Masters Degrees (Plant Pathology)
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Browsing Masters Degrees (Plant Pathology) by Author "Da Graca, John Vincent."
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Item Control of insect-transmitted viruses in cucurbit crops in KwaZulu- Natal.(1998) Cradock, Kenwyn Roan.; Da Graca, John Vincent.; Laing, Mark Delmege.The production of cucurbits (Cucurbitaceae) in KwaZulu-Natal faces the constant threat of viral diseases. These can be so severe as to severely limit or prevent production in the latter part of the growing season (December-April). The important viruses in this regard are zucchini yellow mosaic potyvirus (ZYMV), watermelon mosaic 2 potyvirus (WMV2), watermelon mosaic potyvirus - Morocco strain (WMV-M), papaya ringspot potyvirus - type W (PRSV-W), cucumber mosaic cucumovirus (CMV), and squash mosaic comovirus (SqMV). The potyviruses and CMV are vectored by aphids (Homoptera: Aphididae) and SqMV is vectored by cucumber beetles (Coleoptera: Coccinelidae). PRSV and SqMV were found to be absent from the region, while CMV was found not to be a serious threat to cucurbit production. ZYMV, WMV2 (now confirmed to occur in South Africa) and WMV-M are the major viral pathogens of cucurbits in KwaZulu-Natal. The distribution of these viruses and methods for their control were investigated. Investigations of aphid morphology using the scanning electron microscope were undertaken to determine if taxonomic studies could be conducted using this form of microscopy. The best form of specimen preparation was the cryo-fixation technique, which resulted in less collapse of the body wall and general damage to the specimen when compared to the critical point drying technique. Due to the lack of mobility of the specimen while viewing, this form of microscopy is rejected as a means of identifying aphids to the species level. ZYMV was found to occur in a number of weed species (Galinsoga parviflora, Malva parviflora, Amaranthus sp., Solarium spp.), which could serve as reservoirs of virus. WMV-M and CMV were also found in some weed species. All tests for the potyviruses and SqMV were done using the double-antibody sandwich (DAS) ELISA technique. CMV was tested for using indirect ELISA tests. A third of the plants tested were found to be infected with more than one virus which could have implications for disease severity. Disease severity was found to increase at about midway through the growing season (December- January). This was concurrent with a massive increase in the general aphid population in the experimental area. As no aphids were seen on the cucurbits in the fields, these vectors are believed to be transient inhabitants of the crop at first testing and then rejecting the plants as a food source. All control measures applied in the trial were aimed at reducing the numbers of aphids in the plots. Aphids were trapped using yellow sticky traps. Cucurbita pepo (zucchini) was used in the trials due to its bush growth habit and good virus symptom expression. The success of the treatments was determined by monitoring the numbers of aphids present in the plots, and the use of a rating scale which assessed the severity of virus disease in the plots. The two best treatments were the white reflective mulch and the straw mulch. In the cultivar trial which assessed ten different cultivars for their virus resistance/tolerance. The best three cultivars were 'SQ 229', 'Puma', and 'SQ 228'. 'SQ 229' and 'SQ 228' were withdrawn from the market by the seed company for unknown reasons. From the results obtained from these investigations, a disease management programme can be suggested. All cucurbit crops should be grown over a white reflective mulch, drip irrigation should be used to reduce agitation of the plants which could unnecessarily disturb feeding aphids, and a resistant or tolerant cultivar should be used in the latter half of the growing season. The effectiveness of any treatment can be assessed by comparing the number of aphids caught with the number caught in the control plots.Item The epidemiology and control of Capsicum viruses in Natal.(1995) Budnik, Krzysztof.; Da Graca, John Vincent.; Laing, Mark Delmege.Virus diseases pose a serious threat to commercial pepper (Capsicum annuum L.) production in Natal. Following a survey of the principal capsicum-producing areas, potato virus Y (PVY) was found to be the predominant virus infecting peppers, often reaching 100% incidence. Currently, TSWV incidence and CMV levels are insignificant with respect to pepper crops in Natal. Thus, the diverse ecological and epidemiological factors which determine PVY infection of peppers were investigated. The potential host range of PVY was established in a glasshouse study. Seed from solanaceous weeds commonly occurring in vegetable-producing areas of Natal was collected. Seedlings were mechanically inoculated with a pepper strain of PVY and assayed for infection using double-antibody sandwich ELISA. Nicandra physaloides L., Solanum elaegnifolium Cav., S. nigrum L., S. velosum L. and S. aculeastrum L. were found to be susceptible to PVY infection. In addition, a field survey of over 100 samples of commonly occurring weed species growing in or adjacent to capsicum crops in the Pietermaritzburg and South Coast regions of Natal was carried out. Several weed species were found to be naturally infected with PVY, including Acanthospermum hispidum DC., Bidens pilosa L., N physaloides and S. nigrum. The spread of PVY into a pepper crop on the Natal South Coast was monitored during 1993. Virus spread was rapid, with PVY first detected in pepper seedling one week after planting, suggesting a nearby source of the virus. A survey of the wild vegetation prior to planting of the crop, revealed that populations of N physaloides may be the primary sources of PVY infecting the crop. Large virus-infected S. nigrum populations appeared later in the season, suggesting its role in maintaining high levels of PVY during periods when no pepper cultivation takes place. In addition to identifying possible virus reservoirs, several virus control measures were investigated, demonstrating ways of avoiding or minimising infection. The effects of insecticides, oil sprays (Virol), insect repellents (AzatinTM), yellow polyethylene traps and plastic mulches on virus incidence within peppers were evaluated in field experiments. Results of weekly sprays of the insecticide mercaptothion at 5%, increased virus incidence in peppers by 15% when compared to the untreated control. Similarly, the effects of insecticide applications on pepper yields and quality were negative. Results of applications of Virol at 1 % and Azatin™ at 1.5% did not differ from those of the unsprayed control. Mulching was most effective by reducing virus incidence in treated plots by 50% and resulted in a yield increase of 62% and a 40% increase in fruit quality. The use of yellow sticky traps reduced virus incidence by 35%, with a yield increase of 25% and a 24% improvement in fruit quality, when compared to the untreated control. Both mulching and the use of yellow sticky traps reduced the number of aphids trapped within the plots. In order to assist the development of capsicum cultivars resistant to PVY infections, a screening method was developed to determine susceptibility levels of a breeding population. Two rating procedures were investigated based on disease severity of the whole plant and on the fruit (chilli pods). The technique was effective in detecting small incremental increases in susceptibility within a breeding population, provided that an adequate positive selection pressure is applied. Using this technique breeders may be able to define a large breeding population to those parents exhibiting a genetic base most suitable for resistance development and eliminate those which exhibit low frequencies of resistance genes. Based on the results obtained, an integrated virus management strategy is suggested, including the elimination of virus sources and the use of cultural practices which facilitate a reduction in virus spread.