Abstract:
Buttercup squash (Cucurbita maxima), which has been Tongas' major export crop since 1987, is severely affected by viral mosaic disease. This study determined the incidence of the common viruses and associated aphid vectors infecting the crop, and the effects of time of planting, proximity to major virus sources, and insecticide application on disease development. Symptomatic plants were tested for Cucumber mosaic virus (CMV), Watermelon mosaic virus (WMV) and Zucchini yellow mosaic virus (ZYMV) by ELISA. The most frequently detected virus was ZYMV followed by WMV and CMV, often as mixed infections. It was concluded that CMV and WMV were of minor economic importance but ZYMV was consistently associated with stunted growth and reduced yields. Tongan ZYMV isolates showed a very high (>96%) coat protein (CP) gene nucleotide sequence identity to each other and phylogenetic analysis indicated they were closely related to isolates from the South East Asian region. Aphids were collected from squash plots using yellow pantraps. Fifteen different species were identified including Aphis craccivora, Aphis gossypii, Myzus persicae and Rhopalosiphum maidis which are known vectors of potyviruses. There was strong correlation between the presence of aphids and the incidence of viral mosaic symptoms. A field trial was conducted to determine the effects of time of planting and insecticide treatment on virus incidence and crop yield. Early planted crops became virus infected later in the crop cycle and produced higher yields than late planted crops. Insecticides application had no significant effect on virus incidence or crop yield, irrespective of planting time. Watermelon, rockmelon and cucumber crops, and 11 weed species were tested for CMV, Papaya ringspot virus-\N (PRSV-W), Squash mosaic virus (SqMV), WMV and ZYMV. In watermelon ZYMV, PRSV-W, WMV, and CMV were detected with ZYMV and WMV being most frequently detected. ZYMV and WMV were detected in the weed species Chamaesyce sp. and Sonchus oleraceus, and ZYMV was also detected in the weed species Synedrella nodiflora , Momordica charantia, Malvastrum coromandelianum and Coccinnia grandis.