Pythium spp. cause root and stem rot in cucurbits, but no formal surveys have been conducted to identify which species are responsible. A panel consisting of bottlegourd, cucumber, Hubbard squash, and watermelon was transplanted in May, July, September, and November into four and five different fields in 2017 and 2018, respectively, in South Carolina to identify associated Pythium spp. Isolates (524) were collected and identified by sequencing the mitochondrial cytochrome oxidase I region. Of the 11 species recovered, the 4 most common were P. spinosum (38.9% of isolates), P. myriotylum (23.1%), P. irregulare (16.4%) and P. aphanidermatum (15.1%). P. myriotylum and P. aphanidermatum were predominantly isolated in May through September, whereas P. spinosum and P. irregulare were predominantly isolated in November. Representative isolates (63) from each species were screened in vitro against oxathiapiprolin. All isolates were insensitive to oxathiapiprolin, except those of P. ultimum var. ultimum and P. sylvaticum, both members of Pythium Clade I. Additional isolates of remaining Pythium Clade I species were obtained. EC₅₀ values for Clade I species ranged from 0.50 to 1.05 µg/ml. Four Pythium spp. made up 93.5% of all isolates recovered from diseased cucurbits, and the community of pathogenic Pythium spp. changed throughout the year. Oxathiapiprolin is effective for managing many oomycetes but appears to selectively target only those Pythium spp. in Clade I.