Phytophthora blight, caused by Phytophthora capsici Leonian, is an economically important disease of cucurbit crops (Cucurbita spp.). Michigan accounts for 20% of the squash produced in the U.S., with total cash receipts of $12 million in 2009. Field and greenhouse trials were conducted to compare soil drenches and foliar sprays of eleven fungicides for control of Phytophthora crown and root rot on summer squash. Soil drenches were more effective than foliar sprays at limiting plant death caused by P. capsici. Mean plant death 42 days post inoculation (dpi) was 41% for soil drenches and 92% for foliar sprays. Drenches of fluopicolide, mandipropamid or dimethomorph limited plant death to ≤10%, and prevented yield loss associated with crown and root rot. Similarly, disease progress was slower and crown rot was less severe following soil drenches compared to foliar sprays in greenhouse trials. Most fungicide treatments were more effective on the cultivar `Leopard' than `Cougar', which is more susceptible to P. capsici. A laboratory study evaluated age-related resistance to Phytophthora fruit rot in `Dickenson Field' processing pumpkin and `Golden Delicious' winter squash. Hand-pollinated fruits were harvested 3, 7, 10, 14, 21, 28, 42, or 56 days post pollination (dpp) and inoculated with P. capsici. Susceptibility to Phytophthora fruit rot decreased with fruit age in `Dickenson Field' processing pumpkin, whereas `Golden Delicious' winter squash remained susceptible to fruit rot even as fruit reached full maturity. Lesion diameter and pathogen growth were generally greater on younger fruit than older fruit. Less than 15% of `Dickenson Field' fruit 21 dpp or older became diseased. Conversely, about 80% of `Golden Delicious' fruit 21 dpp or older became diseased. Several morphological and physiological changes were observed as fruit matured. Soluble solids content and exocarp firmness of both cultivars increased with fruit age, and were negatively correlated (ρ = -0.29 to -0.73) with lesion diameter and pathogen growth density. Field trials were conducted to evaluate the effects of bed height, mulches, dried poultry litter, and cultivars on Phytophthora crown and root rot of summer squash. Differences in cultivar susceptibility to P. capsici accounted for most of the variation in the observed disease levels. Mean incidence of plant death 35 dpi was 87% for `Payroll' and 99% for `Cougar'. Plant death of `Payroll' was greater in flat beds than raised beds. Disease was not affected by bed height, mulches, or rates of dried poultry litter application. Crown rot severity differed significantly among thirty-two summer squash cultivars and ten cucurbit germplasm accessions in a separate greenhouse trial. Mean crown rot ratings were 4.3 on commercial cultivars and 2.2 on germplasm accessions. Crown rot was least severe on the cultivar `Spineless Beauty'. No disease developed on four accessions of Cucurbita moschata (PI 442262, PI 442266, PI 458740, and PI 634693) previously reported to be crown rot resistant.
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