Resistance in watermelon rootstocks to crown rot caused by Phytophthora capsici

Crown rot caused by Phytophthora capsici is becoming an increasingly important disease of vegetable crops in southeastern U.S. In recent years, the practice of grafting watermelon onto rootstocks belonging to other Cucurbitaceae genera has also been slowly gaining adoption in the U.S. However, it is not known how these rootstocks, developed mainly in Asia, will respond to diseases prevalent in local production areas. We evaluated the available commercial watermelon rootstocks for resistance to Phytophthora crown rot by inoculating them with a zoospore suspension of P. capsici in four different trials. Disease development on rootstocks was rated on a 1–9 scale (1 = no symptoms, 9 = plant dead). Based on all the four trials, the commercial Lagenaria siceraria (bottle gourd) rootstocks, FR-Strong, Emphasis, Macis and WMXP-3938 were resistant (Mean rating ≤3) to crown rot when compared to susceptible watermelon checks (mean rating >8). All Cucurbita inter-specific hybrid rootstocks and a watermelon rootstock Ojakkyo, were highly susceptible to crown rot (mean rating >8). Real-time quantitative polymerase chain reaction (qPCR) using two different P. capsici specific primers (ITS and β-tubulin) indicated the presence of significantly (P < 0.0001) greater amounts of P. capsici DNA g−1 plant tissue in susceptible Cucurbita inter-specific hybrid rootstocks (β-tubulin, mean = 2895 ng) and watermelon (2665 ng) compared to the L. siceraria rootstocks (357 ng). Crown rot resistant bottle gourd rootstocks may be useful in areas where P. capsici is a recurring problem. The present study identified several commercial bottle gourd rootstocks with resistance to Phytophthora crown rot, and confirmed their levels of resistance using qPCR.

► Diseases caused by Phytophthora capsici especially crown rot are a serious problem on vegetable crops in southeast U.S.
► Grafting watermelons on various rootstocks is relatively new in the U.S. and is currently being evaluated and adopted.
► Commercial Cucurbita inter-specific hybrid and watermelon rootstocks and cultivars were highly susceptible to crown rot.
► Several Lagenaria siceraria (bottle gourd) rootstocks were resistant to crown rot.
► Resistance in bottle gourd rootstocks was confirmed by quantifying the levels of P. capsici infection using qPCR.