Maize damage has been observed from time to time in many parts of South Africa where registered herbicides were applied. Differential cultivar tolerance to certain herbicides was identified as an important factor in many cases where herbicide selectivity was inadequate. A study was done to investigate the existence of differential tolerance of maize genotypes to selected herbicides. Several maize genotypes were screened for tolerance to selected herbicides, both in the greenhouse and in the field. Maize genotypes demonstrated significant differences in their tolerance to all herbicides. Some genotypes were severely injured by certain herbicides while others were not affected at all by the same herbicide. This suggests that maize genotypes should be screened for tolerance to herbicides in order to reduce crop injury by using only appropriate ones for specific genotypes. Generally, hybrids were more tolerant to herbicides than inbreds, indicating that screening may be most necessary in seed production, where inbreds are used, than in commercial production where only hybrids are used. Tolerance of maize to herbicides was more variable to metazachlor than to other herbicides, and metazachlor also injured more genotypes than other herbicides. This was probably due to differences in herbicide mechanism of action. Shoot or root dry mass reduction of some of the maize genotypes occurred without visual injury symptoms, thus suggesting that visual injury may not reliably indicate susceptibility or tolerance to herbicides. The degree of correspondence of herbicide effects on maize in the greenhouse and the field was determined. Comparison of results from the greenhouse and the field showed that there is positive correlation between herbicide effects in the greenhouse and in the field. There was generally good correspondence of major parameters, such as shoot dry mass and injury symptoms, in the greenhouse and in the field. Similarly these parameters were positively correlated with the grain yield obtained from the field. It appears that shoot dry mass and visual injury symptoms could be good predictors of the yield. This indicates that reliable data could be generated through quicker screening at greenhouse level. A total of four herbicides, metazachlor, dimethenamid, acetochlor and the combination atrazine / metolachlor / terbuthylazine, had significant correlations while only two, flufenacet and acetochlor + atrazine/sulcotrione, had no significant correlations for major parameters with the yield. This indicates that the correlation of data was herbicide-dependent. The influence of temperature on maize tolerance to alachlor, metazachlor and metolachlor was investigated. Results showed that low temperatures reduce the tolerance of maize to these herbicides. This could mean that low temperature may reduce the selectivity of these herbicides. Fluctuating temperature conditions of 10°C at night and 35°C during the day, which are found in some maize producing areas, did not affect maize tolerance to the herbicides. The possibility of improving metazachlor tolerance in maize was also investigated. Evidence provided for possible gene effects on the tolerance of metazachlor indicates that maize tolerance to the herbicide could be improved by crossing tolerant parents. The results suggest that it may be possible to improve metazachlor tolerance by crossing appropriate parent lines with dominant genes for tolerance to metazachlor. Ultrastructural changes in the maize seedling root and shoot cells caused by metazachlor were investigated. In susceptible genotypes root cell nucleoli were found to be abnormally large, empty and more abundant than those in untreated control plants. In susceptible plants the chromatids appeared disorganised in cell nucleoli, and both the nuclear and plasma membranes showed signs of disintegrating. There were more and larger vacuoles in the herbicide-susceptible plants. Leaf cells from the susceptible plants had more empty vacuoles and more chloroplasts with generally disorganised content. The bundle sheath chloroplast membranes were dilated in susceptible plants, and the orientation of the grana was disrupted. In the herbicide-tolerant plants, the ultrastructure was not different from that of all the untreated plants. The established differential tolerance of maize to herbicides necessitates the screening of all genotypes to all registered herbicides in order to recommend specific herbicides for certain maize genotypes. Due to the large number of genotypes that would require screening, techniques that yield reliable data quickly have obvious merit. Pot experiments under controlled conditions, which could be selected to promote herbicide bioactivity, are likely to provide data with which the best possible predictions on the risk of herbicide damage in the field could be made. Based on this requirement, environmental factors that should be considered for greenhouse work are: soil with low adsorptive capacity, soil water content close to the field capacity level, and cool temperatures. When screening for herbicide tolerance, the use of herbicide rates in excess of the recommended rate could obviate the need for special environmental conditions, since all the aforementioned factors basically promote the accumulation of higher than usual amounts of herbicide at the site of action in the plant. Therefore, the use of at least a 2X-herbicide rate in screening experiments is advised. / Thesis (PhD (Plant Production: Weed Science))--University of Pretoria, 2006. / Plant Production and Soil Science / unrestricted

