightly higher in ZM607 with estimates ranging from 0.45 to 0.59 and 0.26 to 0.29 respectively. Correlation coefficients of grain yield with the disease scores were generally negative with values ranging from 0 to -0.32. Correlation for stem lodging was generally small to the point of insignificance, the values ranged from 0.08 to -0.03. These results were interpreted to mean that selection for resistance for these diseases would result in low gains and that the diseases negatively affect grain yield in these two populations.
Survival of Pseudomonas syringae pv. tomato in Association with the Soil, Hosts and Non-Hosts Plants - Zira Mavudzi
Supervisor: Mrs J. Sibiya
A study was carried out to determine the survival of Pseudomonas syringae pv. tomato in the soil, host and non-host plants. Sandy, loam and clay soils were collected from Chinhamora communal areas and sterilized before inoculation with an estimate of 5 x 1010 colony forming units (cfu)/ml P. s. pv. tomato bacterium. The inoculated soils were incubated at room temperature in the laboratory. Two soil conditions wet and dry were maintained throughout the experiment for each of the soil types. Bacterial population in the different soils was measured by the plate dilution method on a monthly basis for four months. To determine survival on host and non-host plants, two crops were used in rotations: okra and cabbage, two solanaceous crops: paprika and potato, two weed species: Amaranthus hybridus and Bidens pilosa, and two tomato varieties: floraded and rossal, which were inoculated with a population of 1 x 105 cfu/ml. The bacterial counts were made at 1, 2, 4, 8 and 12 weeks after inoculation by washing the surface and then plating the washing on King's Medium B. Soil type, soil moisture significantly (P<0.01) affected the survival of P. s. pv. tomato in the soil. The bacterium survived well in wet soils as was shown by higher of colony forming units than in dry soil. Sandy soils had the highest number of colony forming units then loam and clay soils had the least. The bacterium decreased significantly from January to April in all the soil types, in both dry and wet soil. By the fourth month, April, there were no surviving bacteria in all the soil types. P. s. pv. tomato was detected on the surface of tomato cultivars, rossal and floraded, A. hybridus, paprika, potatoes and okra. The population increased from the first week after inoculation. Rossal had the highest number of surviving bacterial than all the other species. Also P. s. pv. tomato survives on leaves of host and non-host plants as an epiphyte since it could be recovered by surface washing. P. s. pv. tomato did not cause any disease on these non-host plants, though it was present. No colonies of the bacterium were detected on cabbage and Bidens pilosa leaves.
It was concluded that as the texture becomes finer the bacterial growth decrease as air spaces decreases. Therefore, sandy soils support more bacterium than loam and clay soils respectively. Wet soils had a higher population of the bacterium than dry soils. Therefore, the best combination was wet sand soil. The conclusion on host and non-host plants was that P. s. pv. tomato survives on the surface of rossal, floraded, A. hybridus, okra, potatoes and paprika as an epiphyte. Therefore, okra, potatoes and paprika should not be rotated with tomatoes. Volunteer tomato plants and weeds should not be left in the field, as these can be initial sources of inocula.
Stability of Yield and Yield Components for Five Long-season and Four Short-season Groundnut Cultivars in Zimbabwe - Lawrence Mjere
Supervisor: Dr Z.A. Chiteka
Stability refers to performance uniformity of a cultivar across seasons and environments and is classified as the euclidean distance from the environmental index. A desirable genotype is one that has a high mean, a regression co-efficient of 1.0 and zero deviations from the regression. The main objective of the study was to evaluate stability of yield and yield components of five long-season and four short-season groundnut cultivars in Zimbabwe. Five long- and four short-season groundnut genotypes were evaluated using data from yield trials conducted at six sites for two years (long-season genotypes) and at five sites for three years (short-season genotypes) in Zimbabwe. Analysis of variance for seed yield, seed size and shelling percent were carried out. Regressions were also carried out using site means as environmental index. Chisumbanje and Panmure produced highest seed yields for short-season genotypes at 2.5 t/ha and Matopos lowest seed yields at 1.3 t/ha. Harare produced the highest seed yields for long-season genotypes at 3.0 t/ha while Kadoma produced the lowest seed yields at 2.0 t/ha. The short-season genotype ICGMS/56 was stable and high yielding while the variety Valencia R2 was relatively stable and low yielding. The short-season genotype 93/7/2 produced exceptionally higher yields at low potential sites and lowest yields at high potential sites. The long-season genotypes ICGM-339 and 97/8/4 were stable and high yielding while the varieties Flamingo and Makulu Red produced yields below average at most sites. Stable higher yield varieties could be recommended for production in high input areas and low yielding varieties in low input areas for sustainable production.
Diallel Analysis as a Tool for Analyzing Resistance to Maize Streak Virus Disease in Selected Maize Lines - Rose Machiridza
Supervisor: Dr Z.A. Chiteka
Maize streak virus disease (MSV) causes severe yield losses in maize in many countries south of the Sahara. The use of tolerant cultivars is the most economical and environment friendly method of managing the disease. This study analyses resistance to MSV of maize lines obtained from a diallel cross involving seven parents which varied in their levels of resistance to MSV. The seven parents were all CIMMYT maize lines and they were CML197, CML202, CML205, CML206, CML216, CML221 and CML312. These were planted in the greenhouse and in the field at the CIMMYT Research Station in the 1997/98 season. In order to develop resistant cultivars, knowledge of the type of gene action that is operative and the performance of the lines in the greenhouse and in the field is important. The genotypes were inoculated with the MSV virus by artificially infesting them with viruliferous Cicadulina mbila leafhoppers. Genotype responses were rated for MSV on two dates using a 1 to 5 scale where 1 was the rating for the genotypes not showing the streak symptoms. A rating 2 was of the genotypes showing light streaking on the old leaves, which decreased on the young leaves. For genotypes with moderate streaking on the old and young leaves a rating of 3 was given and 4 was for the genotypes that were severely streaked and were displaying some stunting. A rating of 5 was for genotypes that had severe streaking, were stunted and were dying. An analysis of variance was carried out for both the greenhouse and the field scores and for general combining ability (GCA) and specific combining ability (SCA). There were significant differences (P<0.05) for both the greenhouse and the field tests for the second date of scoring but not for the first date of scoring. Significance was also observed for GCA (P<0.05) and not for SCA for the second score in both the greenhouse and the field. Additive genetic variability ratio for GCA to SCA was 4.25 for the greenhouse and 3.92 for the field and this indicates that additive variance was more important than non-additive variance in the determination of resistance to MSV. Early generation selection for resistance to MSV would be an effective strategy in population improvement. There was no correlation between the greenhouse and the field ratings for MSV, indicating that greenhouse ratings for MSV cannot be used as a predictor of field resistance to MSV.
An Investigation into the Tissue Culture of Paprika for the Purpose of Rapid Regeneration Of Disease-free and Resistant Material for use in Commercial Production - Temesai S. Chimsoro
Supervisor: Dr A.I. Robertson
Paprika (Capsicum annuum), a solanaceous crop, is fast becoming one of the best income earning crops in Zimbabwe. It is mainly grown in Natural Regions IIa, IIb and III, but can also be grown in Natural Region IV. This production is carried out by both large scale and small-scale commercial farmers. Because of this, extensive work should be done so as not to be caught unaware by rapid increases in bacterial, fungal or viral diseases as well as pest infestations, when paprika becomes a major crop. Some farmers are already experiencing serious losses due to these diseases and pests.
Tissue culture techniques have been incorporated in the development and production of "clean" and resistant plant material in other commercial solanaceous crops like tomato (Lycopersicum esculentum), potato (Solanum tuberosum) and the eggplant (Solanum melongena). The same should therefore be done for paprika. As this is a new crop to the Tissue Culture Team in the Department of Crop Science, initially the standard Tissue Culture techniques were embarked on, to discover how they respond in this relatively new crop. These involved: (i) germination of seed in vitro (sterile), (ii) production of plantlets in vitro from sterile nodal cuttings from (i), (iii) production of calli from cotyledonal and leaf tissue from selected genotypes; (iv) production of calli from apical meristems or stem tissues and (v) regeneration to produce clonal plantlets (somatic embryos) from calli or from leaf or stem parenchyma.
Of the experiments carried out on paprika, (i) to (iii) were carried out successfully. There was partial success in the regeneration as only roots were obtained. Paprika seed that was planted during pilot tests on both media and cotton wool germinated. This allowed for the other experiments to be carried out under controlled aseptic tissue culture conditions. Seed was then sterilized and planted so as to carry out experiments on germination, that is, effect of pH and effect of temperature on production. For the callus production one square centimetre discs were obtained from the cotyledonal and first leaf ex-plant. On obtaining the callus the masses of cells were viewed under the microscope and the early stages of embryogenesis were observed.
Key words: Paprika, tissue culture, germination, callus, sub-culture, regeneration, embryos.
Interaction Between Maize Streak Virus and Maize Dwarf Mosaic Virus in Maize Plants - Amos Tendere
Supervisors: Mrs J. Sibiya and Dr E.R. Kunjeku
Interaction between maize streak virus (MSV) and maize dwarf mosaic virus strain A (MDMV-A) was studied in a susceptible variety of maize (Zea mays L.) R201. Maize plants were inoculated with either MSV, MDMV-A or both at two growth stages (3 to 4 leaf stage or 5 to 7 leaf stage). MSV was transmitted by the leafhopper Cicadulina mbila while MDMV-A was sap transmitted from MDMV-A infected plants to healthy plants by rub-inoculation. Plants were monitored weekly for symptom development and growth. Indirect antigen-coated, enzyme linked immunosorbent assay (IAC-ELISA) tests were done to monitor virus concentration in singly and co-infected plants. Single infection by MDMV-A had the least effect on plant growth and the plants in this treatment showed typical mosaic symptoms which were less severe than those with co-infection or single infection by MSV. The greatest effect on the growth rate of plants was due to co-infection by both viruses and consequently plants in these treatments were severely diseased. Plants which were previously exposed to either MSV or MDMV-A were not protected against subsequent infection by the other virus. The concentration of MSV was 1.4 times higher in plants doubly infected where MSV was inoculated first than in plants inoculated with MSV alone after 35 days. The concentration of MDMV-A was 1.5 times higher in single MDMV-A infection than in co-infection where MSV was inoculated first. MSV and MDMV-A were thus found to enhance each other's effect on the host plant thus inducing more severe disease symptoms. In the case of co-infection, the whole crop may be destroyed and the symptoms that may occur may be very misleading thus making it difficult to adopt effective disease control measures.
The Potential of Wild Sunflowers (Tithonia rotundifolia (Mill.) Blake) and Mexican Sunflower (Tithonia diversifolia (Hemsley) A. Gray) as Source of Nitrogen and Phosphorus in Maize Production Around Harare - Obert Jiri
Supervisors: Dr O.A. Chivinge and Dr S.R. Waddington
Two species of Tithonia were used in this experiment and these were wild sunflower (Tithonia rotundifolia (Mill.) Blake), an annual herb, and Mexican sunflower (Tithonia diversifolia (Hemsley) A. Gray), a perennial shrub. Their distribution and growth around Harare was measured and evaluated, respectively. They grew along field and farm borders and roads and in indigenous fallow systems and were located at Domboshava and Thornpark Estate near Harare. They both have a very high growth rate and produced large quantities of leaf biomass. Green manure from the Tithonia species was applied to sole maize crops. Two application times were made, first at 4 weeks after emergence (WAE) and then at 6 WAE of maize. Both species were applied as fresh leaf prunings. Tithonia rotundifolia was applied at rates of 1, 2 and 4 t/ha while T. diversifolia was applied at rates of 2, 3 and 4 t/ha. All plots were treated with a compound D (8% N, 14% P2O5 7% K2O) basal dressing fertiliser. Maize yield, on average, increased with increase in the amount of Tithonia prunings applied. Applying fresh T. rotundifolia at 1 t/ha increased yields from an average of 0.86 t/ha in the control to 0.96 t/ha. This was 11.6 per cent higher than the control. Application of T. rotundifolia at 2 t/ha led to yields 61.6 per cent higher than the control. There was an increased of 3.5 and 36 per cent when T. diversifolia was applied at rates of 2 t/ha and 3 t/ha, respectively. Applying T. rotundifolia at 4 t/ha produced a maize grain yield 154.7 per cent higher than the control and only 9 per cent lower than the yields of the Ammonium nitrate (AN) treatments. Tithonia diversifolia applied at 4 t/ha produced yields 168.6 per cent higher than the control and only 5 per cent lower than the yields of the AN treatments, on average. Tithonia rotundifolia produced a yield 10.3 per cent higher than the AN treated maize when the prunings were applied at 4 WAE while T. diversifolia produced yields 7.8 per cent higher than the AN treated maize when the prunings were applied at 6 WAE. The amount of Tithonia biomass available for cutting and transfer to fields could be limiting but the shrubs can be planted to the desired amounts along field borders and on contours. This would reduce the labour costs incurred in Tithonia usage. However. T. rotundifolia was very common in and around Harare.
Determining Stage of Cutting for Acacia boliviana, Calliandra calothyrsus and Leucaena leucocephala for Optimising Leaf Production - Ignatious Matimati
Supervisor: Mrs B.V. Maasdorp
The project was design to determine fodder harvesting schemes for optimum leaf yields of Acacia boliviana, Calliandra calothyrsus and Leucaena leucocephala. Well established trees growing at the University of Zimbabwe Farm, were cut at a height of 50 cm above the ground and the regrowth monitored over 132 days. The diameter of fresh cut stems was measured for use as a co-variate to adjust for the different tree sizes. Parameters measured were leaf, stem and total dry mass yields, leaf % and shoot length. Growth curves of these parameters enabled deductions on the harvesting schemes to be made. Leaf growth rate of C. calothyrsus was a maximum at 13 weeks after complete defoliation, with leaf area of 701.9 dm2 and LAI of 3.51. Shoot length (five average longest) was 150 cm. Estimated leaf yield at this point was 2.8 t/ha. Maximum leaf yield could be obtained by frequent lenient harvests that maintain shoot length as close as possible to 150 cm. Maximum leaf yields under a complete defoliation scheme (maximum cumulative growth) were attained after 17 weeks, and at a shoot length and a LAI of 210 cm and 7.41 respectively. Estimated leaf yield at this cutting stage was 6t/ha. Maximum leaf yield of A. boliviana can be obtained after week 13 when the shoot length was 160 cm. Maximum leaf yield can be obtained by frequent lenient defoliations that maintain shoot length close to 160 cm. At this point leaf area, LAI and leaf yield estimate were 1377.8 dm2, 6.89 and 6.9 t/ha respectively. Leaf proportion at this stage was about 60 %. Maximum leaf yield under a complete defoliation scheme (at maximum cumulative growth) are attained after 18 weeks, at shoot length and LAI of 220 cm and 12.11 respectively. Maximum leaf yield and leaf area were 12.1 t/ha and 2421.5 dm2 respectively. Leaf proportion of the regrowth was 50 %. Maximum leaf yield of L. leucocephala was after 12 weeks when the shoot length, LAI, and leaf % were 130 cm, 1.18 and about 60 % respectively. The estimated leaf yield was 3.4 t/ha. Frequent lenient leaf harvests are recommended that maintain shoot length as close as possible to 130 cm. Maximum leaf yield under a complete harvesting scheme (at maximum cumulative growth rate) was obtained after 15 weeks, when shoot length, LAI and leaf area were 180 cm, 3.31 and 662.7 dm2 respectively. Leaf % at this stage was about 50 %.
Effects of Temperature, Light, Planting Depth and Soil Structure on Germination and Emergence of Sweet Corn (Zea mays L.) Seeds - Terence Tendayi Tigere
Supervisor: Dr R. Madakadze
A study was carried out at the University of Zimbabwe to find the effects of planting depth, soil type, temperature and light on the germination and emergence of sweet corn seeds. In the study four experiments were carried out. Experiment 1 was to determine the effect of temperature, light and darkness on sweet corn seed germination. Germination did not take place possibly because of fungal and bacterial growth in the plates. The second experiment was to determine the effect of planting depths (0.5, 1.5, 3, 6 and 9 cm) on seedling emergence. Percent emergence decreased with an increase in depth, with 10 % at 0.5 cm and no emergence at 9 cm. The third experiment was designed to determine the effect of soil type on seedling emergence. Sandy soil had the lowest percent emergence (2%) followed by UZ red clay (18%), black vlei/sand, black vlei soil and UZ red clay/sand (22%) soils, respectively. The fourth experiment was to determine the effect of sterilization in seed germination. There was no pathogen growth in the sterilized plates but growth occurred in non-sterilized plates. Germination percentage was similar for all treatments. It was concluded that sweet corn seeds should be sterilized before planting to get a good stand. Emergence was better in clays and sand/clay soil mixtures compared to sandy soils.
