The epidemiology and control of Capsicum viruses in Natal.
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.