Summer squash (Cucurbita pepo) is a significant vegetable crop in the United States with an annual value exceeding $216 million. The production of summer squash is significantly hindered by powdery mildew (PM), a fungal disease caused by Podosphaera xanthii. Management of PM relies on costly and routine application of fungicides. Moderate resistance to PM (designated PM0) in C. okechobeensis is widely deployed in commercial summer squash cultivars. However, it is important to expand the repertoire of alleles against PM in squash to complement and reduce the risk of PM0 resistance breakdown. In the current study, the USDA core collection of C. pepo (n= 207) was evaluated for PM resistance in Florida (greenhouse), New York (greenhouse), and Michigan (field) using a randomized complete block design across three reps, each with five plants. ‘Success PM’ (carrying PM0) and ‘Early Prolific’ Straightneck cultivars were used as resistant and susceptible checks, respectively. Pathogen inoculum was provided through naturally infected plants. At the 6th true-leaf stage, symptom severity data were collected on a scale of 0-100% based on visible pathogen sporulation on the surface of ‘top 4th leaf’, ‘bottom 4th leaf’, ‘stem above 4th leaf’, ‘stem below 4th leaf’, and ‘whole plant’. Across locations, ‘Success PM’ and ‘Early Prolific’ were consistently tolerant and susceptible, respectively. On the other hand, wide phenotypic variation was observed across the C. pepo core collection with accession 189 showing resistance across locations. The multi-location phenotype data was combined with genome-resequencing data (4 million SNPs) for the core collection to conduct a genome-wide association study using three statistical models (MLM, FarmCPU, and Blink). GWAS analysis for the FL dataset revealed significant genomic loci associated with PM resistance for ‘top 4th leaf’ (Chr 11 and 20), ‘stem above 4th leaf’ (Chr 4, 14 and 16), and ‘whole plant’ (Chr 13, 15, 18 and 20). Resistance loci for ‘top 4th leaf’ and ‘whole plant’ co-located on Chr 20, suggesting potential linkage/ pleiotropy for the two traits. For NY, significant hits for PM resistance were detected for the ‘top 4th leaf’ (FarmCPU: Chr 2, 4, 7, 13 and 19; Blink: Chr 3, 4, 5 and 19) and ‘bottom 4th leaf’ (Chr 6, 14 and 19). However, no significant GWAS hits were observed using MI data. The significant loci detected in this study will be validated and deployed in marker-assisted selection to improve PM resistance in squash.
