Screening the Cucumber Plant Introduction Collection for Young Fruit Resistance to Phytophthora capsici

Fruit rot caused by Phytophthora capsici is a major constraint in cucumber (Cucumis sativus) production. In an effort to identify a source of resistance, we developed a more streamlined detached fruit method for high-throughput screening and tested the U.S. cucumber PI collection for fruit rot resistance. A total of 1076 PI collections, from 54 geographic locations around the world, along with the susceptible commercial cultivar, Vlaspik, were grown in the field and tested for resistance to P. capsici. Using the knowledge gained from our prior studies regarding greater susceptibility of young fruits compared with older fruits, very young fruits (approximate to 3 to 4 days post-pollination) were collected and inoculated with zoospore suspensions of P. capsici isolate OP97. From the screens performed in 2011 and 2012, 99% of the tested PIs were rated as moderately or highly susceptible based on symptom development and pathogen growth at 5 days post-inoculation. The cv. Vlaspik control showed consistent high susceptibility to P. capsici with a mean symptom rating of 8.0 on a 9-point scale. A set of 28 PIs was chosen for further testing in the greenhouse or field in 2013. The disease ratings of PIs rescreened in 2013 were much lower compared with that of the full collection of Pis. Three accessions, PI109483, PI178884, and PI214049, showed consistent low mean disease ratings and may be considered as possible sources of resistance to young cucumber fruit infection by P. capsici. Evaluation of the S-1 progeny of PI109483 suggests that the resistance is heritable and should allow for development of useful breeding materials that can be used for developing P. capsici-resistant cucumber cultivars.