Effect of Trellising, Basal Fertiliser Application and Pinching on the Vegetable Leaf of a Pumpkin - James Marume
Supervisor: Dr R. Madakadze
The effects of three cultural practices (trellising, basal fertiliser application and pinching) on the vegetable leaf of pumpkins were studied. The plants were trellised using three treatments: wire and stakes, strings, and a control that was not trellised. Basal fertiliser rates of 0, 500 and 1000 kg/ha compound C were used. The plants were either pinched or not pinched. The trellising method of using strings improved the total leaf number of the plant. Basal fertiliser application at rates of 500 and 1000 kg/ha improved the total vine length, branch length, leaf number, branch number, number of male flowers, fresh and dry leaf weights. There was no interaction between the effects of the three cultural practices.
Evaluating Effects of Temperature, Light and Storage Length on Germination and Emergence of Ammi majus and Buplerum graffithi - Michael Vambe
Supervisor: Dr R. Madakadze
The main objective of the study was to evaluate the effects of light, temperature and storage period on germination and emergence of A. majus and B. graffithi seeds. Seeds had been stored for seven, six and less than one year at a temperature range of 5 to 70C and 30 to 50 % relative humidity. Germination tests were carried out in light and total darkness at 150C, 200C and 250C for the three storage durations. Emergence tests were only carried out for the three storage durations at a depth of 0.5 cm with five replications for each storage duration. Early seed germination (T10 = 0.38 for A. majus and T10 = 0.12 for B. graffithi) was recorded at high temperatures; however, high temperatures had a low final percentage germination for both species. B. graffithi seeds germinated better in darkness than in light (37% vs. 23%). Length of storage period did not significantly affect seed emergence of A. majus. B. graffithi seeds failed to emerge. Storage improves germination of B. graffithi seeds. A temperature of 20OC was established to be the optimum for germination of both flower species in this study.
A Survey on Vegetable Processing in Seke Communal Land, Mashonaland East Province - Takawira Joseph
Supervisor: Mr S. Masimbe
A questionnaire-based survey was carried out in Seke Communal Land in June 1998 to determine the current vegetable processing practices, quality standards and constraints faced by the farmers in processing vegetables. The survey revealed raw data on current vegetable processing practices in the smallholder horticulture sector. The data are going to be used in phase two of the experiment which will focus at the effect of boiling duration before drying on the nutritive value and acceptability of vegetables by the consumer and the effect of drying vegetables under different shade intensities on nutritive quality. Data analysis was done using the Statistical Package For Social Sciences (SPSS). Cowpea leaves are the most commonly processed vegetable with 63.3% of the respondents processing the vegetable. Rape is second to cowpea leaves with 58.3% of the respondents processing it. Rotting and rainfall have been observed as the major constraints in vegetable processing. The survey revealed that processed vegetables are part of the diet for the old, young, rich, and the poor. The main method of vegetable processing is by boiling vegetables and then drying them under full sunlight. There is need to come up with optimum conditions that should be used as measures in processing, such as boiling time, stage of slicing and conditions of drying. Hygiene should be considered if consumption of processed vegetables is to be promoted.
Varietal Evaluation of Field Beans (Phaseolus vulgaris) Performance in Maize/Bean Intercropping - Sikhanyiso Matshalaga
Supervisor: Dr I.K. Mariga
An on-farm trial was conducted during 1997/98 season at Domboshava (natural region IIa) and Chinyika (natural region IIb) to investigate the effects of dry bean (Phaseolus vulgaris L.) and bean yield components in a maize/bean intercrop. Maize and beans were in mixed and pure stands. Seven bean varieties were planted with a very early three-way maize hybrid SC401. A split plot design was used with the cropping system as the main plot and the bean variety as the subplot. Yield data were collected and statistically analyzed at p = 0.05. The season was a low rainfall one with poor distribution of precipitation. Grain yields of maize and beans were significantly reduced by intercropping. There was a significant interaction between bean variety and cropping system at Domboshava. The total biological output was higher in the intercrop than the pure stand. Natal sugar, the control had consequently high yields of 580 kg/ha and 564 kg/ha at the two sites. Maize/Light Red Kidney, maize/Dark Red Kidney and maize/Natal Sugar depressed maize yields by 11 %, 27 % and 28 % respectively, compared to sole maize at both sites. EX-RICO had the largest percentage yield reduction of 67% at Domboshava and 85% at Bingaguru. in Chinyika. The shelling percentage, number of seeds/pod and the number of pods/plant were not affected by the cropping system. Land equivalent ratio (LER) values were higher for the intercrop for all bean varieties indicating that it was advantageous to intercrop. Carioca and Light Red Kidney had the highest total return in the intercrop at Bingaguru. There were two promising bean varieties, namely EX-RICO and Light Red Kidney, but they were not stable across the sites. The experiment results suggest that Natal Sugar and Carioca are still the best bean genotypes for use in intercropping because of stable yields in adverse conditions such as moisture stress and disease.
A Review of the Effect of Auxins on Rooting of Stem Cuttings - Nqobile Ncube
Supervisor: Mr S. Masimbe
The review develops the argument of rooting as a process consisting of successive physiological phases, with different exogenous requirements. The auxin hormone is identified as the principal rooting hormone on which the rate and extent of rooting is dependent. The auxins exist in a complex environment within the plant and generally their effect is dependant on the condition of the endogenous environment and other external factors especially those that are related to the physiological condition of the plant from which cuttings are taken. The Zimbabwean propagation industry is also reviewed looking at the level of research and some of the factors that have determined the direction of the research.
Effect of Residual Atrazine on Subsequent Weed Density and Spectrum after Three Consecutive Years of Application in Maize (Zea mays L.) at Different Fertilizer Levels - Melody W. Kwanayi
The effect of residual atrazine at different fertilizer levels on the weed seedbank was evaluated in the herbicide and fertilizer interaction trials conducted in Chinyika Resettlement Area from the 1993/4 to 1996/7 rainy season. The evaluation was done over two sites of the resettlement area (Chinyudze and Govakova). The herbicide was applied at 1.75 kg a.i. ha-1 either as full-cover spray or banded 45 cm on the crop row in combination with different fertilizer rates. Hand weeding at two and five weeks after crop emergence was used as a control. Weed counts were then done at five and ten weeks after crop emergence during the 1997/8 season, then at harvest soil samples were taken for incubation in the greenhouse at the Crop Science Department of the University of Zimbabwe. Weed counts were also done for these samples. Three weed classes were observed which were the grassy, broadleaved and sedges classes. Atrazine was persistent enough to keep the total number of weed seeds in the soil low such that a general decline was observed for the broadleaved, reduced decline with grassy but not significant for the sedges. Residues of site 1 were more effective in reducing weed populations than those at site 2 and this was with application of half the recommended rate for compound D and ammonium nitrate with herbicide applied as a full-cover spray.
Survival of Pseudomonas syringae pv. tomato in Association with the Soil, Hosts and Non-host Plants - Zira Mavudzi
Supervisor: Mrs J. Sibiya
A study was carried out to determine the survival of Pseudomonas syringe pv. tomato in the soil, and host and non-host plants. Sandy, loam and clay soils were collected from Chinhamora communal areas and sterilized before inoculation with an estimated of 5 x 1010 cfu (colony forming units)/ml P. s. pv. tomato bacterium. The inoculated soils were incubated at room temperature in the laboratory. Two soil conditions, wet and dry were maintained throughout the experiment, for each of the soil types. Bacterial population in the different soils was measured by the plate dilution method on a monthly basis for four months. To determine survival on host and non-host plants, two crops used in rotations: okra and cabbage; two solanaceous crops: paprika and potato; two weed species: Amaranthus hybridus and Bidens pilosa; and two tomato varieties: floraded and rossal were inoculated with a population of 1 x 105 cfu/ml. The bacterial counts were made at 1, 2, 4, 8 and 12 weeks after inoculation by washing the surface and then plating the washings on King's Medium B. Soil type and soil moisture significantly (P<0.01) affected the survival of P. s. pv. tomato in the soil. The bacterium survived well in the wet soils as was shown by higher number of colony forming units than in dry soil. Sandy soils had the highest number of colony forming units, then loam and finally clay soil had the least. The bacterium decreased significantly from January to April in all the soil types, in both dry and wet soil. By the fourth month, there were no surviving bacteria in all the soil types. P. s. pv. tomato was detected on the surface of tomato cultivars, rossal and floraded, A. hybridus, paprika, potatoes and okra. The population increased from the first week after inoculation. Rossal had the highest number of surviving bacteria than all the other species. Also P. s. pv. tomato survived on leaves of host and non-host plants as an epiphyte since it could be recovered by surface washings. P. s. pv. tomato did not cause any disease on these non-host plants, though it was present. No colonies of the bacterium were detected on cabbage and B. pilosa leaves. It was concluded that as the texture becomes finer the bacterial growth decreased as air spaces decreased. Therefore, sandy soils support more bacterium than loam and clay soils respectively. Wet soils had a higher population of the bacterium than dry soils. Therefore the best combination was wet sand soil. The conclusion on host and non-host plants was that P. s. pv. tomato survives on the surface of rossal, floraded, A. hybridus, okra, potatoes and paprika as an epiphyte. Therefore, potatoes, paprika should not be rotated with tomatoes. Volunteer tomato plants and weeds should not be left in the field, as these can be initial sources of inocula.
Jatropha curcas L. as a Rooting Media - Katsvairo Tiwirai Lister
Supervisor: Mr S. Masimbe
Jatropha curcas L. cake, a product of oil extraction from jatropha seed, is high in plant nutrients and can't be used for animal feed due to the toxins that it possesses. Therefore, the areas that it could be used are as an organic fertiliser or as a rooting media. Greenhouse trials were carried out to determine the effect of J. curcas cake (JCL) on germination rate of tomato (Lycopersicon esculentum Mill.) and rape (Brassica napus) seed; to compare germination in J. curcas containing media to that of UZ standard and pine bark, and to determine nutrient uptake in particular nitrogen (N), potassium (K) and phosphorus (P). It was found that media had a very significant effect (P<0.01) on the germination of tomato seed. Germination differences between the different media, with respect to B. napus, were significant (P<0.01). Tomato germination in the different media was significantly different (P<0.05). Medium 5 (pine bark) and medium 2 (25% JCL and 75% sand) were found to have, significantly, had the highest germination. Medium 3 (50% JCL and 50% sand) had, significantly, the highest growth rate. The fact that there were high values with JCL-containing media suggest that J. curcas has great potential and can be used as a rooting media.
A Review of the Symbiotic Relationship between Legume and Rhizobia - Fari Nyashanu
Supervisor: Dr R. Madakadze
This review paper is basically an insight into aspects of biological nitrogen fixation (bnf) that result in increasing the benefits that farmers and consumers of agricultural products derive from bnf. The paper contains such aspects of bnf as the factors affecting bnf, the uses of bnf and future prospects of bnf. Work from different researchers is consolidated, critically comparing different work on the same subject to give one viewpoint. This is with the hope of making easier the task of making the use and benefits of bnf more rewarding and accessible to farmers and consumers. The potential benefits that bnf offers both nationally and individually have prompted this study with the hope of bettering lives worldwide. I have attempted to show that bnf will play an important role in sustainable agriculture for the future, especially in view of the environment constraints that agriculture now faces in the wake of campaigns against use of chemicals in agriculture.
The Potential Use of Summer and Winter Butternut-squash as Minimally Processed Vegetables, with Special Reference to their Germination, Yield and Nutritive Value - Gwendolyn S. Madondo
Supervisors: Dr M.T. Masarirambi, Mr C. Smyth and Mr P. Mills
Experiments were conducted to determine the effect of growing season on fresh fruit yield on four butternut-squash species namely C. pepo, C. moschata, C. maxima and C. ficifolia. The harvested fruits were analyzed in the fresh form and processed form for content of ascorbic acid, beta-carotene, iron, calcium, phosphorus and protein. For processing, the butternut-squash was cut into thin stripes dipped in chlorine, which was used as a biocide, (minimally) processed and packed into salad packs. In all species, higher fruit yields were recorded in summer than in winter.
Unprocessed butternut fruit had a higher nutritional content than the processed fruit in all four species. Pre-soaking the winter squash seeds in hot water before planting them, increased the germination percentage. Germination percentage in all species increased with an increase in seed age, with seeds sown one week after harvesting recording the lowest germination percentages.
Evaluating the Effect of Adding Oxyfluorfen and Penetrator-H on the Efficacy of Glyphosate - Willington Ncube
Supervisors: Mr A.B. Mashingaidze and Dr O.A. Chivinge
The relative efficacy of glyphosate (Roundup) at half-dosage with surfactant Penetrator-H and with herbicide additive oxyfluorfen was compared to the standard full glyphosate dosage on Nicandra physalodes, Richardia scabra, Portulaca oleracea, Eleusine indica, Galinsoga parviflora, Rottboellia cochinchinensis, Commelina benghalensis, Amaranthus hybridus, Ipoemia plebia, Euphorbia heterophylla, Bidens pilosa, Cyperus esculentus and Setaria verticillata.
The effects of adding Penetrator-H, a surfactant that enhances glyphosate activity, and the effect of oxyfluorfen, a herbicide additive that assists in the translocation process of glyphosate, were evaluated by adding the two additives to high and low dosages of glyphosate. Evaluation of the herbicide treatments involved visually assessing the weeds within 1 m2 quadrants of the 28 trial plots, then counting weed numbers showing necrosis and drying. The numbers obtained were then expressed as a percentage of the numbers obtained in the pre-spray assessment. A score of zero (0) was given to indicate no control of the weeds while a score 100 indicated total kill of the weeds. The assessments were made 2, 3 and 4 weeks after herbicide application. Under the same field conditions, glyphosate half dosage with surfactant Penetrator-H or herbicide additive oxyfluorfen achieved a percent weed kill, which was not significantly different from the glyphosate full dosage with or without additives.
Addition of the surfactant Penetrator-H and herbicide additive oxyfluorfen to half dosage of glyphosate enhanced glyphosate performance such that weed control and symptoms induced were similar to those achieved by full glyphosate dosage. This implies that farmers can use half dosages of glyphosate with surfactant Penetrator-H and herbicide additive oxufluorfen added to achieve the same level of weed control attained by the full glyphosate dosage. The use of half dosages of glyphosate with additives will reduce the overall cost of the herbicide spray.
Selection of Sweet Sorghum Varieties with High Sugar (Sucrose) Content and High Fresh Biomass for Ethanol Production - Joseph Sanyangore
Supervisor: Mr S. Masimbe
The variation in sugar (sucrose) content and fresh biomass weight was analyzed in irrigated sweet sorghum varieties. Five varieties (Keller, Mori Edes, IS19674, Rona-H and Cowley) grown under favourable conditions yielded differently in terms of sugar and fresh biomass. The sugar level was analyzed at different growth stages (booting, flowering, milking, soft dough, hard dough and maturity) of the sweet sorghum crop.
Cowley had the highest mean sugar content of Pol% 10.250 and Keller had the second highest of Pol% 8.86. The variety Rona-H had the lowest mean sugar yield of Pol% 3.98. The different stages of growth of sweet sorghum varied in mean sugar content within the five varieties. The hard dough stage had the highest recorded mean sugar content of 8.64 % and the lowest sugar content was at the booting stage, which had a mean sugar level of 3.9%.
The fresh biomass weight varied in the five varieties that were analyzed. Keller obtained the highest mean fresh biomass of 80.8 t/ha, Cowley had the second highest mean fresh biomass of 76.5 t/ha and Rona-H had the lowest mean fresh weight of 40.5 t/ha. It is probable that differences between varieties in sugar content and fresh biomass weight are due to the different genetic constitution of the varieties.
The Use of Multipurpose Trees as Live Fences Around Riverine Gardens by Smallholder Farmers in Chinyika Resettlement Area - John Mutangadura
Supervisor: Dr I.K. Mariga
The study was aimed at investigating the importance of multipurpose trees as live fences around riverine gardens in the smallholder farming sector. The use of dry wooden poles and tree branches in fencing results in high deforestation rates and extinction of the tree species used. Barbed wire used in combination with these poles, or with steel poles, as an alternative is also expensive. The use of multipurpose trees offers other benefits besides a strong fence.
