QTL Mapping of Downy Mildew Resistances in PI 197088 and WI7120 Cucumbers

Downy mildew (DM, Pseudoperonospora cubensis) is a devastating fungal disease of cucumber worldwide. Several plant introduction lines have been identified with high resistance to the post-2004 new DM strain found in the U.S. However, the inheritance of DM resistance is still not well defined. Molecular markers that can be used for marker-assisted selection in DM resistance breeding are not widely available. Here, we report QTL mapping studies with two inbred lines PI 197088 and WI7120 (derived from PI 330628) with high resistance against the post-2004 DM strain. Mapping populations included 154 F₆ RILs from PI 197088 × Coolgreen and 243 F₃ families from WI7120 × 9930. Reponses to downy mildew pathogen in artificial greenhouse or natural field inoculations of the two populations were evaluated in multiple environments over three years and four locations. Genotyping of the RIL and F₃ populations was conducted with 2,086 specific-length amplified fragment (SLAF) markers and 348 SSR markers, respectively. QTL analysis identified six QTLs underlying DM resistance in the RIL population contributed by PI 197088, which were located in cucumber chromosomes 1, 3, 4, and 5, respectively with 3 located in Chr5. These QTL together explained 78.4% observed phenotypic variations. With the F₃ population of WI7120 × 9930, four QTLs for DM resistance contributed by WI7120 were identified in chromosomes 2, 4, 5, and 6 that could explain 75.5% phenotypic variations. Based on the LOD support interval of each QTL, two QTLs, one in Chr4, and the other in Chr5 seemed to be shared by PI 197088 and WI7120 although the magnitude of effects (R²) was different between the two lines at each locus.