Identifying Resistance to Powdery Mildew Race 2W in the USDA-ARS Watermelon Germplasm Collection

Powdery mildew caused by Podosphaera xanthii has recently become an important disease of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] in the United States. The disease can be controlled with fungicides, but it would be more economical and environmentally safe to use genetic resistance. Here, we report sources of resistance to powdery mildew race 2W identifi ed in the evaluation of the entire U.S. watermelon Plant Introduction collection made of four Citrullus Schrad. ex. Eckl. & Zeyh. species and Praecitrullus fi stulosus (Stocks) Pangalo species. A total of 1654 PI accessions, cultivars, and breeding lines (hereafter collectively referred to as cultigens) were tested in the greenhouse using at least seven replications. From that, 54 cultigens including the 44 most resistant and 10 susceptible checks were retested in greenhouse and fi eld experiments. All cultigens showed symptoms of powdery mildew. Resistance was identifi ed in wild PI accessions. Eight cultigens had high resistance and 21 had intermediate resistance. Leaf and stem disease severity ratings were positively correlated (r = 0.86, P < 0.0001). Data were summarized from the screening and retest studies, and the most resistant cultigens were PI 632755, PI 386015, PI 189225, PI 346082, PI 525082, PI 432337, PI 386024, and PI 269365. The most susceptible cultigens were PI 222775 and PI 269677. Many of the resistant cultigens originated from Nigeria and Zimbabwe.