A survey covering a hundred randomly selected villagers from five randomly selected villages was carried out in the Chinyika Resettlement Area. It revealed that 75% of the farmers used live trees on their riverine fences, and 25% did not. Tree species mostly used were Pterocarpus angolensis, Antidesma venosum, Ficus natalensis, Eucalyptus spp., Lannea discolor and Brachystegia spiciformis. Ninety-four percent of the farmers were willing to use live trees on their fences, and the most limiting factor was the unavailability of propagation material. The Government Extension Service supported the use of live fences.
The Effect of Quality and Method of Application of Manure on Yield and Nutrient Uptake by Rape (Brassica napus) - Nyaradzo E. Kaseke
Supervisors: Dr M T Masarirambi and Dr H.K.Murwira
The initial and residual effects of the application method and quality of manure on yield and nutrient uptake of Brassica napus were investigated by conducting two experiments at each of two sites, Mangwende and African Center for Fertilizer Development.
Manure of 0.63%N, 1.08%N, 1.19%N and 2.6%N was applied by broadcasting, banding and station placement prior to planting of the B. napus. After harvesting the first crop, B. napus was again planted on the same plots. In both experiments, the total dry matter yield and leaf area were measured at two-week intervals for the six weeks that each crop was in the field. The nutrient content in the plants was measured at harvest. The second experiment was to find out the residual effects of the previously applied manure.
Results indicated that the application method is a strong tool that can be used in nutrient management. Broadcasting of manure resulted in higher dry matter and leaf area production as well as nutrient uptake compared to banding and station placement in the first experiment. The residual effects of the application method on yield and nutrient uptake were greater than the initial response of B. napus to the application method of the manure. The residual effects of banded and station placed manure will, under favourable conditions, be superior to those for broadcast manure. Under water-logged conditions, B. napus responds best to broadcast manure.
The Heritability of Grey Leaf Spot Resistance, Correlation of Grey Leaf Spot and Maize Streak Virus Resistances in Two Maize Populations - Rudo D. Masvodza
Supervisor: Dr P. Tongoona and Dr K. Pixley
Grey leaf spot (GLS) and maize streak virus (MSV) diseases reduce maize yields in Sub-Saharan Africa. The use of resistant cultivars is the most economical control method. In order to develop disease resistant cultivars, knowledge of heritability of resistance is important.
Broad-sense heritability estimates of grey leaf spot for two populations of S1 lines were determined in the 1997/98 season at the International Maize and Wheat Improvement Centre (CIMMYT). The genotypes used were S1 progeny from two crosses, Ecuador 573 X SNSY[ETO-B-90], and Kitale II X SNSYN[N3/TUX-90]. Kitale II and Ecuador were obtained from Kenya and they are high yielding, white-seeded maize lines. They were crossed with maize streak virus resistant CIMMYT synthetic lines to improve their maize streak resistance.
Grey leaf spot was evaluated in the field at CIMMYT (Harare) and was rated on 1 to 5 scale where, 1 = no symptoms on all leaves and 5 = severe streaking on all leaves, and older leaves dead. Maize streak virus was evaluated in the greenhouse at CIMMYT (Harare), and was also rated on a 1 to 5 scale where 1 = no symptoms on all leaves and 5 = severe streaking on all leaves, plants severely stunted, some dead. Disease scores were taken on three separate dates for GLS and on two separate dates for MSV. Parental grey leaf spot scores were available from the 1996/97 season.
Broad-sense heritability was determined using two methods, parent/offspring regression and parent/offspring correlation. Correlation analysis between grey leaf spot in the field and maize streak in the green house was also done.
There were significant differences among S1 lines for GLS and MSV resistance at all dates. Broad-sense heritability values for the three disease scores for the lines from Ecuador 573 had a mean of 0.624, and those from Kitale II had a mean of 0.544, by the parent/offspring correlation method. They were higher than those obtained with parent/offspring regression method, which were 0.443 +/-0.037 for lines from Ecuador 573 and 0.335 +/-0.028 for lines from Kitale II. There was no correlation between MSV and GLS disease for both of the populations at all dates.
Inheritance of Grey Leaf Spot Resistance in Some Maize Populations in Zimbabwe under Field Conditions - Nyamande Mapope
Supervisor: Dr P. Tongoona and Mr M. Caulfield
Eleven inbred lines, twenty four F1 hybrids and twenty four F2 progeny were planted at Rattray Arnold Research Station in a field that had maize the previous season. The objectives were to screen inbred lines and their F1 progeny for resistance to grey leaf spot (GLS) and to determine the number of gene pairs controlling GLS resistance. About 420 F2 progeny from each F2 were planted to enable estimation of the number of genes controlling this trait. Scores for GLS were done as from day 55 after planting to day 97 at weekly intervals. Inbred lines 6VR1, 120VH1 and WA24AW and MB1089 showed no or slow disease development. F1 hybrids PNG12W/SQ8, WA24W/6VR1, WA24/SQ8, WA24W/CML202, WA24W/MB1200-9, 6VR1/120VR1, 6VR1/SQ8, 6VR1/MB1089 and 6VR1/MB1200-9 showed reduced disease development. F1 hybrids PGN12W/RT254P, PGN12W/6VR1 and 6VR1/LR190P were susceptible to GLS. From F2 progeny segregation, genes controlling GLS resistance were estimated to be around three to five.
MSc Abstracts (1995)
Studies in the Control of Scirtothrips aurantii (Faure) in Citrus, Frankliniella occidentalis (Pergade) in Chrysanthemums and F. Occidentalis, Haplothrips gowdeyi (Franklin), and Megalurothrips sjostedti (Trybom) in Runner Bean - Joshua Karuma
Supervisor: Prof. D.P. Giga
A range of pesticides was screened against thrips, Scirtothrips aurantii (Faure), in citrus, Frankliniella occidentalis (Pergande) in chrysanthemums and Haplothrips gowdeyi (Franklin), Megalurothrips sjostedti (Trybom) and F. occidentalis in runner bean, Phaseolus coccineus.
The chemicals, dicarzol, methiocarb and isofernphos were evaluated for citrus thrips, S. aurantii control and compared with the recommended chemicals, triazophos and tartar emetic. The insecticides were applied on two-year old Washington Navel trees and randomly selected fruitlets were assessed for adult and nymph thrips presence. Dicarzol effectively controlled citrus thrips for 38 days while methiocarb and isofenphos were effective for 24 days. The two latter pesticides were similar to triazophos from 32 to 38 days after spraying. Tartar emetic was not effective as it was washed off trees by rain one day after spraying. After 45 days of spraying, all treatments were relatively ineffective.
An observation was conducted at two farms, Trelliseck and Three Crowns near Harare, to compare the different methods of F. occidentalis control in the greenhouses. Population levels of adult thrips were monitored in selected bays of greenhouse using PVC sticky cards hung over the different varieties of chrysanthemums. The farmer at Trelliseck applied in rotation; dichlorvos, abamectin, endosulfan and lambdacyhalothrin weekly for nine weeks. Thrips density was monitored every week. At Three Crowns, pyrazophos, dichlorvos, endosulfan and triazophos were applied in rotation every three days and thrips density was monitored every three days following spraying. At both farms, F. occidentalis was successfully controlled. However, F. occidentalis control was better at Trelliseck Farm because of adequate air-vent screening, a complete plastic roof and good hygiene. The former two ensured exclusion of the pest and the latter reduced multiplication of the pest in the greenhouse.
Following the observation, nine chemicals were evaluated in the laboratory against F. occidentalis collected from Three Crowns Farm. The pesticides were applied at 25% and 10% less than the recommended rate, at the recommended rate and at 10% more than the recommended rate. An immersion technique was used in the tests. The adult thrips were immersed in each chemical solution and immediately filtered and placed in 3 cm diameter petri dishes lined with damp tissue paper for 24 hours. The chemicals methiocarb and thiocyclam, which are not registered against F. occidentalis in Zimbabwe, were the most efficacious at all the rates tested. Dichlorvos, a chemical registered for use in chrysanthemums in Zimbabwe, when applied at the recommended rate or at 10% more than the recommended rate, elicited a significantly similar kill of the pest as methiocarb or thiocyclam. Diazinon, teflubenzuron, triazophos and pyrazophos elicited less than 60% mortality of the pest when applied at 10% more than the recommended rate.
An integrated thrips control trial was conducted at Chabwino Farm where the effectiveness of thiocyclam, teflubenzuron, Beaveria bassiana, grass mulch and the combination of each pesticide with grass mulch were investigated in a runner bean crop. Twenty-five flowers were randomly sampled from each plot, 1, 4 and 10 days after treating plots, to determine the efficacy of the treatments. Thiocyclam was most effective in reducing thrips populations and least were B. bassiana and teflubenzuron. All the pesticides plus mulch combinations resulted in significantly similar levels of thrips control.
Since thrips pupate in the soil, the vertical distribution of pupae was also determined. Soil samples were randomly taken from unmulched plots in a runner bean field from 2, 4, 6, 8, 12 and 15 cm depths using a soil auger. The samples from each depth were placed in plastic containers lined with black paper and the lids were smeared with ant-bar to trap the emerging thrips. The traps were placed in the greenhouse and observed after 48 hours for adult thrips emergence. Most thrips emerged from soil sampled at 2 cm followed by 4 cm. The lowest number of thrips emerged from soil sampled at 12 and 15 cm depths.
MSc Abstracts (1996)
Efficacy of Two Trichoderma Isolates as Bio-control Agents of Soil-borne Fungal Pathogens and Their Plant Growth Enhancing Properties in Selected Solanaceous and Graminaceae Crops - Susan Dimbi
Two Trichoderma harzianum isolates, T77 and TM4 and a mixture, T77/TM4, were assessed for soil-borne disease control against the pathogens, Phytophthora nicotianae, Sclerotium rolfsii, Sclerotinia sclerotiorum and the soreshin complex (Fusarium solani and Rhizoctonia solani). The Trichoderma treatments had less disease compared to the untreated control, producing less stem lesions as well as higher plant masses. There was significant disease control by Trichoderma isolates in S. rolfsii, S. sclerotiorum and soreshin complex (R. solani and F. solani) (P<0.05).
The two Trichoderma isolates, T77 and TM4, and the mixture, T77/TM4, were further tested for growth enhancement in four crops: maize (Zea mays), tomato (Lycopersicum esculentum), tobacco (Nicotiana tabacum) and sorghum (Sorghum bicolor). Experiments were done in seed trays as well as pots. All Trichoderma treatments produced higher dry masses although these were not always significant compared to the control in maize and sorghum. Dry mass increases of as much as 23% were obtained in sorghum and up to 40% increase were obtained in maize. In tobacco seedling trays Trichoderma treatments always produced significantly higher dry masses of treated seedlings (P<0.001). In pots, however, differences were not always shown. Whereas dry masses were high for the T77 and the TM4, there tended to be a less growth stimulation by T77/TM4 in tray treatments.
Eight T. harzianum T77 isolates, produced by liquid fermentation (LF) and plate culture formulations (DY) and denoted LF5, LF10, LF15, LF20, LF25, 7DY, 14DY and 21DY, were tested for disease control and plant growth enhancement in tobacco. All Trichoderma treatments produced higher plant masses as well as better disease control than the control although the results were variable, with some treatments such as LF15 getting no disease at all in some experiments but diseased in others. T77 spent grain formulation had excellent disease control at all times. Treatments 7DY, 14DY, 21DY, LF15 and LF10 had dry masses not significantly different from the standard T77 treatment. Treatments 14DY, 21DY and LF25 each had disease scores not significantly different from T77 in at least two experiments out of the three done.
In an experiment to determine the optimum rate of Trichoderma sp., TM4 at the three rates of 50 g/m2, 100 g/m2 and the recommended rate for T77 (71.5 g/m2) were used. The three rates were tested against the soreshin complex (R. solani and F. solani) as well as plant growth enhancing properties on tobacco. The rate 71.5 g/m2 was the optimum rate for TM4, both in terms of disease management and plant growth stimulation. Significant increases in plant fresh mass (P<0.005) were obtained. A lower or higher Trichoderma rate always produced plants, which were stunted, and disease scores that were not different from the controls.
A Study on the Availability of Entompathogenic Nematodes and their Potential in Controlling the Maize Stalkborer Busseola fusca (Fuller) - Shepherd Musiyandaka
Supervisor: Dr S.L.J. Page
A survey was carried out in three areas around Harare (viz. Domboshawa-Chinamora communal area, Fambidzanai Permaculture Centre (a farm under organic practices since 1988), and the University of Zimbabwe) to find the availability and incidence of entomopathogenic nematodes during the months of February to June 1996. The nematodes were isolated using the method of Bedding and Akhurst (1975) with maize stalkborer (Busseola fusca) as the baiting insect. The isolated nematodes were first characterised then used in the laboratory to kill the maize stalkborer.
Of the 41 samples tested, 12.20% were positive for the entomopathogenic nematodes. The two genera Steinernema and Diplogaster were isolated. The Steinernema sp. culture could not be maintained in the laboratory while Diplogaster sp. survived.
Diplogaster sp. was found in sandy loam textured soils under both wet and dry conditions in the Chinamora communal areas. It was also found in arable land in a maize field, a pine plantation, fallow and in an uncropped land under indigenous woodland populated by muhacha (Parinari curatalifolia), munondo (Julbernadia globiflora), musasa (Brachystegia spiciformis) and mufuti (B. boehmii) trees. Steinernema sp. was found in one fifth of the positive samples, collected 24 hours after rain, with the land previously cultivated under maize but left fallow for the 1995/96 season.
The entomopathogenic nematode, Diplogaster, effect on B. fusca was evaluated in the laboratory. It had a mortality range of 22.22 %, 37.78 %, 55.56 % and 68.89 % using nematode concentrations of 50, 100, 200 and 400 larva pipetted in one millimetre of water onto a moist filter paper in petri-dishes containing 10 late instar larva incubated at 25OC. There was a significant difference between B. fusca percentage mean mortalities caused by nematode dosages of 50 to 400 larvae per mm compared to the control (P<0.01).
Differentiation and Characterisation of the Maize-infecting Maize Dwarf Mosaic Virus Strain A (MDMV-A)/Sugarcane Mosaic Virus Strain MDB (SCMV-MDB)-like Isolates in Zimbabwe and their Effect on Resistance of Local Maize Hybrids - Cyril M. Nyanhete
Supervisor: Mrs J. Sibiya
A virus isolate related to the characterised maize dwarf mosaic virus strain A (MDMV-A) and sugarcane mosaic virus strain MDB (SCMV-MDB) was isolated and identified from maize leaves showing mosaic-type symptoms. The MDMV/SCMV-like isolate was mechanically transmissible from maize plants to other members of the Poaceae family and had an incubation period that ranged from 6 to 10 days depending on host plant species and/or temperature. The flexuous filamentous virus isolate was morphologically identical to the characterised MDMV/SCMV isolates. The particles had an average length of 750 nm and a width of 12.5 nm.
In direct antigen-coat enzyme-linked immunosorbent assay (IAC-ELISA), the local virus isolate reacted positively with antisera to MDMV-A and SCMV-MDB, implying the presence of similar epitopes on the coat proteins of the local virus isolate and the characterised MDMV and SCMV from the USA. Electroblot immunoassay (EBIA) showed no difference in migration distance between the local virus isolate and SCMV-MDB from the USA. The coat protein molecular weight (Mr) of the local virus isolate was greater than the coat protein Mr of MDMV-A from the USA.
Symptoms induced in maize by the local virus isolate were light-green/dark-green mosaics/mottles depending on the maize cultivar. In sorghum cultivars, the local virus isolate incited symptoms that ranged from local necrosis, mosaics/mottles to leaf reddening.
A wide range of grass species in the Poaceae were hosts to the local virus isolate. Some grass species such as Setaria homonyma, Phalaris minor, Sorghum bicolor cvv. Marcia, SAR33 (dwarf), Atlas, and FA10 differentiated between strains of the local virus isolate, implying that the local virus isolate was a mixture of strains. Johnson grass was immune to the local virus isolate.
Local maize hybrids reacted differently to infection with the local virus isolate. Maize cultivars were ranked: highly susceptible, susceptible, tolerant or resistant on the basis of the nature and extent of disease symptoms occurring in each cultivar. Overall, the MDMV/SCMV-like isolate retarded growth of maize plants. Growth was stimulated in PAN 473. Co-infection of the maize cultivar R201 with maize streak virus (MSV) and the MDMV/SCMV-like isolate had a synergistic effect on disease development.
