Evaluation of selected Cucumis sativus accessions for resistance to Pseudoperonospora cubensis in Egypt

Downy mildew [Pseudoperonospora cubensis (Berk. & Curt.) Rostov] is a major destructive disease of cucumber (Cucumis sativus L.) worldwide. Resistant cultivars were recently introduced into Egypt, but yield losses were high if no fungicides were used. The objective of this study was to identify sources of resistance to downy mildew among Plant Introduction (PI) cucumber accessions from the U.S. National Plant Germplasm System. We evaluated 133 such accessions for downy mildew resistance under natural field epidemics during the summer 2013 at El-Beheira Governorate, Egypt. Mean ratings for downy mildew leaf damage ranged from 1 to 9 on a 0 to 9 scale. We classified 18 PI accessions (13.5%) as highly resistant (rating of 1.0-2.0), 46 (34.5%) as moderately resistant (rating 2.1-4.0), 40 (30%) as intermediate (rating 4.1-6.0), 12 (9%) as moderately susceptible (rating 6.1-7), and 17 (13%) as highly susceptible (rating > 7.1). The most resistant PI accessions were PI 432870, PI 432873, PI 432878, PI 432884, and PI 432886 with a rating of 1, which originated from China. The most susceptible PI accessions were Ames7736, PI 211979, PI 288991, PI 288992 and PI 289698 with a rating of 9. The five most resistant and five most susceptible accessions were further evaluated in replicated experiments during the summer 2014. Results from the repeated test confirmed the results from the first screening. No PI accession was found immune to downy mildew. However, high levels of resistance were observed in several PI accessions that could be useful for the breeding for resistance to P. cubensis in cucumber.