Maize contributes substantially to food security in the Eastern Cape province of South Africa. It is a staple food to many of the province’s rural and urban inhabitants. Insect pests are one of the factors that hamper its productivity and as a result, deprive farmers of good yields. The adverse effects of insecticides and the high cost associated with them and the cost of transgenic seeds are some of the challenges faced by small-scale farmers in rural areas. Alternative control methods which include the use of indigenous techniques to control pests are now sought. A study to identify and document ethnobiological means used by rural farmers to manage insect pests of maize was conducted in the rural areas of the Eastern Cape Province. A total of 217 participants were interviewed on the matter, using semi-structured but detailed questionnaires. Rural farmers due to their linkage to agriculture activities and the fact that they are considered as custodians of agricultural indigenous knowledge were selected as respondents. Only maize producing and IsiXhosa speaking people were chosen to contribute. Main focus was on the demography of respondents, crop production activities and insect pest control. Pretesting of the questionnaire in order to assess the appropriateness of questions and comprehension by both farmers and enumerators was done. Data was analysed using descriptive statistics. Fifty five percent (55 %) of the respondents were females and the highest number of participants was from the Chris Hani District Municipality. Majority of the people were unemployed or pensioners. Most had only attended primary school and the mean age was 59 years. Apart from maize, respondents were cultivating other crops such as cabbage, Swiss chard, potatoes etc. Stalk borers followed by cutworms were the main pests of maize in these areas. Respondents used mainly insecticides, followed by alternative substances, which also included cultural control methods such as planting date manipulation. Few respondents used plants as control agents for insect pests. Some people did not control pests even though they were a problem in their fields. The most used plant was Chenopodium ambrosiodes L, while the most used substance was Madubula (a detergent). The most used insecticide was carbaryl from the carbamite family. Respondents listed different preparation techniques for all the control methods mentioned. These techniques revealed different times of preparation, quantities of ingredients, amounts applied on plants, modes of application and intervals of application. Rural farmers in the study areas used different atypical methods which may play a significant role in pest management today. Some of the products may have a positive influence on agriculture, while some are dangerous to humans and environmental health. Further research which will investigate their potential use in pest control needs to be done.

The common maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), a well known pest of stored-maize grain in most parts of the world, was identified as one of the major constraints of harvested maize grains in the Eastern Cape Province, South Africa. The use of plants or their products is one of the recent methods being investigated for insect pest control worldwide. Therefore, the main aim of the present study was to identify available plants in the Eastern Cape that could be used to combat the problem of Sitophilus zeamais in stored-maize grains. For the realization of the aims of this research, the following studies were carried out; a preliminary survey was conducted to obtain baseline information on the farmers’ knowledge and experience of indigenous insect pest control methods in the Eastern Cape. Also, studies on the insecticidal potential of the essential oils of some plants were investigated against the maize weevil. The quality parameters of maize grains treated with the essential oils was also studied and, using a rat model, the toxicity of the essential oils was investigated. The outcome from this study revealed that there is awareness amongst the farmers in the Eastern Cape on the use of plants or their products to control insect pests. Unfortunately, such methods are currently being neglected and the knowledge of their application was found to be eroding. Among the various essential oils screened were those from Mentha longifolia L. and Tagetes minuta L. which evoked an appreciable level of contact, fumigant and repellent toxicity on the maize weevil. Further work done to determine the effects of the oils on maize stored over a period of three months revealed that the two oils had no adverse effect on the proximate compositions and some quality parameters of the stored maize. However, the toxicological study conducted on rats showed that the oils at tested concentrations exhibited some level of toxicity. It is, therefore, suggested that the essential oils of M. longifolia and T. minuta should not be used to treat maize grains intended for human consumption.

