Congruent and Differential Responses of Pseudoperonospora cubensis Clades 1 and 2 to Downy Mildew Fungicides

Fungicides are the most common and important management technique for cucurbit downy mildew. Fungicide efficacy against the causal agent Pseudoperonospora cubensis, an obligate biotroph, has been monitored on potted cucumber seedlings exposed to natural inoculum. The objective of this study was to compare the efficacy of nine fungicides on cucumber (Cucumis sativus) against Clade 2 of P. cubensis with their efficacy on butternut squash (Cucurbita moschata) and watermelon (Citrullus lanatus), hosts primarily infected by Clade 1. Bioassays were done in Charleston, SC, United States, with all three hosts in October 2019 and 2020 and with butternut squash in July 2018. Lack of efficacy, determined as relative disease severity not significantly different from the water control, typically >= 50%, was detected with cymoxanil (Curzate), azoxystrobin (Quadris), dimethomorph (Forum), mandipropamid (Revus), or propamocarb (Previcur Flex) more frequently in Clade 2 isolates (7 of 18 times) than in Clade 1 isolates (12 of 43 times). Cymoxanil, azoxystrobin, and dimethomorph were ineffective in over half of the bioassays on all hosts. Mandipropamid was ineffective only against Clade 2 in both cucumber assays, whereas propamocarb was ineffective only against Clade 1 in two of three and one of two bioassays on butternut squash and watermelon, respectively. In all seven bioassays, ethaboxam (Elumin) was effective to moderately effective, and cyazofamid (Ranman), fluazinam (Omega), and fluopicolide (Presidio) were effective.