Effect of Mulches from Selected Multi-purpose Trees on the Germination and Development of Witchweed (Striga asiatica (L.) Kuntze) and Sorghum (Sorghum bicolor (L.) Moench) - Rosemary M. Chanyowedza
Supervisors: Dr O.A. Chivinge and Mr A.B. Mashingaidze
Glasshouse and laboratory experiments were carried out to determine the interactions between multipurpose tree mulches, sorghum (Sorghum bicolor (L.) Moench) and witchweed (Striga asiatica (L.) Kuntze). In experiment 1, the effect of mulches from Acacia nilotica, A. angustissima, A. karoo, Gliricidia sepium and Colophospermum mopane on the emergence, height and leaf area of the sorghum varieties Mutode, Katandanzara, Brown Tsveta, Kasvikisire, Ntelwane, Iganu and Nyamudzi germination and growth of S. asiatica was determined. The varieties were mulched with 20 g/pot of each mulch. In experiment 2, root exudates were extracted from sorghum varieties that had been grown under each of the mulches and for their ability to germinate S. asiatica seeds. The effect of mixing leaf leachates from the multipurpose tree mulches with root exudates from sorghum varieties on the germination of S. asiatica was determined. The number of days to the emergence of the sorghum varieties was determined and leaf area and plant height were measured 30 days after planting. Experiments were carried out to determine the effect of leachates from the mulches on the germination and plumule dry weight of the sorghum. The number of plumules emerged was recorded and were oven dried for 72 hours at 80OC.
Experiment 1 showed that the susceptibility of the varieties to S. asiatica depended on the mulch type. The mulches of A. nilotica and A. angustissima increased the resistance of all the varieties except for Brown Tsveta. Susceptibility of Ntelwane, Nyamudzi and Mutode increased in G. sepium mulch. Brown Tsveta was most susceptible in A. karoo mulch while Ntelwane was most susceptible in A. nilotica mulch. G. Sepium caused a delay in the emergence of all the sorghum varieties while A. karoo, C. mopane, A. angustissima and A. nilotica mulches delayed the emergence of Kasvisikire, Mutode, Katandanzara and Brown Tsveta. Plant height and leaf area were increased by the mulches. The root exudates from Katandanzara when grown under any one of the mulches resulted in an increased germination of S. asiatica. A. karoo mulch caused the greatest reduction in the germination of S. asiatica seeds in rood exudates extracted from Kasvisikire. C. mopane mulch caused the greatest increase in the germination of S. asiatica compared to all other mulches. The germination of S. asiatica was decreased by all other leachates. A. nilotica leachates caused complete inhibition of S. asiatica germination. The leachates decreased the increased germination of sorghum except when leachates from A. karoo increased the germination of Nyamudzi and Katandanzara, while A. nilotica increased the germination of Nyamudzi. Plumule dry weight was decreased by all the leachates except for A. angustissima which increased the plumule dry weight of Katandanzara, and A. karoo which increased that of Brown Tsveta.
Detection and Ecological Studies of Pseudomonas syringae pv. tomato; the Causal Agent of Bacterial Speck Disease of Tomatoes (Lycopersicon esculentum Mill.) - Walter Manyangarirwa
Supervisor: Mrs J. Sibiya
Surveys were conducted on large-scale commercial farms and communal areas near Harare to establish trends in bacterial speck infection on tomatoes and to rank the importance of bacterial speck disease in relation to the fungal diseases; early blight (Alternaria solani) and late blight (Phytophthora infestans).
Laboratory and glasshouse experiments were also conducted to detect Pseudomonas syringae pv. tomato in seed batches and to study survival aspect of P. s. pv. tomato in sterile soils. Commercial area surveys showed a correlation between increases in weekly rainfall totals and increases in bacterial speck infections on fruits. There was a negative correlation between increases in bacterial speck infections and a decrease in maximum and minimum daily temperatures. Bacterial speck disease caused more qualitative than quantitative losses on tomato fruits in the commercial and communal area farms.
On seed detection, no P. s. pv. tomato was isolated from commercial and communal area seed batches through the laboratory assay and the "growing-on" detection procedure. A total of five species of gram-negative bacteria were isolated from commercial and communal area seed batches in laboratory assays. None of the five species caused symptoms of bacterial speck on tomato plants of the cultivar Rodade.
P.s. pv. tomato was detected after eight weeks in a long-term soil survival experiment in three autoclaved soils. There were significant rates of bacterial cell survival in the Mutoko sand than the Chinamora and UZ clay soils (P<0.05). There were significantly higher rates of bacterial cell survival in dry than in wet soils of the same origin (P<0.05).
Bioassays to test the effectiveness of fungicides and bactericides in inhibiting P. s. pv. tomato, Pseudomonas syringae and Pseudomonas solanacearum in vitro showed a highly significant effect of mancozeb, a dithiocarbamate, and ridomil, a phenylamide, in inhibiting bacterial growth when compared to copper oxychloride, the recommended bactericide for use against bacteria on crops. The bactericide oxytetracycline achieved 100% growth inhibition of the three bacterial species but it is not registered for use on crops. Combinations of copper oxychloride plus ridomil, copper oxychloride plus mancozeb plus ridomil were less effective than the single chemicals (P<0.05).
Efficacy of Reduced Dosages of Foliar Applied Atrazine and Nicosulfuron to Control Weeds in Maize (Zea mays L.) - Dzingo Mafuvadze
Supervisor: Mr A.B. Mashingaidze
Field and glasshouse experiments were carried out to determine the efficacy of reduced dosages of atrazine and nicosulfuron on weed control. The effect of weed growth stage, herbicide combinations, integrating tillage with reduced herbicide dosages and timing of application were investigated on weed control. For the field experiments at the Crop Science Department, atrazine and nicosulfuron dosages (33.3%, 66.7% and 100% of the full recommended dosage) were applied at two weed growth stages in plots planted to maize. The reduced herbicide dosages were found to be more effective in controlling weeds at the two- to three-leaf stage than at the five- to six-leaf stage. Effectiveness in weed control was judged on the basis of the following parameters: percent weed kill, weed biomass, weed seed capsule numbers, maize grain yield and net economic return from the maize crop. Maize yield from plots treated with reduced herbicide dosages was not significantly different from full dosages though weed control levels were lower. Reduced atrazine dosages were as effective as full-recommended dosages in the control of susceptible weed species such as Amaranthus hybridus, Galinsoga parviflora, Nicandra physalodes and Oxalis latifolia. For the above species, 100% weed control was obtained for both full atrazine dosages and the reduced atrazine dosages. The reduced atrazine and nicosulfuron dosages produced lower percent weed control for tolerant species such as Cyperus rotondus and Setaria homonyma. For glasshouse experiments, herbicide dosages as low as 12.5% of the recommended dosage integrated with simulated tillage were as effective as full dosages in controlling weeds. Better economic returns were obtained for reduced atrazine and nicosulfuron dosages applied as a mixture than full-recommended dosages of the two herbicides separately. Reduced nicosulfuron dosages applied sequentially were more effective in controlling weeds than the full dosages and reduced dosages applied at once. Weed seed production was not significantly different between weeds treated with reduced herbicide dosages and those treated with full-recommended dosages. However, the reduced atrazine and nicosulfuron dosages were more effective in suppressing weed seed capsule numbers when applied at the two- to three-leaf stage than at the five- to six-leaf stage.
Crop Protection Practices of Smallholder Vegetable Growers in Chinamora Communal Land - Maxwell Sibhensana
Supervisors: Dr A. Turner, Prof. D.P. Giga and Dr O.A. Chivinge
A study was conducted in Chinamora Communal Land to gather information on pest control practices for vegetables. The study was divided into an informal exploratory survey, a formal survey and field observations of the farmer day-to-day operations. Sixty percent of the farmers were able to identify most of the insect pests. However, 32 % of the farmers were unable to identify disease damage such as early blight caused by Alternaria solani, 73 % could not identify bacterial speck damage caused by Pseudomonas tomato, none of the farmers perceived halo blight of beans caused by Pseudomonas phaseolicola as a disease, and 97 % were not aware of root knots of tomato caused by the nematode Meloidogyne javanica. Ninety percent of the farmers used chemical control methods against pests. Forty percent also used fungicides in an attempt to control red spider mite and 4 % used insecticides in an attempt to control fungal diseases such as early blight.
Fifty per cent of the respondents owned knapsack sprayers or other hand-held sprayers; the remainder borrowed sprayers or used brooms for pesticide application. About 86 % of the farmers applied pesticides before a pest was seen, 52 % after the pest had been seen, 10 % every seven days, 10 % every 14 days while another 10 % after each crop picking. Farmers observed various spray-to-harvest intervals: seven days, 14 days and some according to label instructions. Ninety per cent of the farmers surveyed also used seed saved from previous crops even for the crops for which they used certified seed. Twenty five per cent of the farmers selected tomato cultivars that were less susceptible to early blight. Sixty per cent of the farmers did not practise any meaningful rotation against pests. Fifteen per cent of the farmers were aware of biological control of insect pests. Fifty per cent liked the idea of integrating cultural and chemical methods of pest control for the benefit of cutting on chemical costs, but 85 % felt that some of the cultural practices were too labour intensive and unsuitable compared to chemicals which could be administered quickly. No farmer complained of weeds in the vegetable crops although some crops were overgrown with weeds.
Cloning and Expression of the Coat Protein Gene of a Potyvirus: Zimbabwean Isolate of Sweet Potato Feathery Mottle Virus (SPMFV) Designated SPV-Z - Sizolwenkosi Mlotshwa
Supervisors: Dr A.I. Robertson and Dr B.J.M. Verduin
The coat protein-coding region of a Zimbabwean strain of sweet potato feathery mottle virus (SPFMV), designated SPV-Z, was cloned, sequenced and expressed in E. coli BL21 (ED3).
Double-stranded cDNA was synthesised from SPV-Z RNA by reverse transcription-polymerase chain reaction (RT-PCR) using two antigenome-sense, downstream, reverse primers (P89 and Pot1) and a genome-sense, upstream, forward primer (Pot2) derived from potyviral consensus sequences. The PCR products were ligated into pGEM-T cloning vector and sequenced. The deduced sequence incorporated the 3'terminal 2049 nucleotides of the SPV-Z genome comprising an open reading frame of 1803 nucleotides and a 3' non-coding region of 215 nucleotides followed by 31 nucleotides of polyadenylate. Two putative glutamine/alanine and glutamine/serine protease cleavage sites were mapped at amino acid positions 206-207 and 266-267 respectively. The glutamine/serine site was deduced to be the genuine cleavage site, which would result in a 335 amino acid protein with a calculated Mr of 40.2 kDa. This was consistent with the 39-kDa-coat protein estimated by SDS-PAGE for purified SPV-Z. Comparison of the deduced SPV-Z coat protein amino acid sequence with five other strains of SPFMV revealed 74 % similarity, with the C-termini showing more homology than N-termini.
To effect expression of the SPV-Z coat protein in E. coli, two specific primers were designated to flank the coat protein coding sequence. The genome-sense, upstream forward primer (SPVCP1ND) incorporated an ATG translation start codon in the context of a NdeI restriction site and the first 18 homologous nucleotides of the N-terminus of the coat protein gene. The antigenome-sense, downstream reverse primer (SPVCP2BA) incorporated a BamHI site, a TAA translation stop codon and the last 20 complementary nucleotides of the C-terminus of the coat protein gene. The SPV-Z CP clones generated by PCR amplification using these primers were constructed under the regulation of phage T7 RNA polymerase promoter in pT7-7 expression vector and expressed in E. coli BL2 (ED3). The expressed protein had the same electrophoretic mobility on SDS-PAGE and antigenic specificity on immunoelectroblots as purified SPV-Z coat protein.
Oviposition Behaviour of Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) on Mange Tout Pea (Pisum sativum L.) and Bio-assays to Evaluate Efficacy of the Tablet Formulation of Deltamethrin - Ivy Nzuma
Supervisors: Prof. D.P. Giga and Mr A.B. Mashingaidze
The efficacy of a new tablet formulation of deltamethrin was assessed under laboratory conditions against first, third and fifth instar Helicoverpa armigera larvae. The results were compared with an emulsifiable concentrate of deltamethrin. Following exposure by ingestion of four dosages of each formulation applied to mange tout pea leaves, the mortality of different instars of larvae of the pest was evaluated 48 hours post treatment. There were no significant differences between tablet formulation and the emulsifiable concentrate. First instar larvae were highly susceptible to both formulations even at the lowest concentrations tested. The fifth instar larvae were only susceptible to higher concentrations of deltamethrin. The potential of the tablet formulation for commercial use is discussed.
Two greenhouse experiments were done to study the oviposition behaviour of Helicoverpa. Aspects studied included the effects of host phenological stage on oviposition and the spatial distribution of eggs within the plant canopy. Observations were also made on the nocturnal pattern of H. armigera oviposition and distribution of eggs on various plant parts. The results indicate that peak oviposition coincides with flowering of the host, which is consistent with published findings on other crops. Oviposition was not random as egg numbers decreased going down the plant canopy. It appeared that egg numbers were almost equally distributed between the upper and lower surface of the leaves even when some were recovered from buds, open flowers, pods and peduncles. It also appeared that within an observation cycle between 1700 and 0300 hours, most eggs were laid between 1900 and 2200 hours.
MSc Abstracts (1998)
The Effect of Organic Amendments on the Population of Verticillium chlamydosporium (Goddard) for the Control of the Root-Knot Nematode (Meloidogyne) Species - LLyn K. Munjoma
Supervisors: Dr. S.L. Page and Mr A.B. Mashingaidze
Plant parasitic nematodes are important pests of vegetable crops worldwide. An investigation was carried out at Fambidzani Permaculture Centre, a farm under organic practises since 1988, to ascertain the occurrence and damage of Meloidogyne sp. and to screen the bio-control agent, Verticillium chlamydosporium. It was found that most of the vegetable crops sampled from the garden were infected by the root knot nematode. The most infected crops were spinach and pumpkins in the compost treated plots. Maize in the dry land plot was not infected at all. The fungal natural enemy V. chlamydosporium was found in all the amended plots of compost, poultry manure and pig manure. The highest numbers of colony forming units (cfu) per gram of soil and of root tissue were found in the compost treated plots. The fungi were also found associated with the nematode eggs. In the compost treated plots an average percentage of egg parasitism of 4.71% was observed whilst the unamended control treatments only had 1.50% of the eggs parasitised.
Heat sterilisation and organic amendment application in the glasshouse significantly affected population levels of the M. javanica as well as the establishment of V. chlamydosporium. Addition of organic amendments to non-sterilised soil reduced nematode juveniles in the soil than in sterilised soil treated with the same amendment. Galling index of the potted tomato plants was significantly (P<0.05) affected by soil sterilisation and organic amendments. Lower root galling indices were observed in the non-sterile soil than in the sterile soil. There were similar reductions in the root-galling index of plants by poultry and pig manure treatments. The unamended control treatment had the highest root-galling index of 5.8. There was no significant (P<0.05) interaction between the soil treatment and organic amendment addition on the population levels of M. javanica females per gram of root tissue. There were more females (43.6/g) in the sterilised soil than in the non-sterile soil (32.8/g). Poultry manure reduced female population levels more than the other treatments with only 16.5 females/g whilst the other treatments had levels above 20/g.
There was significant (P<0.05) interaction between soil treatment and organic amendment addition on the root weight of the potted plants. There was a large reduction on root weight between non-sterilised and sterilised soils for the amended pots, whilst a small difference was observed for the unamended treatments.
There were no significant (P<0.05) differences in the V. chlamydosporium number of colony forming units per gram of root tissue for the sterilised and non-sterilised soil samples for the unamended control and poultry manure treatments. Compost and pig manure treatments had significantly larger differences in root weight between the sterilised and non-sterilised. The number of colony forming units per gram of soil significantly (P<0.05) differed with soil treatment with sterilised soil having the least number of colony forming units (cfu). Among the organic amendments, compost treatment yielded the highest number of 155.26 cfu/g.
Soil sterilisation did not significantly (P<0.05) affect the percentage number of nematode eggs parasitised by fungi. There were significant organic amendment effects on the percentage number of parasitised nematode eggs. Compost treatments had the highest percentage followed by poultry manure and then pig manure and lastly the unamended control treatments.
The application of the organic amendments at different rates in the Fambidzanai soil affected the occurrence of the M. javanica and V. chlamydosporium in the glasshouse. The type of organic amendment and rate of application had a significant (P<0.001) interaction with the juvenile numbers in the soil. In all the amendments, there was a progressive decline in M. javanica juveniles as the organic amendment rate increased. Compost reduced the juveniles by 43 % between the 0.5 times and 1 times the garden rate. There was no significant (P>0.05) interaction between the type and rate of organic amendment application on the number of nematode females per gram of root tissue. However, there was a significant reduction in the number of females as rate of application of organic amendments increased. Poultry manure treatments, amongst the organic amendments, reduced the female nematodes to the same level with compost. Similar numbers of females were observed between compost and pig manure treatments. There was a significant (P<0.05) interaction of type of organic amendment and rate of application on the root-galling index of the potted tomato plants. There was no further reduction in root galling index from the 1 times to 1.5 times the garden rate of all amendments.
