Efficacy of Fungicides for Pseudoperonospora cubensis Determined Using Bioassays over Multiple Years in the Mid-Atlantic and Northeastern United States

In the United States, fungicides are the primary management option for cucumber growers to protect their crops from Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew. Pathogen resistance to some fungicides can quickly develop with the repeated applications needed to protect yield. In order to determine fungicide efficacy and monitor it over time, bioassays were conducted from 2016 to 2019 in Delaware, Maryland, Pennsylvania, and New York. Potted cucumber plants were either sprayed with fungicides or not treated, placed next to field-grown plants with cucurbit downy mildew for up to 2 days, and then kept in a greenhouse until symptoms developed. Severity of symptoms or number of lesions on leaves was recorded 6 to 14 days after exposure started and used to deter-mine fungicide efficacy. Quadris (azoxystrobin) was ineffective in seven of the nine bioassays, and Revus (mandipropamid) was ineffective in six of seven bioassays. Forum (dimethomorph) and Presidio (fluopicolide) were ineffective in three of eight and four of nine bioassays, respectively. The most effective fungicides were Bravo (chlorothalonil), Zing! (zoxamide 1 chlorothalonil), and Orondis (oxathiapiprolin), all of which consistently suppressed disease severity more than 90% when compared with the untreated control. Previcur Flex (propamocarb hydrochloride) and Ranman (cyazofamid) were also effective in every bioassay.