Characterizing the Economically Important Species of \u3ci\u3ePythium\u3c/i\u3e Affecting Cucurbits in South Carolina and Grafting to Control Pythium Disease in Watermelon

The plant family Cucurbitaceae, informally referred to as cucurbits, is the largest family of cultivated vegetables with 118 genera and 825 species. In South Carolina, the most commonly grown cucurbit crops are watermelon (Citrullus lanatus) (1,982 ha), squash (Cucurbita pepo) (643 ha), cucumber (Cucumis sativus) (398 ha), and cantaloupes and muskmelons (Cucumis melo) (365 ha). In cucurbits, Pythium spp. cause symptoms of root rot, stem rot, and damping-off. Four different cucurbit species were planted repeatedly in sentinel plots in nine fields over three years to isolate and identify Pythium species pathogenic to cucurbits in South Carolina. Isolates (600) were recovered from symptomatic seedlings and identified by sequencing DNA of the mitochondrial cytochrome oxidase I region. The four most common species identified were P. spinosum (45.6% of isolates), P. myriotylum (20.0%), P. irregulare (15.3%), and P. aphanidermatum (12.8%). These four Pythium species were recovered from every cucurbit host each year. P. myriotylum and P. aphanidermatum were predominantly isolated in warmer months, whereas P. spinosum and P. irregulare were predominantly isolated in cooler months. Cool-season species of Pythium were more virulent than warm-season species at 25°C, an average soil temperature for spring cucurbit planting dates in South Carolina. Representative isolates from each species (63 total) were sensitive to two standard fungicides, mefenoxam and propamocarb. However, these same isolates were insensitive to the new fungicide oxathiapiprolin, except those classified taxonomically as species in Pythium Clade I, which includes P. ultimum. Cucurbit rootstocks used in watermelon grafting were evaluated for resistance to pathogenic species of Pythium. Watermelon cultivars were more susceptible to P. myriotylum and P. aphanidermatum than were cultivars of bottlegourd and interspecific hybrid squash (Cucurbita maxima × C. moschata) at both 20°C and 30°C in a growth chamber experiment. Interspecific hybrid squash cultivars were less susceptible to infection by these two species of Pythium than were bottlegourd and watermelon cultivars in a field experiment. The seedless watermelon cultivar Tri-X 313 was grafted to citron, bottlegourd, and interspecific hybrid squash rootstocks, and the fungicides mefenoxam (Ridomil Gold), propamocarb (Previcur Flex), and mefenoxam and propamocarb together were applied at transplanting to compare grafting and commercial fungicide applications for disease management. Grafting to interspecific hybrid squash rootstocks reduced disease incidence compared to non-grafted controls. However, there were no differences in disease development between fungicide and non-fungicide-treated plots. Grafting to the interspecific hybrid squash cultivar Camelforce significantly increased yield and lowered disease development when compared to non-grafted control plants. Interspecific hybrid squash was consistently resistant to inoculation with P. myriotylum and P. aphanidermatum isolates both in the field and growth chamber experiments, demonstrating its resistance and utility as a rootstock for grafting watermelon plants