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

As the causative agent of black shank, Phytophthora nicotianae is a serious threat to tobacco cultivation in South Africa. Research presented in this dissertation describes pathogenicity studies and control measures for P. nicotianae on tobacco. Special attention is given to the population structure of P. nicotianae in South Africa. The implications of these genetic studies in breeding and selection programs against P. nicotianae were also evaluated. The first chapter of this dissertation represents a literature review on black shank and available control measures for P. nicotianae on tobacco. The mechanisms of pathogenicity and the life cycle of P. nicotianae are also treated in detail. Special reference is made to the maintenance of genetic diversity in Phytophthora species and particularly P. nicotianae. This literature review also highlights the fact that very few studies have been conducted to determine the genetic structure of P. nicotianae populations. The success of South African breeding programs for tobacco cultivars with P. nicotianae resistance is to some degree dependent on the selection of isolates with high levels of aggressiveness. The research presented in chapter two provides information on cultivar resistance and selection of P. nicotianae isolates for future breeding programs. Significant differences in levels of aggressiveness were found between P. nicotianae isolates. Furthermore, race 0 and 1 of P. nicotianae occurred in most of the tobacco growing regions in South Africa. Selected Race 0 and 1 isolates were thus used to evaluate black shank resistance of 11 commercially planted tobacco cultivars. Commercially planted cultivars differed significantly in their resistance to race 0 and 1. Cultivars LK33/60 and OD1 were highly resistant to race 0 but susceptible to race 1 while cultivars Vuma/3/46 and LK3/46 were highly resistant to both race 0 and 1. Chapter three reports on the use of metalaxyl treatments combined with resistance in tobacco cultivars for control of P. nicotianae. One hundred and thirty two isolates of P. nicotianae were screened for sensitivity to metalaxyl. P. nicotianae isolates from most tobacco farms were metalaxyl sensitive. The results further indicated that the use of metalaxyl in combination with moderately resistant cultivars effectively reduced black shank in the field. The outcome of this study provided useful information for the implementation of an economically viable combination of disease resistance and metalaxyl as the basis for a P. nicotianae management program in South Africa. Chapter four of this dissertation deals with the development of a rapid seedling-' based screening technique to assay tobacco for resistance to P. nicotianae. This technique was validated by comparing it to a stem inoculation technique commonly used on adult plants. A strong positive correlation was found between results of the seedling assay and adult plant trials for all isolates and cultivars tested. P. nicotianae isolates could also be characterized as race 0 or I using both stem inoculation and the rapid seedling assay. The ability to screen large numbers of tobacco plants rapidly at the seedling stage allows for the testing of large germplasm resources in a systematic manner and under standard conditions. This may help in the timely development and release of more black shank resistant cultivars. In chapter five, a population study on P. nicotianae in South Africa is presented. One hundred and five P. nicotianae isolates were collected from the Northern Highveld and Lowveld regions, as well as from both citrus and tobacco hosts in South Africa. Levels of phenotypic diversity were determined in populations of P. nicotianae using RAPD markers. Among the 105 P. nicotianae isolates analysed 79 different RAPD phenotypes were found, where 35 of the isolates were found to be clonal. The high number of RAPD phenotypes (79) in relation to the sample size (105), the presence of both the Al and A2 mating type and high levels of phenotypic diversity in the P. nicotianae population indicate a sexually outcrossing P. nicotianae population in South Africa. This sexual outcrossing may mean that P. nicotianae is likely to remain a constant threat to tobacco and citrus cultivation, since new genotypes with the potential to overcome resistance genes in commercial cultivars are likely to emerge. All chapters of this dissertation deal with some aspects of black shank control and breeding for resistance to P. nicotianae. This dissertation provides new knowledge on variation in levels of aggressiveness, race distribution and the development of metalaxyl resistance in the South African P. nicotianae populations. This also represents the first study on the genetic diversity of P. nicotianae populations in South Africa. The results presented here not only show the possible occurrence of sexual reproduction, but also indicate the presence of clones and discreet phenotypic groups of P. nicotianae. This information will be applied in future tobacco breeding programs to select breeding lines with resistance against a number of specific P. nicotianae races and phenotypic groups. / Thesis (DPhil)--University of Pretoria, 2006. / Microbiology and Plant Pathology / Unrestricted

The false codling moth (FCM), Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), is an important pest of citrus in South Africa and challenging to manage due to its inconspicuous nature. An effective method currently being employed for the area-wide suppression of the FCM is the Sterile Insect Technique (SIT) and the effective dispersal of sterile moths is very important for success with SIT. This study was conducted in the Addo area of the Sundays River Valley (Eastern Cape) where the programme is commercially used. In this study, sterile male moths were released in different orchards on a citrus farm, and in nearby veld at different times of the year, and their dispersal was monitored through the use of pheromone traps. Various climatic factors were monitored. This provided insight into the local dispersal of sterile male FCM adults in response to abiotic cues (particularly climatic factors). The movement of the FCM in four citrus cultivars, namely lemons, navel and Valencia oranges and mandarins and in the nearby veld (open field), was determined at six different stages of the year. Results clearly indicated that sterile FCM movement is concentrated within citrus orchards, as very few moths were trapped beyond 30 m from the release point, particularly in navel and Valencia orchards. Of the climatic factors measured, minimum and maximum temperatures had the most significant influence on FCM dispersal, and based on the results, various recommendations are made for the releases of sterile FCM in an area-wide SIT management programmes on citrus. A better understanding of the dispersal capabilities of the FCM in an agricultural system, under different conditions and at different times of the year, is invaluable not only in improving release strategies in an SIT programme but in planning future control strategies against the FCM.