Please read the abstract in the section 00front of this document / Thesis (PhD (Microbiology))--University of Pretoria, 2004. / Microbiology and Plant Pathology / unrestricted

Bibliography: pages 175-184. / The occurrence of maize dwarf mosaic virus (MDMV) in field grown maize was investigated. For this purpose, maize showing mosiac symptoms was collected from different maize growing areas in South Africa by Prof. M.B. von Wechmar. These samples from Transvaal, Orange Free State and Natal were then investigated for the presence of MDMV and possible strains of this virus. Three virus isolates were purified and partially characterised. These isolates were serologically compared together with a fourth isolate SCMV 4975, obtained from the U.S., to establish strain relationships.

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Studies were conducted in 1990 and 1991 to investigate two kinds of soil insects that attack field corn in Virginia. Corn rootworms, the most important of which is the western corn rootworm (WCR), feed on corn roots and have been sampled effectively using yellow sticky traps. The spatial distribution of weekly counts of WCR on yellow sticky traps was investigated over three consecutive weeks in July of each year using Pherocon A.M. traps. The traps were cut in half, placed in a seven by seven grid, and separated by an average distance of 18.3 m. Data were collected in a total of 24 corn fields. Dispersion indices showed that the trap counts of WCR were aggregated. Further analysis using geostatistics revealed that the trap counts were nonrandomly distributed at least half the time at all population densities; however, the presence and type of spatial pattern was density-independent. Pests of germinating corn seeds comprise a guild of insects including seedcorn maggots, wireworms, and white grubs. No one method has yet been developed to sample all of the insects in the guild. A new method, the baited wire trap (BWT), was evaluated in 47 fields in 1990 and 1991. No significant differences were found between the proportion of seeds damaged by pests of germinating seeds in the BWT and seeds planted by hand at conventional plant spacings. The proportion of damaged seeds in the BWT was significantly less than that in the commercially-planted fields; however, the proportion of damaged seeds in the BWT was significantly greater than the stand loss in these same fields. Linear regression revealed a significant relationship between the proportion of damaged seeds in the BWT and simulated plantings, and the conventionally planted fields. Because of low coefficients of determination (R²), the BWT could not be used to accurately predict damage by the germinating seed pest guild. However, the BWT detected damage by all three members of the pest guild and was found to be significantly faster to use than the bait station method. / Master of Science

The stalk borer (SB), Papaipema nebris Gn. exhibited from 7 to 9 instars when reared on a meridic diet in a control-led environment. Both sexes went through variable numbers of instars before pupation. Head capsule width measurements did not form discrete sets, and overlap occurred between adjacent instars. In feeding preference tests with first instar larvae using either leaf disks or stem sections, higher numbers of larvae fed on grasses, such as orchardgrass, Dactylis glomerata L. or fescue, Festuca arundinacea Schreb., compared to numbers feeding on other plants. In no-choice laboratory and field tests, larvae tunnelled into plant stalks at the same frequency by which they would feed on foliage, showing a tendency to tunnel into plants they accept as hosts. Third or fourth instar larvae preferred to tunnel into orchardgrass and rye, Secale cereale Lover the other plants present in field cage tests. The SB prefers to oviposit on narrow leaved, perennial grasses, such as fescue and orchardgrass, over wide leaved annual grasses or broadleaf plants. Significantly higher numbers of eggs were laid on upright over prostrate plants in cage studies. The SB also preferred ovipositing on desiccating or dry plant material. Higher numbers of SB infested corn seedlings were found next to field margins compared to numbers found within fields. Contour and transect maps of SB infested fields showed considerable aggregation over three years, and this distribution was confirmed by high variance to mean ratios, and small k values. SB distribution in no-till corn can be adequately described by the negative binomial model. Field collections of SB in corn stalks showed 2, 3, or 4 instar larvae infesting the youngest seedlings. Variation existed in larval development from year to year. Linear regression covariance analysis showed that larval development was different between field collected larvae from year to year. / Ph. D. / incomplete_metadata