There was a significant (P<0.05) interaction of the two factors on the root weight of the potted plants. A progressive increase in root weight as the organic amendment rate increased was observed. Among the amendments, compost had the highest root weight of 19.83 g/plant.
There was a significant (P<0.001) interaction of the two factors on the number of V. chlamydosporium, colony-forming units per gram of root tissue and of soil. The colony forming units per gram of root tissue in the compost treatments increased as the rate of application increased. In contrast, for poultry and pig manure treatments, an increase was observed between 0.5 times and 1 times the garden rates and no further increase was observed from the 1 times to 1.5 times the garden rates. In the soil for compost and pig manure treatments, there was a significant increase in the number of colony forming units from 0.5, 1 to 1.5 times the garden rates. In poultry manure treatments there was no further increase between 1 and 1.5 times the garden rate.
Among the type of the amendments used, nematode egg parasitism was highest in the compost treatment (13 %) whilst pig and poultry manure treatments had similar percentage numbers of parasitised eggs. Also, there was an increase in the percentage numbers of parasitised eggs as the rate of organic amendment application increased from 0.5, 1 to 1.5 times the garden rates. Thus, in this study compost has proved to be effective in the stimulation of V. chlamydosporium for the control of the root knot nematode, Meloidogyne sp.
Reduced Metsulfuron-methyl Dosages for Weed Control in Wheat (Triticum aestivum L.) Grown Under Rainfed Summer Conditions - Tanaka Gumunyu
Supervisors: Dr. P. Tongoona and Mr A. B. Mashingaidze
Field and glasshouse trials were conducted during the 1997/98 summer season to investigate the efficacy of reduced metsulfuron-methyl dosages against weeds in wheat (Triticum aestivum) grown under rainfed summer conditions. The field trials were conducted at three sites namely Harare (University of Zimbabwe), Marondera and Nyanga. These sites differed in altitude, soil type, weed pressure and distribution and in climatic conditions. Supporting glasshouse experiments were conducted at UZ to investigate the effect of surfactants and rain-free period on the efficacy of reduced metsulfuron-methyl dosages. Tank mixtures of metsulfuron-methyl and bentazone were also assessed under glasshouse conditions. Measurements taken during the study included weed counts, before and after herbicide treatments, weed shoot dry weight and leaf area. Field results obtained from all the three sites indicated no significant (P>0.05) differences between reduced and recommended metsulfuron-methyl dosages. Significant (P<0.01) interactions were observed between weed species and weed growth stage. In general the one- to two-leaf stage of weeds like Galinsoga parviflora, Nicandra physalodes and Hibiscus meeusei was easier to control than at the five- to six-leaf stage. Growth stage did not have any effect on the control of tolerant grass species such as Eleusine indica and sedges such as Cyperus esculentus. Glasshouse experiments results showed that inclusion of the surfactant Sanawett does not improve efficacy of reduced metsulfuron-methyl dosages on N. physalodes, G. parviflora, E. indica and Setaria pumila. The results also showed that N. physalodes, Richardia scabra and G. parviflora require different lengths of rain-free period after application of metsulfuron-methyl. Efficacy of two thirds of the recommended metsulfuron-methyl dosage was observed not to be affected by rain-free period length. No synergism or incompatibilities were observed between metsulfuron-methyl and bentazone. Mixtures of reduced dosages of metsulfuron-methyl and bentazone were observed to cause similar weed control when compared to the recommended dosages used either in mixture or separately. Reduced metsulfuron-methyl dosages were also observed to retard growth of S. pumila, G. parviflora and N. physalodes under glasshouse conditions.
Integrated Insect Pest Management in Crucifers - Shonagh C. Taruza
Supervisor: Prof. D.P. Giga
A series of experiments were carried out to evaluate the effect of a range of new insecticides on the insect pests of crucifers. The two main species of aphids attacking crucifers in Zimbabwe are Brevicoryne brassicae (L.) and Myzus persicae (Sulzer). Two chemical and three botanical insecticides were evaluated for their effects on these aphids and the aphid parasitoid Diaeretiella rapae (M'Inctoch) five and 21 days after spraying. The botanical insecticides BTG504, BTG505 and neem were compared with the chemical insecticides pymetrozine and dimethoate and two controls, namely water and the wetter and plain tap water. It was found that pymetrozine and dimethoate were the most effective in reducing aphid numbers after both five and 21 days, however they also caused the greatest loss of the parasitoid D. rapae. The next most effective insecticide was neem, which reduced aphid numbers markedly after five days and allowed parasitoid numbers to increase, however it was ineffective after 21 days. BTG504 and BTG505 were ineffective in controlling aphids both after five and 21 days, however they had no effect on D. rapae whose numbers increased.
Two insecticides were evaluated for their efficacy in controlling diamondback moth larvae, one was a formulation of Bt, Biobit7, and the other was a formulation of neem, Nico neem7. The formulation of Bt was found to be very effective in controlling the diamondback moth larvae five days after spraying, whereas the neem formulation was found to be ineffective.
An investigation was also carried out evaluating the effects of different doses of dimethoate on aphids and the parasitoid D. rapae. It was found that halving the dose of dimethoate was just as effective in controlling aphids as the full dose both five days and 21 days after spraying. Halving the doses, however, did not result in greater survival of D. rapae, and there was no difference between the full dose and the halved dose in their effects on survival of D. rapae.
A survey was carried out at a communal farm in Chinamora Communal Land and at the University of Zimbabwe Farm (UZ) in the month of September 1988. Diamondback moth larvae and pupae were collected and taken back to the laboratory, reared and the emergence of parasitoids was recorded. Two types of parasitoids emerged from both samples. The first one emerged from larvae only and was identified as Cotesia plutellae Kurdjumov. The second one emerged only from larvae and was identified as Oomyzus sokolowskii Kurdjumov. C. plutellae accounted for 2.8 per cent parasitisation at the communal farm and 25 percent at the UZ farm. O. sokolowskii accounted for 24.7 percent parasitisation at the communal farm and 19 percent at the UZ farm.
Studies on the Efficacy of Vegetable Oil Pesticide Formulations on Aphis gossypii (Glover) in Cotton, and their Effects on Aphid Predators and Parasitoids - Casper Masvikeni
Supervisor: Prof. D.P. Giga
Studies were carried out on the efficacy of vegetable oil formulations on cotton aphids, Aphis gossypii mortality in the cotton fields at the Cotton Training Centre in Kadoma between January and May 1998. Different spray coverages were applied to individually marked leaves using hand-held sprayers. In the first experiment, the aphid mortality caused by the oils was compared with that caused by traditional aphicides, and a soap solution, Tepol.
The spray application of the oils to run-off showed a highly effective A. gossypii control, which was as good as that given by the traditional aphicides, dimethoate and benfuracarb. The soap solution afforded a better control of the cotton aphids. A spray application which simulated a tailboom sprayer was less effective in killing the cotton aphids in the field. This could have been caused by poor spray coverage (droplet density) on the leaf surfaces. Oil insecticides are contact poisons and have to cover the insect body to be effective.
Five oil formulations were sprayed to run-off seven days after they were diluted with water. Their efficacy on A. gossypii mortality remained very high and similar to that afforded by the soil sprays made soon after dilution with water. Two observational experiments to evaluate the effect of the oil chemicals on aphid natural enemies (coccinellids and parasitoids) were set up in the laboratory under room temperature conditions of 22 to 34 OC. For the experiment involving coccinellids, adults, pupae and larvae of Cheilomenes lunata were dipped into the chemical for five seconds and kept in pill boxes for up to three days. Observations showed that the oils were toxic to the three coccinellid stages and two of the plant oil formulations were relatively less harmful.
Mummified aphids were sprayed with the chemicals in the field using two spray coverages, collected and kept in the laboratory for up to 12 days. Observations showed that mummies that received a fine spray had a higher parasitoid emergence than those that were sprayed to run-off. Eight of the formulations were evaluated for their repellent effect on A. gossypii. The chemicals did not elicit any repellent effect on the aphids introduced on both the treated and untreated halves of individual leaves.
The Effects of the Dwarfing (Brachytic-2) Gene on Maize (Zea mays L.) Roots, Dry Matter Partitioning and Performance of Brachytic-2 Hybrids in Contrasting Environments in Zimbabwe - Effie C. Chinhema (September 1996)
A study was carried out in the greenhouse and in the field to investigate the effects of the dwarfing (brachytic -2) gene on maize (Zea mays L.) roots and drought tolerance using six pairs (six tall and six dwarf) of near-isogenic hybrids. The brachytic-2 (br-2) gene caused significant (P<0.05) decreases in root dry mass (RDM), root volume (RV), shoot dry mass (SDM) and grain yield in some dwarf hybrids while in others, significant (P<0.05) increases in leaf dry mass (LDM), proportionate amount of dry matter allocated to the leaves and grain yield were realised.
The gene had no effects on root depth but pleitropically it induced the maize root system to become finer and more fibrous irrespective of the genetic background to which the gene incorporated. Root volume and RDM were either reduced or not affected depending on the genetic background to which the gene was incorporated. The superiority in RV and RDM of some tall hybrids was a result of thicker roots that were of the same depth as those of dwarf hybrids. Except for one dwarf hybrid (dw211DR x K64r), where the gene caused a significantly greater allocation of dry matter to the leaves, dry matter partitioning was not altered in the other five dwarf hybrids.
The effect of the br-2 gene on grain yield was influenced by both the genetic background of the hybrid and the genotype x environment interaction. The gene caused hybrids dw2N3d x SC and dw211DR x 2N3d to be better adapted to the high yield potential area, hybrids dw211DR x K64r and dwSC x K64r to be less adapted to the low yield potential area and did not affect the yield of dw2N3d x K64r and dwSC x K64r in both high and low yield potential areas.
Inorganic Fertilizer and Cattle Manure Management for Dryland Maize (Zea mays L.) Production Under Low Input Conditions - Muchemedzi Wilfred Munguri (June 1996)
Supervisors: Dr I.K. Mariga, Dr O.A. Chivinge and Dr S.R. Waddington
On-farm experiments were conducted at Domboshava and in Chinyika Resettlement Area to develop and test technologies to improve soil fertility management for low input dryland maize production. The effects of the use of 250 kg ha-1 ammonium nitrate (AN), and 8 t ha-1 manure, 300 kg ha-1 compound D and 300 kg ha-1 compound Z applied either alone or in combination with 250 kg ha-1 AN on maize grain yield were evaluated in Chinyika Resettlement Area. Application of manure and fertilizer significantly increased maize yield in 1993/94 except for manure applied alone. Compounds D and Z applied at planting and AN applied 5 WACE (weeks after crop emergence) achieved the highest grain yields which were significantly greater than for the control in the two seasons. However, in the 1994/95 season only treatments with manure or inorganic fertilizer and AN at 5 WACE significantly increased maize yield over the control. During the 1994/95 season 8 t ha-1 manure only, compounds D and Z applied at planting and AN only achieved similar yields to the control. In the dry 1994/95 season, it was largely uneconomic to use fertilizer.
The effects of two rates of nitrogen (43 and 86 kg N ha-1) applied at five weeks after crop emergence, at initiation of tasselling and split at five weeks after crop emergence and initiation of tasselling were also evaluated. There were significant (P<0.05) interactions at two sites combined in the 1993/94 season. Splitting 43 kg N ha-1 significantly (P<0.05) outyielded all other treatment combinations except for 86 kg N ha-1 applied at initiation of tasselling. At Govakova, there was a tendency towards a high but non-significant maize grain yield when N was applied at five weeks as a side-dressing, and, at Pfumoiguru, maize grain yield tended to be increased when AN was split at five weeks and initiation of tasselling. In the 1994/95 season, rate and time of N application significantly (P<0.05) influenced maize grain yield. The higher rate of 86 kg N ha-1 significantly outyielded the lower rate of 43 kg N ha-1 and significantly (P<0.05) higher maize grain yield was obtained when N was applied five weeks after crop emergence. There were also significant interaction effects of N rate x time of application on grain nitrogen and phosphorus concentrations. Applying 86 kg N ha-1 at five weeks after crop emergence resulted in significantly (P<0.05) higher grain N and P concentrations. From the study, the variable response to N rate and application time was mainly influenced by moisture conditions prior to N application and the native status of the sites used in the study.
The effects of two rates of manure and three application methods on maize performance were evaluated during the 1993/94 and 1994/95 rainy seasons. Residual effects of manure quantity and method of application on maize grain yield and on N, P and K % in grain were determined at one site during the 1994/95 season. Manure quantity (4 and 8 t ha-1) had no effect on maize grain yield while method of manure application significantly affected grain yield in both the 1993/94 and 1994/95 seasons. Station placement of manure was superior to broadcasting which was similar to banding in the planting furrow in both seasons. Residual effects of manure applied during the 1993/94 season significantly influenced maize yield in the 1994/95 season at Domboshava. Yield on station-placed plots was significantly higher than where manure was previously broadcast, but similar to that of plots where manure had been banded in the planting furrow. Method of manure application significantly influenced grain nitrogen concentration in the residual trial, with N content being significantly (P<0.05) lower when manure was broadcast.
Manure samples were collected from unroofed kraals to which stover had been added (SAK) and to which no stover had been added (NSAK) and heaped for periods varying from zero to five months. They were analyzed for N, P and K. Nitrogen (%) in samples from NSAK rose from 0.56 % in unheaped manure to 0.90 % after four months for the 1993/94 samples and N% increased from 0.8 % in unheaped manure to 1.7 % in the same period for the 1994/95 samples. Phosphorus content in manure samples was high in freshly dug manure, with contents falling after two months of heaping in both samples for the two seasons. Potassium increased from 0.04 % in unheaped samples to 2.27 % and 1.58 % in both samples after two months of heaping in the 1993/94 season. Potassium content in the 1994/95 samples was generally high in both samples with a peak of 3.2 % after two months of heaping in SAK samples and 2.7 % in NSAK samples. There was a general decline in nutrient content in both samples more than two months after heaping. Therefore, farmers in Chinyika can dig and heap their manures in August such that this does not interfere with other critical farm operations such as harvesting in May and June.
Weed Management Studies in Maize (Zea mays (L.)) for the Resettlement Smallholder Farming Sector - Edgar Rupende (June 1997)
Supervisors: Dr O.A. Chivinge and Dr I.K. Mariga
Diagnostic surveys were conducted in Chinyika Resettlement Area (CRA) to determine resource base, crop production trends and constraints that limit maize (Zea mays (L.)) productivity in the 1992/93 rainy season. Two glasshouse experiments were conducted at the University of Zimbabwe during the 1993/94 and 1994/95 rainy seasons to determine the effect of rapoko grass (Eleusine indica (L) Gaerth) and Mexican clover (Richardia scabra (Moq.) Gomez) competition in binary and ternary mixtures with maize. Experiment I considered interference of E. indica on maize and R. scabra on maize in binary mixtures using an additive series. One maize plant and each weed species varied from 1:1 to 1:6. Experiment II evaluated the performance of maize in ternary mixtures also using the additive series. Species mixtures of 1:1:1 to 1:1:5 maize/E. indica/R. scabra and 1:1:1 to 1:1:5 maize/R. scabra/E. indica were evaluated. In addition, 1:2:1 maize/E. indica/R. scabra; 1:2:3 maize/R. scabra/E. indica and 1:3:2 maize/R. scabra/E. indica mixtures were evaluated. Data on maize leaf number, stem height, stem circumference and grain yield were collected.
An on-farm study of maize competition against weed infestation was undertaken to determine the effect of weed competition on maize growth and grain yield, critical competition period of the natural weed community in maize and establish the optimum time and number of weedings necessary to achieve economic maize yields under dryland conditions in 1994/95 rainy season. The experiment consisted of two sets of six treatments, with the crop either weed-free or weedy for the first two to twelve weeks after crop emergence (WACE). A glasshouse study to identify weed species that commonly occur in manure, determine weed seedling numbers, nutrient and temperature changes in manure heaped for varying durations was undertaken for two seasons, 1993/94 and 1994/95, at the University of Zimbabwe. On-farm weed management trials were carried out to evaluate efficacy on weeds, and to compare the effect on maize grain yield, labour requirements and economic returns of different weeding methods during the 1993/94 and 1994/95 rainy seasons. Manual (hand-hoeing), chemical (atrazine - a herbicide) and combination mechanical (ox-cultivation and ox-mouldboard plough 'ridging') methods were evaluated.
