Cucurbit downy mildew map, July 11, 2022. cdm.ipmpipe.org
Today OSU plant diagnostician Francesca Rotondo diagnosed the first cucumber sample of the 2022 growing season with downy mildew. Downy mildew has been a bit slow to appear, likely due to the high temperatures and often sunny and dry conditions. Last year our first report of cucumber downy mildew in Ohio was on July 12. I expect that conditions last week – rain and overcast skies in northern Ohio -promoted spore transport, deposition and infection. The sample came from an organic farm in the Homerville area, in Medina County – it was just getting started in the field and disease incidence and severity were low. Cucumber and melon growers in northern Ohio should ramp up their spray programs to include highly effective fungicides against downy mildew such as Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin (see efficacy table below). Curzate was effective in our 2021 tests but has been variable in efficacy over the years. It is recommended to tank mix these products with chlorothalanil. Check the labels carefully for use instructions and restrictions. Remember to alternate products in different FRAC groups. Fungicides must be applied preventatively – they are far less effective if applied after infection.
Growers in central Ohio should intensify scouting of cucumbers and melons and apply a protectant fungicide. Look for yellow or tan angular lesions delimited by veins on the top surface of leaves, and fuzzy grey/brown growth on the undersides of the lesions. With a good hand lens or a smartphone camera with high magnification you may be able to see small dark brown/purple spots within the fuzzy growth. These are the spores of the downy mildew pathogen.
Smartphone image of a downy mildew lesion with the pathogen sporulating on the underside of a cucumber leaf.
If you suspect downy mildew in cucumber or melon please text or email pictures to Sally Miller (330-466-5249; miller.769@osu.edu) of both sides of lesions, with the underside in the highest possible magnification. I can often confirm downy mildew from photos, but if not will ask you to send a sample to the OSU Vegetable Disease Diagnostic Lab for confirmation. Instructions for sample submission are here. Thanks to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program, there is no fee for this service for Ohio vegetable growers.
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