Control of insect-transmitted viruses in cucurbit crops in KwaZulu- Natal.
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.