Relative susceptibility of commercial watermelon varieties to powdery mildew

Powdery mildew (PM) of watermelon caused by Podosphaera xanthii has been occurring more frequently in recent years than in the past and now growers routinely apply fungicides to manage PM. Powdery mildew is known to be able to cause a significant yield reduction in watermelon. The current study was conducted in 2014, 2015 and 2016 to determine the relative susceptibility of twenty six watermelon varieties (seeded and seedless) and three pollenizers to P. xanthii populations prevailing in Charleston, South Carolina. USVL677-PMS, which is highly susceptible to PM and USVL531-MDR, which is resistant to PM were included as controls. A randomized complete block design with three replications was used each year and plants were rated on a 0-10 scale of increasing disease severity. During all three years, USVL677-PMS was the most susceptible line with the highest values for area under disease progress curve (AUDPC). Disease severity at the last ratings for USVL677-PMS ranged from 61 to 82%. In comparison USVL531-MDR was very resistant to PM based on disease severity (1-3%) and AUDPC. The commercial pollenizers, SP5, SP6 and Lion were all resistant to PM (1-5% disease severity) and had significantly lower AUDPC compared to USVL677-PMS and most other varieties evaluated. Among the red fleshed varieties, Suprema (seedless variety), was relatively resistant compared to other seeded and seedless varieties. Most of the seeded varieties evaluated (e.g. Malali, Black Mama, Mickey Lee) were highly susceptible to PM, however, some were relatively less susceptible (e.g. Declaration) under field conditions. Currently very few to no varieties with high levels of resistance to PM are available and there is a need to develop newer resistant varieties as PM is occurring more frequently during the growing season.