Informal and formal surveys revealed that weed management was the major constraint to increased maize grain yields. E. indica and R. scabra were the most dominant and difficult to control weed species. In binary mixtures interspecific interference on maize vegetative components and grain yield was dependent on the density of weed species with the maize components drastically affected at higher densities of E. indica and R. scabra. E. indica showed a greater negative impact on vegetative components and maize grain yield than R. scabra at all the different mixtures with maize. Competition in ternary mixtures resulted in a non-linear maize grain yield reduction showing that the behaviour of the third species cannot be predicted. A weed species with a high proportion of plant in the mixture or competitive ability even at lower proportions has more influence on the outcome of competition.
Studies on competition of maize with natural weed infestation showed that the maize critical competition period is agro-ecological region dependent. Hand-hoeing twice during the critical competition period was adequate to promote a weed-free respite. Cattle manure applied to fields as a source of nutrients can disseminate seeds of several weed species. Manure heaping is an effective method of reducing weed infestation to crop fields and enhances availability of some nutrients. Heaping for three months significantly reduces weed numbers, while N and K2O nutrients are relatively highly available during this period. Use of atrazine was more economical (time, labour and costs) and effective against a wide spectrum of weeds resulting in a higher maize grain yield than with manual mechanical weeding. Economic analysis showed that applying atrazine pre-emergence full-cover had the highest return of 394 percent. In view of the importance of conserving the limited moisture and soils and achieving sustainable crop production, farmers preferred ox-cultivation and hand-hoeing at 2 WACE plus ox-mouldboard plough 'ridging' at 5 WACE and atrazine pre-emergence full-cover spray plus ox-cultivation at 4 WACE weeding combinations.
Maize Witchweed (Striga asiatica (L.) Kuntze) Management Technologies in the Smallholder Farming Sector of Zimbabwe - Darlington Musambasi (December 1997)
Supervisors: Dr O.A. Chivinge and Dr I.K. Mariga
Five on-farm trials were conducted during the 1994/95 and 1995/96 rainy seasons in Chinyika Resettlement Area (Natural Regions IIb and III) to compare the effectiveness and economics of some witchweed (Striga asiatica (L) Kuntze) management practices. Two pot trials were also carried out at the University of Zimbabwe, Department of Crop Science. The objective of the pot experiments were (i) to investigate the response of four maize (Zea mays L.) cultivars to different ammonium nitrate levels applied as topdressing in pots infested with S. asiatica and (ii) to compare the effect of S. asiatica infestation on the grain yields of eight early maturing maize cultivars.
Experiment I compared the effects of 2,4-dichlorophenoxy acetic acid (2,4-D), 3,6-dichloro-methoxybenzoic acid (dicamba), hand weeding at two and five weeks after crop emergence (WACE) and hand weeding at 2, 5 and 8 WACE on S. asiatica density and maize grain yields. The four treatments gave a similar maize grain yield averaging 4790 kg/ha during the 1995/96 season. However, during the 1994/95 season the maize grain yield was lower due to severe moisture stress that occurred at the silking, tasselling and grain-filling stages. Maize grain yields from plots treated with dicamba, 2,4-D and hand weeded at 2, 5 and 8 WACE were 43 %, 16 % and 25 % lower than those from plots only hand weeded at 2 and 5 WACE. Application of dicamba resulted in the best control of S. asiatica at 8 and 10 WACE while 2,4-D was effective in suppressing the weed at 12 WACE. During the 1995/96 rainy season, dicamba gave a marginal rate of return of 91 %, which was found to be economic to the farmer while it was uneconomic to hand weed S. asiatica.
In experiment II, maize was intercropped with Bambara nut (Vigna subterranea (L.) Walp), cowpea (Vigna unguiculata (L.) Walp), field bean (Phaseolus vulgaris L.) and groundnut (Arachis hypogaea L.) in fields infested with S. asiatica. Intercropping maize with these trap crops resulted in reduced numbers of emerged S. asiatica plants that emerged. The maize/cowpea intercrop had lower numbers of emerged S. asiatica plants/m2 during the drier 1994/95 season while during the 1995/96 season all the intercrops resulted in similar numbers of emerged S. asiatica plants/m2. The number of S. asiatica plants that emerged in the intercrops averaged 15 plants/m2 and this was similar to that in the sole maize. The partial land equivalent ratios indicated that it was more advantageous to intercrop than to plant sole maize in fields infested with S. asiatica.
In experiment III, planting maize cultivars R215 and SC501 on the flat resulted in 95 percent less emerged S. asiatica plants/m2 than planting on ridges during the 1994/95 season. However, during the 1995/96 season planting both cultivars on ridges resulted in the emergence of 34 percent less S. asiatica plants than planting on the flat and ridging at 5 WACE. Planting SC501, on the flat and ridging plots planted to SC501 at 5 WACE gave 66 percent more grain yield than planting the cultivar on ridges during the 1994/95 season. During the 1995/96 season R215 planted on the ridges and SC501 in plots ridged at 5 WACE had similar yields that were 25 percent higher than the yield of both cultivars planted on the flat.
The benefit of trap crops in reducing the density of S. asiatica that emerged was tested by growing maize on plots previously planted with the trap crops in experiment IV. The number of emerged S. asiatica plants/m2 averaged 11 plants/m2 and maize grain yields averaged 3000 kg/ha in all the treatments. The number of emerged S. asiatica plants was similar in all treatments at both sites during the 1995/96 rainy season, but S. asiatica only emerged in plots with maize during the 1994/95 rainy season. There were no treatment differences even when the data was combined over the two sites during the 1995/96 rainy season. Maize grain yield was similar when the maize crop was grown after the different trap crops during the 1995/96 rainy season.
Experiment V tested the effects of varying ammonium nitrate levels at 5 WACE on the yield of three-way maize hybrids and the number of emerged S. asiatica plants. Increasing nitrogen levels at topdressing (5 WACE) from 0 to 86.25 kg N/ha increased both maize stover and grain yields by 33 percent. Maize grain yield and stover yields were similar when 34.5, 51.75, 69.0 and 86.25 kg N/ha were applied and were lowest when there was no nitrogen applied as a topdressing. The number of S. asiatica plants/pot that emerged at 10 WACE decreased as the nitrogen level was increased up to 69.0 kg N/ha. However, the number of S. asiatica plants that emerged increased when the nitrogen level was increased from 69.0 to 86.25 kg N/ha. Maize cultivars CG4141 and R215 had the lowest grain yields of 82 g/pot while PHB3435 and PNR473 had similar and higher yields of 111 g/pot.
Experiment VI compared the agronomic performance of eight three-way hybrids in fields infested with S. asiatica. The eight early maturing maize hybrids grown in the fields had similar grain yields and emerged S. asiatica plants/m2 in both seasons. The hybrids had similar grain yields per pot and numbers of emerged S. asiatica plants per pot when the experiment was repeated in pots. However, CG4141, R215 and SC501 had the least Striga damage score of 3 in both seasons.
A Study of the Response to Selection for Drought Tolerance of Two Maize (Zea mays L.) Populations at Germination, Emergence and Flowering - Cosmos Magorokosho (January 1998)
Supervisors: Dr. P. Tongoona and Dr. K. Pixley
Drought is a major factor limiting maize yields in many parts of the world. The need to breed maize cultivars with improved drought tolerance is apparent, but optimal selection strategies are not clear. This study compared two maize populations, ZM601 and ZM607, for drought tolerance during germination, early establishment and flowering. ZM601 had been selected for two cycles for drought tolerance at flowering under rainfed drought stress while ZM607 has been improved for high yield potential for three cycles under favourable growing conditions.
Eighty-nine random S2 lines from ZM601 and 76 from ZM607 were evaluated under moisture stress imposed by applying a gradient of water available for germination and early establishment using a line source irrigation set-up. Another set of 143 random S1 lines from ZM601 was compared with 94 from ZM607 at three drought-stressed and one well-watered environment for yield potential and drought tolerance at flowering. Each of these S1 lines was then crossed to two sisters of similar heterotic pattern but reputedly differing in drought tolerance. Testcross hybrids were evaluated for yield potential and drought tolerance during flowering at five well-watered and drought-stressed environments.
The S2 lines from ZM601 established slightly better (maximum of 77 %) than those from ZM607 (maximum of 69 %) under drought stress at germination. However, there were no differences for survival (maximum of 86 % for both populations), drought-induced leaf rolling (mean score of 2.6 for both populations) and percent of severely drought-stressed lines able to recover after irrigation (maximum of 48 % for both populations). This study confirmed that selection for these traits as indicators of germination and seedling establishment ability under drought is difficult because environmental variation is high under such field conditions. Agronomic solutions may be more practical and lead to more short term impact than genetic solutions in improving maize seedling drought tolerance.
The S1 lines from ZM601 had an average of 1.06 ears per plant (EPP), whereas lines from ZM607 had 1.02 EPP. For the two populations, frequency distributions for anthesis to silking interval (ASI) and leaf rolling were different; ZM601 had more lines with shorter ASI and less leaf rolling as compared to ZM607. The S1 lines from ZM601 did not differ significantly from those from ZM607 for yield and most of the traits studied under drought stress at flowering. Selecting for drought tolerance using rainfed sites (random stress) for population ZM601 was, thus, considered inefficient, and it is recommended that selection be conducted under managed drought-stress levels which reveal increased variability for drought-adaptive traits such as ASI and EPP.
Testcross hybrid performance differences were ascribed to drought tolerance of the tester used, with no differences in population means for all the measured traits between ZM601 and ZM607. Testcrosses to K64R had mean ASI values of 0 and 3 days under intermediate and severe drought stress respectively, while those for CML202 had means of two and four days under the same respective conditions. Testcrosses to K64R flowered about six days earlier than those testcrosses to CML202. Drought escape through early maturity did not result in yield advantage for early maturing testcrosses to K64R under severe drought stress. Drought tolerance through reduced ASI and increased EPP however, resulted in superior yield of testcrosses to K64R relative to those to CML202 under the intermediate drought stress level. Data adjusted to a constant flowering date for tester revealed superior yield of testcrosses to CML202 under severe drought (SS) while under intermediate drought stress (IS) there were no yield differences between testcrosses from the two testers. Thus, this study demonstrates that drought escape through earliness is not always the best solution to maize varietal requirements for drought prone areas when the stress coincides with flowering.
Probabilities of obtaining drought tolerant maize hybrids were higher when S1 lines were selected based on a combination of yield, ASI and EPP under drought stress (61 and 80 % for testcrosses to K64R and CML202 respectively) than when lines were selected based on yield under favourable conditions (48 and 49 % for testcrosses to K64R and CML202 respectively). Hence selection for increased yield along with reduced ASI and barrenness of S1 lines under drought stress can be used to select drought tolerant hybrids.
Characterisation, Epidemiology and Management of Bacterial Wilt of Tobacco, Caused by Ralstonia solanacearum in Zimbabwe - Alison E. Robertson (August 1998)
Supervisors: Dr D. L. Cole and Mrs J. Sibiya
The physiological and genetic diversity in Zimbabwean isolates of Ralstonia solanacearum (Smith) Yabuuchi et al. (1995) were determined using carbohydrate oxidation and DNA fingerprinting, respectively. It was established that Zimbabwean isolates of the bacterium isolated from infected tobacco (race 1) and from potato and tomato (race 3) belong to biovar 1 and biovar 2, respectively. DNA fingerprints of the Zimbabwean isolates from potato and tomato were similar to race 3 isolates from Kenya, Israel and Colombia. Comparisons of Polymerase Chain Reaction (PCR) fingerprinting profiles of tobacco isolates from Zimbabwe, the Americas and Australia, showed that the Zimbabwean isolates differed from the American and Australian isolates. The Zimbabwean isolates could also be divided into two sub-groups, those from Mvurwi/Concession and those from Burma Valley. On a wilt-susceptible flue-cured cultivar, there was no difference in virulence between isolates from Concession and Burma Valley, as determined by infectivity titration, but, on a wilt-susceptible burley cultivar, the isolate from Burma Valley was more virulent.
The effects of planting date and topping (removal of the apical bud) on the development of bacterial wilt of tobacco in a naturally infected field over two seasons, 1995/96 and 1996/97, were investigated. Infection was greater in the November-planted tobacco for the two cultivars BAG 3-11 and RK 3, (75 % and 21 % infection (1995/96) and 83 % and 51 % infection (1996/97)) than September-planted tobacco (34 % and 17 % infection (1995/96) and 38 % and 11/40 % infection (1996/97)), respectively. The time when the first symptoms of bacterial wilt infection were observed differed in the two seasons. Bacterial wilt occurred 18 days earlier and disease severity was greater for both planting dates in the 1996/97 season. This could be attributed to the greater rainfall which fell in the second season (972.5 mm vs. 701.5 mm). The effect of topping on disease development was both cultivar- and season-dependent.
A calibration curve (y = -0.0149 + 0.875/1 + e(-0977*[x-8.860]); r2 = 0. 973) of the absorbance at 450 nm plotted against populations of R. solanacearum detected by an ELISA technique was used to estimate the population of the bacterium in the soil to a depth of 105 cm. The minimum population that could be detected using this technique was 105 colony forming units (cfu) g-1 of soil. R. solanacearum was first detected (at 106 cfu g-1 of soil) in the top 30 cm of a naturally infected soil two weeks before wilt symptoms developed in the plant. Disease development depended on increasing soil moisture and minimum air temperatures above a critical minimum of 15OC. As disease severity increased, so did the numbers of the bacterium detected in the soil to a depth of 105 cm. Concomitantly, with maximum disease incidence the number of R. solanacearum peaked at 107 cfu g-1of soil, then, as the infected plants died, the population declined to undetectable levels. The bacterium was detected six weeks later in the top 30 cm of soil (c. 106 cfu g-1 of soil) after the infected tobacco plants were disked into the field. Bacterial numbers declined to undetectable levels as the plant debris decomposed over a period of four months.
In summary, only race 1 biovar 1 of R. solanacearum infects tobacco in Zimbabwe. Disease severity can be significantly reduced by planting tobacco in September/October when environmental conditions are less conducive to disease development. Numbers of the bacterium in the soil increase with wilt severity and then decrease to undetectable levels with the death of the host. Infected crop residues aid the survival of the bacterium between crops.
Inorganic Fertilizer Management to Improve Dryland Maize (Zea mays L.) Grain Yield for the Smallholder Farming Sector in Chinyika Resettlement Area, Zimbabwe - Munyaradzi Jonga (February 1998)
Supervisors: Dr I.K. Mariga and Dr O.A. Chivinge
On-farm experiments to investigate the effects of inorganic fertiliser application method, time and rate on maize (Zea mays L.) grain yield were conducted in Chinyika Resettlement Area (CRA) and at Domboshava Training Centre (DTC) in the 1994/95 and 1995/96 rainy seasons. The soils were sandy, typical of Zimbabwean communal areas. A monitoring exercise to gather information on fertilizer management by farmers in CRA was also carried out in the two seasons.
Experiment I tested the effect of basal fertilizer (compound D: 8 % N: 14 % P2O5: 7 % K2O: 6.5 % S) application method and rate on maize yield. Three rates of compound D (150, 300 and 450 kg/ha) were applied using three methods: dolloping at planting (station placement), dribbling in the planting furrow and dolloping on soil surface two weeks after crop emergence (WACE). There were significant (P<0.05) grain yield differences due to basal fertilizer application method during the two seasons. Dolloping at planting achieved a similar yield to the farmer practice of dolloping on soil surface 2 WACE. Dolloping at planting and 2 WACE performed better than dribbling where moisture was not limiting. Dribbling outyielded dolloping at planting and on soil surface 2 WACE by 375 kg/ha and 348 kg/ha of grain, respectively, under moisture stress conditions during planting and after crop emergence. Although dolloping was labour intensive, it resulted in higher net benefits than dribbling. In the drier 1994/95, season basal fertilizer application rate resulted in similar yields at sites 1 and 2. A combined analysis over sites 3 and 4 showed that an application of 150 kg/ha of compound D achieved a similar yield to 300 kg/ha. Both rates were significantly (P<0.05) outyielded by the yield at 450 kg/ha of compound D. In the wet 1995/96 season, application of 300 kg/ha of compound D yielded 6779 kg/ha and was similar to the yield achieved using 450 kg/ha. Application of 150 kg/ha achieved the lowest yield of 5843 kg/ha. In the wetter 1995/96 season, 300 kg/ha had the highest net benefit of ZW$ 4963.00/ha.
