Cucurbit downy mildew map – click on map to enlarge it. https://cdm.ipmpipe.org/
Downy mildew was found on cucumbers in Haldimand-Norfolk, Ontario on July 1 and Chatham-Kent, Ontario on July 4. Even though it has been hot recently, cooler nighttime temperatures and cloudy skies can allow the spores of the downy mildew pathogen to survive during transport from source plants in Ontario on air currents, and to infect cucumbers and melons. The spores are usually deposited on plants during rainfall events. In our experience, we can find downy mildew in northern Ohio 1-3 weeks after outbreaks in these counties in Ontario, depending on the weather. Normally we only see Clade 2 isolates in northern Ohio during this time. Clade 2 isolates attack only cucumbers and melons e.g. cantaloupe, and circulate primarily in the Great Lakes region. Clade 1 isolates attack watermelon, pumpkins, squash and other cucurbits and work their way up the east coast from Florida and into the eastern parts of the Midwest. We see them mainly in southern and central Ohio in August or later in most years. The clades of the downy mildew pathogen differ somewhat in sensitivity to fungicides, so Ohio growers should use recommendations for fungicides from labs in this region.
Before symptoms appear in northern Ohio, cucumbers and melons should be protected with a chlorothalanil or mancozeb product. Fields should be scouted often; once downy mildew is reported in the northern Ohio counties, growers should add an effective downy mildew fungicide to their protectant (chlorothalanil or mancozeb) spray program. Good options for Ohio are Orondis Opti (FRAC 49+M05; contains chlorothalanil so don’t tank mix with additional chlorothalanil), Previcur Flex (FRAC 28), Elumin (FRAC 22), Ranman 4SC (FRAC 21), Omega (FRAC 29), and Zampro (FRAC 45+40). For more information on these fungicides, see here (OSU data until 2021) and here (Michigan State University 2022).
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 growers thanks to a grant from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.