Genetic Control of Age-related Resistance to Phytophthora capsici in Cucumber

Infection by the oomycete pathogen, Phytophthora capsici, is a major problem affecting cucumber (Cucumis sativus) production in eastern and midwestern production areas.  Cucumber fruit, which are the most susceptible part of the plant, become unmarketable due to fruit rot.  Our previous work showed that cucumber fruit (cv. Vlaspik) exhibit an age-related resistance (ARR) to P. capsici. Young fruits are highly susceptible, but as they reach full size (at 12-16 days post pollination (dpp)) they become resistant, exhibiting minimal or no visible pathogen growth and symptom development limited to the site of inoculation. In this study we tested whether there was genetic variation for ability to develop ARR.  A set of 17 cultivars was examined for ARR by zoospore inoculation of 16dpp fruits.  Three of the cultivars exhibited ARR while others remained highly susceptible. To understand the genetic basis of ARR expression, F₁ progeny from reciprocal crosses between Vlaspik (V, ARR-expressing) and Gy14 (GY, non-ARR expressing) were evaluated using a 9-point disease rating scale [(1-3=resistant with no symptom or symptoms limited to the site of inoculation; 4-6=moderately susceptible; 7-9=highly susceptible)]. At 5 days post-inoculation (dpi), Gy14 had a mean disease score of 6.9± 0.1 while Vlaspik had a mean disease rating of 1.9±0.3. The F₁ progeny responded similarly to Vlaspik with resistant mean disease scores of 2.4±0.2 (V x GY) and 3.2±0.4 (GY x V), suggesting that ARR may be inherited as a dominant trait. F₂ progeny are currently being tested for ARR response. Progeny that are ARR expressing and non-ARR expressing will be selected for bulked segregant QTL Seq analysis.