Experiment II evaluated the effect of time and method of application of topdressing fertilizer (ammonium nitrate 34.5% N) on maize grain yield. Ammonium nitrate (AN) at 250 kg/ha was applied at 5 WACE or at initiation of tasselling (IT) either by dolloping or dribbling along the crop row in the 1994/95 rainy season. However, in the 1995/96 season, a third application time was included whereby half of the AN was topdressed at 5 WACE and half at IT. In both seasons dolloping and dribbling achieved similar yields, but dolloping required more labour than dribbling. The application of AN at 5 WACE significantly (P<0.05) outyielded that applied at IT by 320 kg/ha of maize grain in the drier 1994/95 season. In this season (1994/95), the application of AN at IT coincided with the time when rains were tapering. However, the two application times achieved similar yields during the wetter 1995/96 season. Results of the second season suggested that splitting AN can result in a higher maize grain yield than a non-split application and may justify the increased labour cost of splitting the application on this type of soil.
The third experiment that focused on the management of both basal and topdressing fertilizers was conducted at DTC during both seasons. The experiment had two basal fertilizer rates (150 and 300 kg/ha of compound D), three application times (at planting, 2 and 4 WACE) and three AN topdressing times (5 WACE, IT and splitting - half at 5 WACE plus half at IT). The rate of AN applied was 250 kg/ha. In the drier 1994/95 season, there were no significant (P<0.05) treatment effects on maize yield. This was attributed to the lower and poorly distributed rainfall. In the wetter 1995/96 season, the basal fertilizer applied at planting or at 2 WACE gave similar maize yields, and both outyielded that applied at 4 WACE. There were significant (P<0.05) interaction effects between the basal fertilizer rate and AN application time. Application of 300 kg/ha of compound D resulted in higher grain yield when AN was applied at IT. An economic analysis indicated that the lower rate of compound D was economically viable and that a split application of AN was uneconomic because of the higher costs. These results, which are contrary to those obtained in experiment II, are attributed to the higher inherent soil fertility of the site than is the case for most of the smallholder (SH) sector soils.
The results obtained from experiment II and III indicated that the effects of AN on maize grain yield can be variable depending on climatic conditions, time and method of application. However, because the response of maize to AN is partly dependent on moisture availability, the split application method offers a way of reducing wastage of AN fertilizer in drier seasons by withholding the second application when moisture conditions during tasselling become non-conducive to maize plant growth. This calls for flexibility in N management.
The monitoring exercise showed that farmers in CRA applied lower quantities of fertilizer than is recommended. The impact of the previous season drought on the smallholder (SH) farmers' cash income was also reflected in the further reductions of fertilizer use in the 1995/96 season.
Preliminary Investigations into Runner Bean Phaseolus coccineus L. Production Aspects in Zimbabwe - Tapiwanashe Ruhode (April 1998)
Supervisors: Mrs B.V. Maasdorp and Dr A.I. Robertson
Trials on runner bean (Phaseolus coccineus L.) production were carried out on the highveld (1400 m above sea level) of Zimbabwe from 1991 to 1993. The trials were conducted in order to improve runner bean yield and quality for the export market. The overall objective was to define cost effective management and agronomic techniques to enable the production of runner beans of high quality all year round. The factors which were investigated include cultivars, detrussing, plant population, shade, mist, photoperiod extension and its light intensity, side dressing fertilizer, commercial feedlot manure and soil moisture stress.
The export yields of the red-seeded cultivars Enorma, Lady Di and Streamline were not significantly different but they were greater than that of White Emergo. These red-seeded cultivars could be better for production during the hot and dry season. There were no quality differences between the four cultivars.
The removal of later-formed flowers from runner beans was shown to be an ineffective management practice for improving pod setting. There was no yield response to increasing the plant density from 40 000 to 80 000 plants per hectare.
Runner beans are quantitative long day plants. Under a sub-tropical environment, they require artificial day length extension for production. There was benefit in artificially extending the day length by more than two hours beyond a 14-hour day length. The light intensity of the artificially extended photoperiod affected (P<0.05) the yield of runner beans. More beans were produced under a light intensity of 20 to 60 lux as compared to a lower light intensity. The response of runner beans to additions of manure and inorganic fertilizer was affected (P<0.05) by the intensity of supplementary light.
The use of shade and mist in the hot and dry season did not improve the amount or quality of green beans produced. Under the shaded environment this was due to increases in temperatures above ambient.
That too much fertilizer is used in the production of beans on commercial farms was shown by high levels of residual fertility that could sustain a runner bean crop without any additional fertilizer. The use of nitrate fertilizer in the form of ammonium nitrate and potassium nitrate was shown to be more effective in improving runner bean than ammonium fertilizer alone (P<0.05).
The use of manure as a soil amendment in runner bean production enhanced the utilization of residual fertility. Commercial feedlot manure increased the amount of biomass produced (P<0.05). More leaf, stem and total aerial dry matter was produced when manure was used as a soil amendment.
Drought stress due to reduced application of irrigation water reduced the amount of green beans produced (P<0.05). Soil moisture depletion of 75 percent of available water capacity was rather too high for runner bean production since it led to flower and pod abortion. Drought stress from the first trifoliate leaf stage should be avoided.
The Effect of Different Cowpea (Vigna unguiculata (L.) Walp) Cultivars and Time of Ridging on Witchweed (Striga asiatica (L.) Kuntze) Management in the Smallholder Farming Sector of Zimbabwe - Elbert Kasembe (April 1999)
Supervisors: Prof. O.A. Chivinge, Mr. S. Mabasa and Dr. I.K. Mariga
A green house experiment (experiment 1), whose objective was to screen several cowpea cultivars for S. asiatica germination, was carried out at Henderson Research Station. Cowpea cultivar ITK-4-5-6-1-5 caused the highest germination while most cultivars caused germination percentages lower than 5. On-farm experiments were conducted in Chinyika Resettlement Area (Natural regions IIb and III) during the 1996/97 and 1997/98 rainy seasons to determine the effectiveness of different Striga asiatica control technologies in the smallholder farming sector.
Experiment II compared the effectiveness of six cowpea cultivars to suppress S. asiatica in maize/cowpea intercropping system. All the cowpea cultivars significantly reduced the number of emerged S. asiatica plants in the first season with ITK-76 and IT82D-849 being the most effective. However, the effects of these treatments were not apparent during the 1997/98 rainy season as a result of the long dry spells experienced.
In experiment III, the effect of four cowpea population densities on S. asiatica in a maize/cowpea intercropping was investigated. The high population densities of 74 074 and 55 556 cowpea plants ha-1 were the most effective. They however, resulted in lower maize yields (averaging 2.5 t ha-1). The 44 444 cowpea plants ha-1 treatment caused maize yields of above 3 t ha-1.
The effect of time of ridging on S. asiatica emergence and maize grain yield was investigated in experiment IV. Ridging at three and five weeks after crop emergence (WACE) significantly (P<0.05) reduced S. asiatica incidence compared to planting on the flat and ridging at planting. These two treatments also resulted in the highest maize grain yield averaging 4.5 t ha-1 in the 1996/97 rainy season and 2.5 t ha-1 in 1997/98.
From the results of the study, S. asiatica management can be achieved using low cost technologies as outlined above. However, it is recommended that further work be done to ascertain whether the technologies generated in CRA can be applied to other agro-ecological zones. Investigations should also be conducted to determine the optimum density of each of the promising cowpea cultivars. Other areas that also need attention include the effect of delaying the sowing of either intercrop, the use of multipurpose trees and the effects of proceeding catch and trap crops.
Gene Action Determining Maize Weevil (Sitophilus zeamais Motsch.) Resistance in Maize Hybrids - John Derera (September 1999)
Resistance of maize (Zea mays L.) to maize weevil (Sitophilus zeamais Motsch.) was long ago identified as an ideal component of integrated control for use in rural storage, but progress towards finding and using such resistance has been limited. This research was conducted to study gene action determining the inheritance of non-preference and antibiosis resistance of maize weevil in maize hybrids. Knowledge of gene action determining inheritance of maize weevil resistance will be helpful in designing breeding strategies for developing maize with improved resistance.
The study was initiated by forming experimental hybrids among 18 maize inbred lines, six each from Southern Africa, Mexico and CIMMYT-Zimbabwe. Lines were crossed in sets according to a North Carolina Design II mating scheme to form 54 hybrids, at the Agricultural and Rural Development Authority (ARDA) Estate in Muzarabani, during winter of 1996. Hybrids were advanced to F2 generation by full-sib mating at the University of Zimbabwe farm, near Harare, during summer of 1996/97. The F1 and F2 hybrids were advanced for resistance to maize in free- and no-choice tests to measure the levels of non-preference and antibiosis effects, respectively. A local commercial hybrid was used as a susceptible control, while a Mexican composite Oaxacal79 was the resistant check in the weevil-resistance tests as standard checks. Main samples of the grain were frozen at –20 OC for 14 days prior to initiating experiments, in order to kill any insects that may have infected the grain in the field, and to destroy any eggs that have been laid in the grain.
Non-preference tests were conducted at 28±2 OC and 70±5 % relative humidity, using four replications, for 70 days. Fifty-gram samples of seed of each hybrid, in perforated paper envelopes were randomly placed on a bulk of susceptible grain, which was previously infested with 500 weevils. The study showed that weevils preferred some hybrids for feeding as hybrids differed significantly (P<0.01) for grain weight loss. Grain weight loss ranged from 0.85 to 8.29 %. Four percent of the hybrids, which incurred less than two percent grain weight loss (significant at P<0.05) less than the susceptible check) were classified as resistant. Weevil preferred the susceptible check (6.62 % loss) and the local commercial hybrid (6.02 to 6.80 % loss) to the resistant check (3.5 % loss). Male and female additive effects showed similar importance for determining non-preference of grain by weevil, indicating that both parents were contributing equal levels to resistance in F2 hybrids. Non-additive effects were also significant for non-preference of grain by weevil. Total additive (47 % of genotypic sum of squares) and non-additive (53 % of genotypic sum of square) effects were of similar importance in explaining inheritance of non-preference in F2 generation hybrids.
Tests of antibiosis of grain to weevil were conducted at 28±2 OC and 70±5 % relative humidity, using four replications, for 45 days. Fifty-gram samples of each hybrid were infested with 32 weevils for 10 days, and the emerging F1 progeny were counted at 2-day intervals. A Dobie (1974; 1977) index of susceptibility which measures the survivorship of weevils in the grain of the hybrids was calculated for each hybrid, and expressed as a ratio of index of susceptibility of SR52 (Relative Index of susceptibility). The index of susceptibility data was significantly (P<0.05) correlated with the adult weevil mortality, grain weight loss, number of progeny emerged and the median development period of the progeny in both F1 and F2 hybrids. Results showed that some F1 hybrids (5x3, 5x1 and 8x1) and F2 hybrid (6x3) had detrimental effects on weevil biology and fecundity as expressed through high parent weevil mortality rates, long progeny development period, few progeny emerged, low grain weight loss percentage and the small indices of susceptibility. Adult weevil mortality ranged from 0.8 to 169 in F1, and from zero to 42 % in F2 hybrids. The total number of progeny that emerged from the grain sample during the 45-day incubation period ranged from 0.8 to 169 in F1, and between 18 and 188 in F2 hybrids. Weevil median development period ranged from 29 to 44 days in F1, but was similar for all F2 hybrids. Relative susceptibility index ranged from 0.3 to 11.1 in F1 and between 4.4 to 10.6 in F2 hybrids, whereas grain weight loss ranged from 0 to 18% in F1 and from 0.13 to 13 % in F2 hybrids. The resistance characters were significantly (P<0.05) correlated, hence hybrids were classified based on relative index of susceptibility data. Hybrids that had a relative index of susceptibility less than four, and less than the susceptible check (relative index = 10.0, by definition were classified as resistant, while those with a relative index of susceptibility greater than eight and similar to SR52 were classified susceptible. Four percent of the hybrids were resistant in F1, but none were resistant in F2. The local commercial hybrids were susceptible to maize weevil. Female parents accounted for the greater proportion of additive effects for grain weight loss and index of susceptibility, indicating the importance of maternal effects for antibiosis in both the F1 and F2 hybrids. Non-additive effects were significant (P<0.01) for grain weight loss and index of susceptibility, indicating importance of dominance gene action for determining antibiosis in both the F1 and F2 hybrids.
F2 hybrids differed significantly (P<0.01) for grain weight loss during two to five months under simulated storage. Final grain weight loss ranged from 19 to 57 %. The hybrids which incurred grain weight loss than 25 % and less than the susceptible check were classified resistant, while those that incurred grain loss similar to SR52 (>35 %) were classified as susceptible. Eight percent of the hybrids were classified as resistant. Male and female additive and non-additive effects were important for determining weevil resistance during two to three months, but only male additive effects were significant throughout the study. This was difficult to explain, because maternal effects were very important during a 45-day incubation period in the laboratory.
Correlation of hybrid ranks for resistance between laboratory based and simulated storage tests was weak, indicating that laboratory findings should be validated under field conditions. However, the set of hybrids formed between resistant lines was ranked first in all tests, while local hybrids were consistently susceptible in both the laboratory and simulated storage tests. Resistance was not correlated with grain yield, seed size and grain texture in all tests, hence these factors could not be used to explain variation among hybrids. The importance of additive and non-additive gene action for determining both non-preference and antibiosis resistance, suggested that recurrent selection breeding approaches, and hybridization, respectively, can be used to develop weevil-resistant maize. However, importance of maternal effects in addition to additive and non-additive gene action for antibiosis implied that development of weevil resistant-maize would be a challenging task.
Effect Of Spatial Arrangement, Plant Density, Maize and Bean Genotype on Maize/Bean Intercrop Performance - Angeline Mutungamiri (November 1999)
Supervisors: Dr. I.K. Mariga and Prof. O.A. Chivinge
On-farm trials to investigate the effect of spatial arrangement, plant density, and bean and maize cultivar on maize/bean intercrop performance were conducted in Chinyika Resettlement Area (CRA) and at Domboshava Training Centre (DTC) during the 1996/97 and 1997/98 rainy seasons. Farmer evaluation of the trials was conducted during the two seasons.
Maize density, bean cultivar and bean spatial arrangements were evaluated on the performance of the maize/bean intercrop. Two maize densities (90 cm x 30 cm and 90 cm x 45 cm), two bean cultivars (Natal Sugar and Carioca, both of type IIb indeterminate bushy growth habit) at three spatial arrangements (one and two bean rows in between maize rows, and beans planted in the same rows with maize) were used. Maize density, bean cultivar, spatial arrangement and their interactions significantly (P<0.01) affected maize and bean grain yields in the two seasons. Changing the maize density of 90 cm x 45 cm to 90 cm x 30 cm increased maize yields by 28 and 39 percent in the respective seasons but this reduced bean yields by 11 and 18 percent in the two seasons. Intercropping maize with Natal Sugar reduced maize yields more than Carioca. Natal Sugar had higher yields than Carioca in the 1996/97 season but Carioca out-yielded Natal Sugar in the 1997/98 season. Two rows of beans in between maize rows significantly (P<0.01) increased bean yields but reduced maize yields. Generally, all mixtures, except most of those at DTC in the 1996/97 season, proved beneficial by producing land equivalent ratio (LER) values greater than 1. In the 1996/97 season, the maize cultivar SC501 at 90 cm x 30 cm with Carioca in the same row gave the highest return of $12 469/ha and SC501 at 90 cm x 30 cm with two rows of Carioca produced the highest return of $20 895/ha in the 1997/98 season. The bean cultivar Carioca planted in the same row as maize and at the currently recommended maize spacing of 90 cm x 30 cm is the most ideal approach to dryland maize/bean intercropping in NR II of Zimbabwe since it achieved high yields and allows easy weeding. For non-cattle owners and those who would want to carry out maize/bean intercropping on a small area the spatial arrangement of two bean rows in between can be recommended to them.
Four maize cultivars (SC401, SC501, SC601and ZMS607) at two population densities (90 cm x 30 cm and 90 cm x 45 cm) were tested on maize/bean performance. Maize and beans were simultaneously planted in the same row. Maize density, cultivar and their interactions significantly (P<0.01) affected maize and bean grain yields in both seasons. Reduction of maize plant population from 37 000 to 24 000 plants per hectare reduced maize yields but increased bean yields. During both seasons, the presence of SC601 resulted in the highest bean yields of 508 and 623 kg/ha in the respective seasons. In both seasons intercropped SC501 and SC601 at 90 cm x 30 cm and SC601 at 90 cm x 45 cm produced the highest gross outputs.
Farmers liked the spatial arrangement of two bean rows in between maize rows which was attractive in terms of producing higher bean yields, but indicated that they preferred the same-row arrangement as it facilitates easier weeding using an ox-drawn cultivator.
