Cucumber Downy Mildew Confirmed in Ontario – Cucumber and Melon Growers in Northern Ohio Should Intensify Scouting

Downy mildew was reported on cucumbers in Ontario today. This is the first report for 2021 of cucurbit downy mildew in the Great Lakes region. There have been no reports of cucurbit downy mildew in Ohio or Michigan, although the Hausbeck lab at Michigan State University has detected spores of the pathogen in their spore traps for several weeks now. The storm system currently in the upper Midwest is moving in an east/southeast direction, and as it moves through Ontario may pick up spores and drop them further south. Historically cucumbers in the northern approx. 1/3 of Ohio have been the first in the state to show symptoms of downy mildew. Growers should intensify scouting of cucumbers and melons and apply a protectant

Top left: early downy mildew lesion on cucumber; top right: advanced cucumber downy mildew; lower left: underside of cucumber leaf with spores of the pathogen; lower right: microscopic image of downy mildew spores.

fungicide such as chlorothalanil. Look for yellow or tan angular lesions delimited by veins on the top surface of leaves, and sparse fuzzy 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.

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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