Severe cucumber downy mildew in Medina County, OH, July 24, 2023. Photo by Frank Becker.
Map of cucurbit downy mildew outbreaks. First reports for this year in Ohio: Medina, Wayne and Knox counties. cdm.ipmpipe.org.
Cucumber downy mildew was confirmed yesterday in three Ohio counties (Wayne, Medina and Knox), following a report last week from Michigan and several weeks ago from Ontario. While these reports are later in July than average for Ohio, it is likely that infections occurred at least a week earlier. Thanks to OSU’s Plant Pest and Disease Clinic Director Dr. Francesca Rotondo and Wayne County Extension Educator Frank Becker for these finds. Knox County is in central Ohio, so I’ll amend my July 19 post to include cucumber and melon growers statewide, who should add fungicides very effective against downy mildew to their spray program now if they have not already done so. Waiting until symptoms appear may be too late to avoid yield losses; effective fungicides should be applied preventatively. The best ones, according to research in Ohio, Michigan and other Great Lakes states and provinces are Orondis Opti (FRAC 49+M05), Ranman (FRAC 21), Omega (FRAC 29), Previcur Flex (FRAC 28), and Elumin (FRAC 22). These should be tank mixed with chlorothalanil (Bravo, Equus, etc.) or mancozeb (Dithane, Manzate, etc.). Orondis Opti is a premix already containing chlorothalanil, but at a reduced rate. Fungicides have restrictions on how much product can be applied and how often, so follow the label. The more effective fungicides should be rotated to avoid resistance development in the pathogen. More information can be found here and here.
See my June 24, 2023 post for pictures of symptoms and instructions for submitting live or digital samples to OSU for diagnosis. Diagnosis is free for Ohio vegetable growers thanks to a grant from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.