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Phytophthora capsici is an aggressive pathogen that is distributed worldwide with a broad host range infecting solanaceous, fabaceous, and cueurbitaceous crops. Over the past two decades, increased incidence of Phytophthora blight, particularly in eastern states, has threatened production of many vegetable crops. Cucumis melo L. (honeydew and muskmelon), although especially susceptible to fruit rot, is also highly susceptible to crown rot. Currently, little is known about host resistance to P. capsici in C. melo. To assess crown rot resistance in C. melo seedlings, 308 U.S. PIs, and two commercial cultivars (Athena and Dinero) were grown under greenhouse conditions. Seedlings with three to four true leaves were inoculated with a five-isolate zoospore suspension (1 x 104 zoospores per seedling) at the crown and monitored for 6 weeks. All the susceptible control plants of Athena died within 7 days post-inoculation. The majority of the PIs (281 of 308) were highly susceptible to crown rot and succumbed to the disease rapidly and had less than 20% of the plants survive. Several PIs (PI 181748, PI 182964, and PI 273438) succumbed to crown rot earlier than the susceptible melon cultivars. Eighty-seven PIs selected on the basis of the first screen were re-evaluated and of these PIs, 44 were less susceptible than cultivars Athena and Dinero. Twenty-five of the 87 PIs were evaluated again and of these six PI, greater than 80% of the plants survived in the two evaluations. Disease development was significantly slower on these PIs compared with the susceptible checks. High levels of resistance in S1 plants of P1420180, PI 176936, and PI 176940 were observed, which suggests that development of resistant germplasm for use in breeding programs can be accomplished. Further screening and careful selection within each of these PIs can provide a framework for the development of resistant germplasm for use in breeding programs. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]