Please read the abstract (preface) in the section 00front of this document / Dissertation (MSc)--University of Pretoria, 2002. / Microbiology and Plant Pathology / Unrestricted

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Historically, most twospotted spider mite, Tetranychus urticae Koch, infestations occurred during the late season near maturity, but since 2005, infestations of cotton have become more common throughout the entire season. Several factors may have contributed to the increasing frequency of mites in seedling cotton, one of which is a shift in production practices from furrow applied aldicarb to neonicotinoid seed treatments for thrips control. Another factor that may impact T. urticae densities in seedling cotton is a shift from pre-plant tillage to conservation tillage or no-till cropping systems. Although the literature confirms that T. urticae can cause extensive cotton yield losses, there is a need to further refine potential late-season yield loss. From 2007 to 2009 a series of experiments were conducted to address these issues. In a series of 12 field experiments, neonicotinoid seed treatments resulted in twospotted spider mite densities greater than those in the untreated check, aldicarb, and acephate treatments. Untreated check and aldicarb treatments had the lowest mite densities. Only aldicarb controlled mites compared to the untreated check. A twospotted spider mite host survey determined that henbit, Lamium amplexicaule L., was the most consistent and preferred host. Carolina geranium, Geranium carolinianum L., cutleaf geranium, Geranium dissectum L., vetch, Vicia spp., volunteer soybean, Glycine max L., purple deadnettle, Lamium purpureum L., and spiny sowthistle, Sonchus asper (L.) Hill, were other frequently infested dicotyledonous species. Herbicide burndown timing in reduced tillage cotton production was not critical, given all weeds were killed before cotton was planted. The herbicide glufosinate was efficacious against T. urticae, providing control equal to low- to mid-rates of labeled acaricides. Including glufosinate in a herbicide burndown program was only beneficial for mite control if living weeds were present at planting. Artificial infestation experiments were adversely affected by unseasonably wet and cool weather. Infestations established at the third true leaf resulted in an average yield loss of 44.7%. It is difficult to make any conclusions about infestations made from first bloom and later because of the difficulty in establishing mite populations later in the season.

Mozambique has potential to boost its banana exports. To fully realise this, agronomic practices in production should be fully developed to combat physiological disorders associated with banana within the region. Currently, lower temperatures are being experienced in some production sites, consequently affecting yield and quality. The objective of this study was to evaluate use of bunch protection covers on banana cultivars Grand Nain and Williams banana cultivars, for performance under different fruit protection materials to determine best fruit protection bag suitable for Metocheria, Nampula. Plants were not selected near plantation borders, drainage canals, cable way and roads, as this would influence the growth pattern of plants and fruit development. Treatments consisted of control (no bag on bunches), white perforated polyethylene, white non-perforated polyethylene, blue perforated polyethylene, blue non perforated polyethylene, green perforated polyethylene, green polyethylene non perforated and cheese cloth bags arranged in a complete randomised block designed CRBD with 26 plants replicated eight times. During 2012/2013, bagging treatments did not considerably improve weight in hands, banana finger weight, total fruit weight, marketable weight and percentage marketable fruit weight and box stem ratio (BSR) of Grand Nain. However there was reduction of fruit defects in all bagging treatments compared to control (no bags). In Williams during the 2013 season bagging treatments improved weight but no significant differences were observed on weight of hands in 2012. Bagging of banana bunches reduce defects in both seasons. Both green and blue perforated bags improved box stem ratio. Bagging treatments increased Williams‟s cultivar yield (per ton) in both seasons / Agriculture and  Animal Health / M. Sc. (Agriculture)

Four trap crops were tested for their ability to intercept stinkbugs (Heteroptera pentatomidae) in a cucurbit field crop at Waterpoort, Limpopo Province. The experimental trap crops were: Sunnhemp (Crotolaria juncea), okra (Abelmoschus esculentum), mustard mixture (Brassica hirta and Brassica juncea) and cowpea (Vigna unguiculata). The surveys were based on visual counts of stinkbugs from all the trap crops. Stinkbugs were collected from trap crops in a field crop trial of cantaloupe (Cucumis melo). The correlation between Brassica hirta and Brassica juncea with other trap crops had a great variance. The repeated B. hirta and B. juncea inner row experiment in 2012 interestingly lured the green stinkbug (Nezara viridula) into the cantaloupe field. Amongst the entire trap crops, stinkbugs recovered from Brassica hirta and Bassica juncea, exceeded the average of the other three trap crops. The number of adult N. viridula captured had significantly increased in March 2012 and started to drop on the B. hirta and B. juncea mixture in the first week of April. During this increase, the crops were at the seeding stage. The perimeter trap-cropping system practiced for all four trap crops varied in the attraction of stinkbugs along the perimeter of the field and inner row. B. hirta and B. juncea attracted a high number of N. viridula along the perimeter but fewer in the inner row. The B. hirta and B. juncea recorded the highest number of N. viridula in the inner rows and outer rows, compared to the other four trap crops, followed by V. ungiuculata. This study has demonstrated a significant potential of using trap crops as biological alternative for managing stinkbug pests in cucurbit crops. / Agriculture and  Animal Health / M. Sc. (Agriculture)