Micrograph of a tape mount of spores and sporangiophores of the cucurbit downy mildew pathogen from cucumber leaves. Photo by Francesca Rotondo.
Downy mildew continues to spread in Ohio cucumbers despite the hot and mostly dry weather. Frank Becker, OSU Extension Wayne County IPM Program Coordinator, brought cucumber leaves with downy mildew symptoms to to our Vegetable Pathology Lab on July 23 for confirmation. We do this by placing a piece of scotch tape on the underside of a leaf lesion then transferring to tape to a glass slide and looking for characteristic spores and sporangiophores (branched, threadlike structures that produce the spores) under a microscope. The samples came from commercial cucumber fields in Wooster and Apple Creek in Wayne County, and both were positive for downy mildew.
Cucurbit downy mildew as of July 24, 2020. cdm.ipmpipe.org.
Although we have confirmed reports in only Medina and Wayne counties, cucurbit downy mildew is likely present in most northern Ohio counties. The map of downy mildew reports shows confirmed cases in Ontario, Michigan and western New York as well. All of these reports are from cucumbers; this clade, or strain of the pathogen affects cucumbers and cantaloupe, but not squash or pumpkins. We don’t expect downy mildew on squash and pumpkins until the other known clade, which has a broader host range, migrates to the Midwest from the Southeast.
Downy mildew in cucumber.
Fungicide recommendations are posted here. If you suspect downy mildew in any cucurbit, please send us a sample. This will help us track the disease and provide early warnings to growers to enable timely protection of cucurbit crops. Our diagnostic service is free to commercial growers in Ohio; gardeners may also send cucurbit downy mildew samples to us free of charge. Instructions for sample submission are posted here.