Quantitative Trait Loci for Resistance to Powdery Mildew in Cucumber under Seedling Spray Inoculation and Leaf Disc Infection

Powdery mildew (PM) is an economically important disease of cucumber (Cucumis sativus L.) caused by the obligate biotrophic pathogen, Podosphaera xanthii, in greenhouse and field. In this study, a resistant breeding line S06 (greenhouse European type) and a susceptible cucumber inbred line S94 (Northern China open-field type) and their F-2:3 populations were used to investigate PM resistance under seedling spray inoculation in two growing seasons and leaf disc infection in one season. Quantitative trait loci (QTL) analysis was undertaken based on a constructed molecular linkage map of the corresponding F-2 population using composite interval mapping. Five QTL (PM1.1, PM1.2, PM2.1, PM2.2 and PM5.1) for PM resistance were identified and located on linkage groups (LG) 1, 2 and 5, explaining 3.4-45.0% of the phenotypic variation. Three consistent QTL (PM1.1, PM2.1 and PM5.1) were detected under the three test conditions. The present results of the QTL analysis for PM resistance have laid a basis for breeding PM-resistant cucumber varieties by molecular marker-assisted selection.