Advances in breeding for resistance to whitefly-transmitted viruses in watermelon

Whiteflies, and whitefly-transmitted viruses have become a major problem on vegetable crops including watermelon in the southeastern United States. Three whitefly- transmitted viruses: squash vein yellowing virus (SqVYV; which causes viral watermelon vine decline), cucurbit leaf crumple virus (CuLCrV), and cucurbit yellow stunting disorder virus (CYSDV) are now regularly detected in southeastern states. USDA released an SqVYV-resistant watermelon line 392291-VDR for use by the seed industry. Plants of 392291-VDR inoculated with SqVYV remained symptomless even though virus presence was detected. In contrast, inoculated plants of susceptible commercial watermelon variety ‘Crimson Sweet’ (CS) produced typical symptoms of watermelon vine decline, had higher virus titers, and succumbed to virus infection. QTLseq analysis of extremes from an F₂ population identified five QTL’s significantly associated with resistance. Global expression profiling using RNA-seq identified 5,114 differentially expressed genes (DEGs) in 392291-VDR and 7984 DEGs in CS with log 2-fold change >1. Of these 1947 DEGs were upregulated in 392291-VDR compared to 3657 DEGs upregulated in CS. Resistance to SqVYV has been identified in watermelon, but not to the two other viruses. To reliably screen watermelon accessions, we have developed infectious clones of CuLCrV that are capable of causing symptoms on susceptible plants even at relatively low concentrations. The ultimate goal of our project is to develop multiple-virus resistant lines and molecular markers for use in watermelon breeding programs.