Physiological Races of Cucurbit Powdery Mildew Pathogen (Podosphaera xanthii) based on Watermelon Differentials

Watermelon is a major cucurbit crop grown across the world and in 44 states in the USA. Powdery mildew (PM) caused by Podosphaera xanthii is an important disease of watermelon that can severely limit fruit production and reduce yield. Extensive work has been done on races of P. xanthii based on melon (Cucumis melo) differential hosts and seven major races of have been identified. However, physiological races of P. xanthii have not been classified for many other cucurbits including watermelon. The objective of this study was to develop a set of watermelon PM race differentials and identify potential races. Ten P. xanthii isolates collected from various states were evaluated using a newly developed set of watermelon (Citrullus lanatus) differentials that included a highly susceptible germplasm line 'USVL677-PMS', a commercial cultivar 'Mickey Lee', two resistant germplasm lines 'USVL608-PMR', 'PI 189225' and resistant pollenizer 'SP5'. The well-studied melon (C. melo) powdery mildew differentials 'Iran H' and 'PMR-45' were also included. All isolates colonized and produced abundant conidia on the melon Iran H, and watermelon USVL677-PMS. SP5 and PI 189225 were resistant to all isolates. USVL608-PMR was resistant to seven of the ten isolates and displayed intermediate resistance to three isolates. Of the ten isolates that grew well on USVL677-PMS, only five colonized and produced abundant conidia on Mickey Lee which was highly resistant to the remaining five. Based on reactions of the ten powdery mildew isolates on Mickey Lee and USVL677-PMS, at least two distinct physiological races (0 and 1) can be identified. These two races based on watermelon differentials did not correlate with race structure based on the two melon differentials. As new watermelon cultivars with resistance to powdery mildew are being released, it will be important to determine their reaction to P. xanthii isolates as a race structure based on watermelon differentials clearly exists.