Genome-Wide Association Study of Resistance to Pseudomonas syringae in the USDA Collection of Citrullus amarus .

Pseudomonas leaf spot (PLS) caused by pv. is an emerging disease of watermelon in the United States. To date, there are no reported watermelon cultivars with resistance to PLS. Since is an important reservoir of resistance genes, we screened the USDA plant introduction (PI) collection of for resistance to PLS. Accessions (n = 117) were phenotyped for their level of resistance to PLS in two separate tests. Phenotypic variability, based on percent leaf area affected, across the two experiments ranged from 1.5% to 99.4%. Whole-genome resequencing generated 2,126,759 single nucleotide polymorphisms (SNPs), which were used to perform a genome-wide association study (GWAS) aimed at identifying SNPs associated with resistance. Three different models, BLINK, FarmCPU and MLM were included in the GWAS analyses. BLINK and FarmCPU found a total of 8 peak SNPs, located on chromosomes Ca01, Ca05, Ca06, Ca08 and Ca10. Two of these peak SNPs were found by both BLINK and FarmCPU. A total of 43 candidate genes were discovered within the genomic intervals of 7 of the 8 peak SNPs. Of these, eleven have been reported to be involved in resistance to in other plant species. Two significant SNPs were within resistance genes previously documented to play important roles in resistance specific to in other pathosystems. The identified peak SNPs will be instrumental in finding causal genes involved in PLS resistance in watermelon and developing resistant germplasm through breeding.