Response of Pseudoperonospora cubensis to Preventative Fungicide Applications Varies by State and Year

Abstract

A cucumber bioassay to assess the response of Pseudoperonospora cubensis to preventative applications of fungicides is a useful, rapid method to determine which fungicides are most likely to be effective at the beginning of a cucurbit downy mildew epidemic.In bioassays on cucumber (Cucumis sativus) in New York, Ohio, and South Carolina in 2015 and 2017, mandipropamid (Revus) and azoxystrobin (Quadris) failed to suppress downy mildew caused by Pseudoperonospora cubensis. Propamocarb (Previcur Flex), cymoxanil + famoxadone (Tanos), and fluopicolide (Presidio) were ineffective in one or two states in both years. Famoxadone, dimethomorph (Forum), cymoxanil (Curzate), and mancozeb (Manzate) were ineffective in one state one year. Oxathiapiprolin, cyazofamid (Ranman), and fluazinam (Omega) suppressed downy mildew in ≥80% of the bioassays.