QTL Mapping of Resistance to Pseudoperonospora Cubensis Clade 2, Mating Type A1, in Cucumis Melo and Dual-clade Marker Development

Abstract Pseudoperonospora cubensis, causal organism of cucurbit downy mildew (CDM), causes severe necrosis and defoliation on Cucumis melo (melon). A recombinant inbred line population (N = 169) was screened against an isolate of P. cubensis (Clade 2 / mating type A1) in replicated greenhouse and growth chamber tests. SNPs (n = 5,633 bins) identified in the RIL population were used for quantitative trait loci (QTL) mapping. A single major QTL on chromosome 10 was consistently associated with resistance across all tests, while a second major QTL on chromosome 8 was identified only in greenhouse tests. These two major QTL were identified on the same chromosomes (8 and 10) but in different locations as two major QTL previously identified for resistance to P. cubensis Clade 1 / mating type A2. Kompetitive allele specific PCR (KASP) markers were developed for these four major QTL and validated in the RIL population through QTL mapping. These markers will provide melon breeders a high-throughput genotyping toolkit for development of melon cultivars with broad tolerance to CDM.