Reduced Sensitivity of Pseudoperonospora cubensis Clades 1 and 2 to Oxathiapiprolin in South Carolina

Abstract

Oxathiapiprolin has been the most effective fungicide active ingredient to manage cucurbit downy mildew, caused by Pseudoperonospora cubensis, since its introduction in 2015. However, oxathiapiprolin was not always the most effective treatment on cucumber in field plot experiments in 2020 and 2021. Potted cucumber, butternut squash, and watermelon treated with 0.13× or 0.25×, 0.5×, 1×, and 2× the maximum field rate of 17.5 g/ha of oxathiapiprolin were exposed to naturally occurring P. cubensis in the field in Charleston, South Carolina, in fall 2020, spring 2021, and fall 2021. On cucumber, mean severity was 18% at 0.25× in fall 2020 and 21% at 0.13× in fall 2021. On butternut squash, mean severity was 8.4% at 0.25× in fall 2020. The 1× field rate reduced severity compared to the nontreated control on all hosts. With cucumber and butternut squash, the slopes of the linear regression lines relating relative disease severity (disease severity on oxathiapiprolin-treated leaves divided by disease severity on water-treated control leaves) to log-10 concentration of oxathiapiprolin were greater than 0 (P ≤ 0.02). On watermelon, the slope was slightly greater than 0 (P = 0.085). Disease was more severe on cucumber infected with P. cubensis clade 2 than on squash infected with clade 1. EC50 values on cucumber were 4.5 and 13.9 ppm in spring and fall 2021, respectively. To slow development of resistance to oxathiapiprolin, cucurbit growers should increase the number of applications of other fungicides applied in rotation with oxathiapiprolin.