Novel Resistance Determinants from Cucumber PI 197085 Against Pseudoperonospora cubensis

Downy mildew, caused by Pseudoperonospora cubensis, remains a major constraint to cucumber (Cucumis sativus L.) production worldwide. The erosion of resistance conferred by the historic dm-1 gene has heightened the quest for new and enduring sources of resistance. PI 197085, a resistant accession identified under Central European field conditions, remains largely genetically unexplored. In this study, an evenly saturated genetic linkage map was developed using an F2 population derived from PI 197085 × PI 175695, which comprised 164 polymorphic markers spanning all seven chromosomes. Composite interval mapping revealed five quantitative trait loci (QTLs) linked to resistance against P. cubensis, distributed across chromosomes 2, 3, 4, and 5. Candidate gene analysis within the QTL intervals identified clusters of receptor-like kinases, transcription factors, and redox-related enzymes, suggesting that resistance in PI 197085 is polygenic and regulator-rich. The improved resolution of the linkage map enabled more precise localization of resistance loci and uncovered novel genomic regions that were not previously detected in this population. These findings provide a foundation for marker-assisted selection and fine-mapping efforts aimed at developing cucumber cultivars with the robust and durable resistance to P. cubensis.