Cattle Manure and Inorganic Fertilizer Management for Dryland Maize Production in the Smallholder Sector of Zimbabwe -Tarisai H. Mubonderi (November 1999)
Supervisors: Dr. I.K. Mariga, Prof. O.A. Chivinge and Dr. L. Mugwira
Field experiments to determine the optimum rates of cattle manure supplemented with inorganic nitrogen and the effect of manure application method and rate on maize yield, maize dry matter yield and weed density and biomass were carried out in Chinyika Resettlement Area (CRA), at Chinyudze and Bingaguru, and Domboshava Training Centre (DTC) in the 1996/97 and 1997/98 seasons. Residual effects of manure application method and rate on maize grain yield were also determined in the 1997/98 season.
In experiment I four rates of manure (5, 10, 20, 30 t/ha) were applied using two methods - broadcasting and banding. These treatments were laid out as a 4 x 2 factorial in a randomized complete block design with three replicates. Other treatments included were the extension recommendation, farmer practice, control and no basal fertilizer application to make a total of twelve treatments in this experiment. Maize plant samples were taken randomly at five, ten, and fifteen weeks after crop emergence (WACE) for dry matter yield at DTC. Weed numbers and biomass were determined along maize rows. Residual effects of manure on maize grain were tested at Chinyudze.
In experiment 2, three rates of manure (0, 10 and 20 t/ha) were band applied at planting and supplemented with inorganic nitrogen at 5 WACE. The inorganic nitrogen was applied at three rates (0, 50, 100 kg/ha). Three other treatments were included in this experiment. These were the application of 300 kg/ha Compound D supplemented with the three rates of inorganic nitrogen. Experimental data were analyzed using standard analysis of variance (ANOVA) procedures and combined across site where error variances were homogeneous. Grain yield data were also subjected to economic analysis using the procedures of CIMMYT (1988). Experiment 1 was evaluated by farmers at physiological maturity.
In experiment I, there were no significant effects of manure application method and rate on maize dry matter yield at 5 WACE in both seasons although the control had the lowest dry matter yield. At 10 WACE in the 1996/97 season banding resulted in significant higher maize dry matter yield than broadcasting (p<0.05). There was also a significant (p<0.01) increase in dry matter yield with manure application rate. At 15 WACE there were no significant effects of manure application method on maize dry matter yield in both seasons. Maize grain yields increased significantly (p<0.001) with an increase in manure application rate in both seasons. Band application of manure resulted in higher maize grain yield than broadcasting at all manure application rates in both seasons. The highest grain yield was obtained from band applications of 30 t/ha manure. The extension recommendation achieved a grain yield of 4.53 t/ha and 1.60 t/ha in the 1996/97 and 1997/98 season respectively while the farmers' practice achieved grain yields of 3.31 t ha-1 and 1.37 t/ha respectively. The control had the lowest grain yield in both seasons (0.94 t/ha and 0.72 t/ha respectively). Band applications of 30 t/ha manure resulted in the highest net benefit in both seasons. Most farmers (85%) said that they liked the performance of the crop that had received 30 t ha-1 manure banded as it had the biggest maize cobs. Economic analysis indicated that 10 t/ha banded was the most economical manure application rate in both seasons. The method of manure application did not significantly affect maize grain yield one year after manure application although grain yield increased significantly (p<0.001) with an increase in manure application rate.
In experiment 2, maize grain yield increased rates of manure and supplementary inorganic nitrogen in both seasons. There was no interaction between manure rate and inorganic nitrogen in both seasons. The highest grain yields were obtained at applications of 20 t/ha manure supplemented with 100 kg/ha N at both sites in the 1996/97 season but this was not significantly different from Compound D supplemented with 100 kg/ha N at DTC only. In the 1997/98 season Compound D supplemented with 100 kg/ha N was significantly superior to an application of 20 t/ha manure supplemented with 100 kg/ha N at DTC. Grain yield obtained from the application of 10 t/ha manure supplemented with 50 kg/ha N was not significantly different from the application of 20 t/ha manure supplemented with 50 kg/ha N in both seasons. At a supplementary nitrogen rate of 100 kg/ha N, the application of 10 t/ha manure was more economical than the application of 20 t/ha manure or Compound D fertilizer.
There were four major weed species identified in the experimental plots. These were Galinsorga parviflora, Nicandra physalodes, Richardia scabra and Eleusine indica. The method of manure application significantly (p<0.05) affected the mean number/m2 of G. parviflora. Banding increased G. parviflora mean number/m2 by 55 % over broadcasting. E. indica had the highest mean number/m2. Manure application rate significantly (p<0.01) influenced mean number/m2 of N. physalodes. The method and rate of manure application did not significantly affect mean number/m2 of R. scabra and E. indica. The method of manure application did not significantly affect weed biomass but the rate of manure application significantly influenced N. physalodes biomass. Application of 30 t/ha manure increased N. physalodes biomass by 300% over the application of 5 t/ha manure.
Influence of Low Soil and Air Temperatures on Yield and Yield Components of Cowpea (Vigna unguiculata (L.) Walp) - Kebadire K. Mogotsi (June 1995)
Supervisor: Dr A.I. Robertson
In September 1989, one hundred and thirty-three cowpea genotypes supplied by the International Institute of Tropical Agriculture (IITA) and Botswana Cowpea Improvement Project were planted at the University of Zimbabwe's Department of Crop Science fields. Eighty percent of these were found to be either susceptible to viruses, or late maturing or both.
Twenty-seven genotypes were selected and subjected to low temperatures in 1990. Eighty nine percent were found to be susceptible to low temperature at the early vegetative stage. Low air temperature sometimes delayed but did not inhibit cowpea seedling emergence, but it did prolong days from planting to 50% flowering.
The lowest average field air temperatures to which the genotypes were exposed in both 1990 and 1991 at Gwebi, Harare and Kadoma was 2OC and the lowest day/night temperatures under controlled environment were 19O/11OC.
Three genotypes, B038, B242 and IT86D-633, were found to be low-temperature tolerant at seedling stage. Plant dry matter production, number of peduncles per plant and number of pods per plant were greater under cooler environments or planting times. This extension of the growing season to include colder periods in low-temperature tolerant types resulted in increased yields compared with the customary planting times.
Seed yields were greater when plants matured under warmer temperatures and longer days and were generally higher than those reported by earlier investigators. Growth chamber experiments indicated that the early stages of growth and development were more sensitive to low day/night temperatures than were the later stages. However, low temperature applied at later stages produced symptoms comparable to nitrogen deficiency.
It was established, therefore, that the three selected cowpea genotypes B038, B242 and IT86D-633 were able to either withstand low-temperature stress or repair the injury caused by the stress at seedling emergence and early stages of growth and development. Each type grew to maturity, achieving at times record yields.
It is concluded that it is possible to grow cowpeas in winter in regions that experience extremes of temperatures, given the availability of soil moisture. Secondly, cowpea research strategies could now be geared towards the cooler environments that may create increased opportunities for cowpea utilization and production by small scale farmers.
The Effect of Cotton-sorghum Rotations on Verticillium dahliae Soil Population Levels And Cotton Infection Levels in Zimbabwe - Reward Chinodya (August 1995)
Supervisors: Dr D.L. Cole and Dr K. Leushner
In a study of the rooting patterns of nine cultivars of sorghum it was observed that the very tall forage sorghums had more and heavier roots than the open-pollinated cultivars that in turn had more and heavier roots than hybrids. In vitro studies on the stimulation of germination of Verticillium dahliae microsclerotia by sorghum indicated that activity seemed to be in direct proportion to the quantity of roots produced and the proximity of those roots to microsclerotia.
Sorghum induced microsclerotia to germinate into conidia, unlike cotton and water, which encouraged hyphal production, and thus reduced the number of microsclerotia as they perished and the remainder of the microsclerotium was susceptible to desiccation, once the nutrients in the conidia were exhausted.
In vitro studies on the effect of a forage sorghum, an open-pollinated sorghum cultivar and a sorghum hybrid on microsclerotial germination by microscopically observing the number of germinating microsclerotia showed that hybrids appeared to compensate for sparsity of roots by having a greater Germination Induction Index (GII).
The exudates produced by the sorghum cultivars were not water-soluble. By the extraction methods used, the Germination Induction Factor (GIF) was the same for different sorghum cultivars but cotton did not possess a GIF extractable either in water or any of the solvents used. From IR spectrophotometer scans the GIF and its precursor were highly likely to be aromatic ketones and had peak absorbance at 209 nm on the U.V. spectrophotometer. Although they all had the same GIF, the heights of the peaks indicated that not all the sorghums produced the same amount of the GIF and the cultivar LARSVYT 19 produced most.
Forage sorghum, cut forage sorghum and sorghum hybrid were grown in the field and the number of microsclerotia per gram of soil after one year's lay off from cotton was examined with the aid of an Andersen Air Sampler. There was a considerable (31 to 52 %) reduction of microsclerotia but no significant reduction of the number of infected cotton plants except for the forage sorghum treatment at one site (Glendale). Correlation coefficients between soil microsclerotial counts and wilt infection levels ranged from -0.373 to +0.287 but were not significant.
The Manipulation of Somatic Embryogenesis in Selected African Cultivars of Coffea arabica L. and its Use in Rapid Multiplication and Transformation - Munyaradzi V. Masona (December 1998)
Supervisor: Dr A.I. Robertson
Both direct and indirect somatic embryogenesis were induced on field-derived explants as well as explants obtained from in vitro somatic embryo-derived plants of coffee (Coffea arabica L.). The growth regulators used in the different Murashige and Skoog (M&S)-based culture media were 3-indolebutyric acid (IBA) in combination with 2, isopentenyl adenine (2, ip) or benzyl amino purine (BAP) and varying levels of 2,4-dichlorophenoxy acetic acid (2,4-D) combined with varying levels of kinetin. The combination of 2,4-D and kinetin (0.5 and 4 mg l-1 respectively) gave the highest somatic embryo numbers per explant. A novel creamish friable embryogenic callus (termed Type II) was discovered on some explants treated with 2,4-D and kinetin. Similar embryogenic callus was also observed on somatic embryos derived from liquid cultures. Preliminary experiments to establish conditions for germination of coffee somatic embryos were carried out using gibberellic acid (GA3), IBA, naphthalene acetic acid (NAA), kinetin, BAP and abscisic acid (ABA) in M&S medium. Synthetic cytokinin levels from 0.5 to 2 mg l-1 in M&S medium induced cotyledon greening and expansion. These developments were the first steps of somatic embryo germination. ABA was found to be useful in the synchronization of somatic embryo development.
Liquid cultures of both leaf explants and embryogenic callus were used to increase multiplication rates. Somatic embryo conversion to plants was obtained at rates ranging from 10 to 80% depending on the liquid culture medium.
Somatic embryos from liquid media containing auxins like IBA or NAA had a low conversion frequency to plants, but gave rise to Type II callus when they turned brown. Type II callus was found to have spherical densely cytoplasmic cells. The manipulation of Type II callus using 2,4-D, ABA and activated charcoal led to a novel cyclic somatic embryogenesis system for coffee. The cyclic somatic embryogenesis involved the development of somatic embryos for Type II callus followed by the appearance of more Type II callus on the embryos. This created a callus-embryo-callus cycle that resulted in the continuous production of somatic embryos.
Protoplasts were isolated from cell suspensions derived from Type II embryogenic callus using a variety of hydrolytic enzymes. The liberation of protoplasts was evident within six hours and this was independent of the enzyme mixture. The protoplasts were viable and could be cultured to the microcalli stage in M&S-based liquid medium. Type II callus, therefore, provided a suitable source of protoplasts for any future work involving the direct uptake of genes into coffee protoplasts. The plasmid pBI 121 and its derivatives were used in LBA 4404 or C58 strains of Agrobacterium tumefaciens for transformation. GUS-expressing calli were obtained on leaf explants after co-cultivation with A. tumefaciens after two months of culture on selection medium with kanamycin sulphate. Somatic embryogenesis could not be induced on these GUS-expressing calli. This was a problem in that the regeneration of transgenic coffee depends on the production of somatic embryos from transformed calli.
Studies on Aspects of the Biology and Control of Araecerus fasciculatus (Degeer) (Coleoptera: Anthribidae) on Dried Sweet Potato Chips - James A. Agona (November 1998)
Supervisor: Prof. D.P. Giga
The food security and income generation status of sweet potato in the eastern and northern districts of Uganda cannot be over emphasised. Production in unimodal rainfall areas in Uganda is limited, however, to only one season in a year, and the crop once mature cannot store in the ground without deteriorating. Farmers are therefore forced to process the mature roots into dried chips for long term storage. A baseline survey was conducted to establish the post-harvest practices and constraints affecting dried sweet potato chip processing, storage and utilization in Kumi district, Uganda. A total of thirty-six households were surveyed. Survey results suggest that the food security status of dried sweet potato chips is under threat. Insect pests were identified as the major loss causative agents of dried chips in storage and six species viz. Araecerus fasciculatus, Rhyzopertha dominica, Dinoderus minutus, Sitophilus zeamais, Tribolium castaneum and Lasioderma serricorne in order of importance, were identified. A. fasciculatus was identified as the most damaging pest of dried chips at smallholder subsistence farming systems in Uganda. Information on the biology and control methods of the pest on dried sweet potato chips is, however, lacking. Farmers' traditional methods of pest management that included regular inspection, re-drying, opening of the granary roof, and sorting and removal of infested chips were considered insufficient for controlling and maintaining stored chip quality.
Investigations on the biology of A. fasciculatus showed that it is a major pest of dried sweet potato chips, and both adults and larvae feed on them. The adults, however, do not feed on sweet potato flour. All the larval stages feed on flour and successfully develop into pupae and adults. The larvae are capable of re-infesting whole dried chips and developing into pupae and adults. Larval development is more favoured on sweet potato flour than on chips, especially when not disturbed, and this was shown by reduced larval development periods (36.7 + 0.6 days), reduced generation times (50.4 + 0.9 days) and all larvae developing into adults (100 %). Growth is prolonged when larvae are disturbed during growth. Larval disturbance resulted into delayed larval development periods (73.0 + 2.4 days), increased development time into adults (86.0 + 2.5 days) and high mortality (85 %). The mean development period of A. fasciculatus on whole chops from egg to adult was 63.6 + 1.1 days under ambient temperature and relative humidity conditions of about 26 OC and 70 %, respectively. The ability of the larvae to survive on sweet potato flour implies that flour is important in sustaining carry-over residual populations, while physical agitation of developing larvae could reduce subsequent chip damage. The sex ratio of A. fasciculatus on dried sweet potato chips is unitary.
There was a significant reduction in damage by A. fasciculatus when the sweet potato chips were either salted, parboiled or solarized. Total mortality of the pest, low adult emergence, and increased generation time and/or reduced weights of emergent adults, depending on the treatment applied, demonstrated the effectiveness of these control methods. Salting at 2 to 3 % w/w, parboiling for five minutes and solarization for three hours of chips are recommended for on-farm adoption. On-farm trial results showed that the three methods are effective against the insect pest spectrum of dried sweet potato chips, with parboiling and salting methods being the most effective. Furthermore, the consumer-acceptability quality-test results showed that parboiled and salted chips had better organoleptic qualities than solarized chips.
Varietal screening for resistance against A. fasciculatus showed that all the varieties screened were susceptible, but to varying degrees. Differences existed in time required for the pest to complete its life cycle on the different varieties. The susceptibility levels of the varieties varied significantly and variety Mbiyombiyo had the highest Susceptibility Index (SI) (4.00 + 0.65), while variety Haraka had the lowest SI (1.83 + 0.13). The dry matter content (DMC) of the different varieties screened was significantly different, but the relationship between DMC and SI was weak and insignificant. A wide range of sweet potato germplasm is recommended for screening in order to identify resistant varieties and isolate specific mechanisms for resistance.
Five damage-class categories due to A. fasciculatus infestation were established based on the number of adult emergence holes and chip weight reduction. Class 1 chips were not damaged and Class 5 the most severely damaged. The severity of infestation of the different class categories was 0, 0.06, 0.23, 0.60 and 0.74 holes cm-2 of chips, which corresponded to weight losses of 0, 17.6, 24.6, 60.5 and 86.2 % in class categories 1, 2, 3, 4, and 5, respectively. The end-uses of each class category were defined to help small-scale Ugandan farmers in decision-making.
Finally, it is envisaged that the importance of dried sweet potato chips as a form of food security will be significantly increased at smallholder agriculture once the IPM strategies developed in this study are implemented on-farm.