QTL mapping of resistance to powdery mildew race 1 in a recombinant inbred line population of melon.

Powdery mildew, caused by the fungus Podosphaera xanthii, is one of the most important diseases of melon. While there are several pathogenic races of P. xanthii, race 1 is the predominant race in South Carolina and the U.S. We used a densely genotyped recombinant inbred line melon population for traditional QTL mapping, to identify two major (qPx1-5 and qPx1-12) and two minor (qPx1-4 and qPx1-10) QTLs (named according to race - chromosome number) associated with resistance to P. xanthii race 1. QTL mapping of disease severity in multiple tissues (hypocotyl, cotyledons, true leaves and stems) identified the same genetic basis of resistance in all tissue types. Whole-genome resequencing of the parents was used for marker development across the major QTLs and functional annotation of SNPs for candidate gene analysis. KASP markers were tightly linked to the QTL peaks of qPx1-5 (pm1-5_25329892, pm1-5_25461503 and pm1-5_25625375) and qPx1-12 (pm1-12_22848920 and pm1-12_22904659) in the population and will enable efficient marker-assisted introgression of powdery mildew resistance into improved germplasm. Candidate genes were identified in both major QTL intervals that encode putative R genes with missense mutations between the parents. The candidate genes provide targets for future breeding efforts and a fundamental examination of resistance to powdery mildew in melon.