New Sources of Resistance in Winter Squash (Cucurbita moschata) to Phytophthora Crown Rot and Their Relationship to Cultivated Squash

Butternut squash (Cucurbita moschata) is an important vegetable crop grown and consumed in most states in the United States. C. moschata lines and interspecific hybrids between Cucurbita species are also used as rootstocks for grafting watermelon and melon. However, currently most commercially available C. moschata squash varieties are highly susceptible to crown and root rot caused by the oomycete pathogen Phytophthora capsici, especially in the southeastern United States. All available plant introductions (PIs) of C. moschata (319 PIs) were evaluated for resistance to crown rot. Four-week-old plants were inoculated with 104 zoospores from a local South Carolina isolate of P. capsici. Plants were rated for disease severity 3 weeks after inoculation using a 0 to 5 rating scale (0 = no symptoms and 5 = plant dead). The majority (87%) of the C. moschata PIs were highly susceptible to crown rot in the first evaluation and were rated as 5. Reevaluation of the promising PIs identified several potential new sources of resistance (e.g., Grif 935, PI 442272, PI 442264, PI 512142, PI 438724, PI 438778, and PI 442280). Variability in resistance reaction among plants within a PI was also observed, and not all plants exhibited resistance. Further evaluation of S1 generation from the most resistant plants (rated ≤1) demonstrated that highly resistant plants could be selected from these PIs to develop lines for use in breeding programs. These new sources of resistance can be utilized for developing new crown and root rot resistant rootstocks for watermelon grafting and for developing resistant varieties for human consumption.