Oxathiapiprolin, a novel oomycete fungicide recently registered by DuPont, was reported to have high intrinsic activity against cucurbit downy mildew (Pseudoperonospora cubensis). The goal of this study was to characterize disease control attributes of oxathiapiprolin-based fungicides critical to effective management of cucurbit downy mildew. In growth chamber and greenhouse studies, oxathiapiprolin-based fungicides were compared with mandipropamid, mefenoxam + mancozeb, fluopicolide + propamocarb, cymoxanil + mancozeb, and ametoctradin + dimethomorph products for pre- and postinfection activity, local systemic movement, and protection of new growth produced after fungicide application. In preventive application, oxathiapiprolin-based fungicides significantly (P < 0.0001) inhibited downy mildew development, with the highest level of disease observed being 0.4% compared with 86.7% observed for mandipropamid. When applied postinfection, oxathiapiprolin-based fungicides significantly (P < 0.0001) suppressed disease development, but disease control was reduced relative to that observed for preventive application. There was a significant effect of formulation on the postinfection activity of oxathiapiprolin, whereby the oil dispersion (OD) formulation was more inhibitory than the water-dispersible granule formulation (0.001 ≤ P ≤ 0.049). Disease severity on the outer half leaf portion protected from spray deposition during fungicide application was lower for oxathiapiprolin-based fungicides (1.6 to 6.6%) than observed for fluopicolide + propamocarb (10.9 to 23.7%), mefenoxam + mancozeb (40.3 to 51.4%), and the nontreated controls (83.3 to 84.9%), which indicates significant acropetal movement within the leaf. Postinfection applications of oxathiapiprolin-based formulations had the greatest effect on lesion growth and sporangia production compared with the other fungicides in the experiment. When applied preventively to rapidly growing plants in a greenhouse, oxathiapiprolin-based fungicides consistently protected new growth that was not present at the time of application, with the OD formulation reducing disease severity by >75% relative to nontreated plants. The practical implications of these observations are discussed.

Christopher P. Shepherd

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