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Phytophthora crown rot, caused by Phytophthora capsici Leonian, is a devastating disease in commercial squash (Cucurbita pepo L.) production across the United States. Current management practices rely heavily on the use of chemical fungicides, but existence of fungicide-resistant pathogen populations has rendered many chemicals ineffective. Host resistance is the best strategy for managing this disease; however, no commercial cultivars resistant to the pathogen are currently available. Resistance to Phytophthora crown rot in PI 181761 (C. pepo) is an important genetic resource for squash breeders worldwide; however, the underlying genetic basis of resistance in PI 186761 that would allow designing of sound breeding strategies is currently unknown. The goal of the current study was to determine the inheritance of resistance in breeding line #186761-36P, a resistant selection of PI 181761, using phenotypic data from F₁, F₂, and backcross populations derived from a cross between #181761-36P and a susceptible acorn-type cultivar, Table Queen. The results indicated that resistance in #181761-36P is controlled by three dominant genes (R4, R5, and R6). Introgression of these genes into susceptible cultivar groups of C. pepo will provide an important tool in the integrated management of Phytophthora crown rot.

Phytophthora capsici Leonian, the causal agent of Phytophthora crown rot in squash (Cucurbita pepo L.), is an economically important pathogen worldwide. Currently, no C. pepo cultivars immune to the pathogen are commercially available, but sources of resistance to Phytophthora crown rot have been identified in a set of 16 C. pepo plant introductions (PIs). Knowledge of the genetic relationships among these accessions and their relatedness to economically important morphotypes of C. pepo would inform breeders’ best strategies for introgressing resistance; however, this information is currently lacking. The goal of the current study was to determine genetic diversity among the resistant accessions and their genetic relatedness to susceptible morphotypes of subspecies pepo (Zucchini and Pumpkin) and texana (Acorn, Straightneck, and Crookneck) using 39 SSR markers. The markers revealed 132 alleles averaging 4.40 alleles per locus and had a mean polymorphic information content (PIC) and gene diversity of 0.44 and 0.49, respectively. CMTp235 had the highest PIC and gene diversity of 0.80 and 0.82, respectively. Hierarchical clustering by UPGMA and principal coordinate analysis (PCOA) revealed grouping into two major C. pepo subspecies, texana and pepo, with all the resistant accessions grouping in the latter. In order of increasing genetic distance (GD), the resistant accessions were least distant to Zucchini (GD = 0.34), followed by Pumpkin (GD = 0.40), Crookneck (GD = 0.56), Acorn (GD = 0.60), and Straightneck (GD = 0.61) morphotypes. Mean GD among the resistant accessions was 0.31 and was highest between PIs 615142 and 615132 (0.61). Based on genetic similarity, PIs 174185 and 181761 (disease severity ≤1.4) would be the best sources of resistance for transfer into subspecies texana and pepo, respectively. Overall, the results presented here support a closer genetic relationship between the resistant accessions and morphotypes of subspecies pepo than those of subspecies texana.