QTL Analysis for Downy Mildew Resistance in Cucumber Inbred Line PI 197088.

Downy mildew (DM), caused by Pseudoperonospora cubensis, is one of the major foliar diseases prevailing in cucumber-growing areas. The mechanism of DM resistance in cucumber, particularly the plant introduction (PI) 197088 from India, is presently unclear. Quantitative trait locus (QTL) mapping is an efficient approach to studying DM resistance genes in cucumber. In this study, we performed QTL mapping for DM resistance in PI 197088 with 183 F-derived F (F) families from the cross between PI 197088 (DM resistant) and Changchunmici (DM susceptible). A linkage map was constructed using 141 simple sequence repeat markers. Phenotypic data were collected from seven independent experiments. In total, five QTL were detected on chromosomes 1, 3, 4, and 5 with DM resistance contributed by PI 197088. The QTL on chromosome 4, dm4.1, was reproducibly detected in all indoor experiments, which could explain 27% of the phenotypic variance detected. Additionally, dm1.1 and dm5.2 showed moderate effects, while dm3.1 and dm5.1 were minor-effect QTL. This study revealed the unique genetic architecture of DM resistance in PI 197088, which may provide important guidance for efficient use in cucumber breeding for DM resistance.