Cucurbit downy mildew was just confirmed on a pickling cucumber sample from our sentinel plot in Dane County, Wisconsin. This is the first confirmation of this disease in Wisconsin for 2021. This non-fungicide-treated plot includes multiple cucurbit types (winter squash, summer squash, pumpkin, cucumber, watermelon, cantaloupe, gourd) for the purpose of twice-a-week evaluations for downy mildew. The tops of leaves exhibited pale angular lesions and sporangia were present on leaf undersides after 24 hour incubation in a moist chamber. Sporangia were abundant. No other cucurbits showed visible symptoms of downy mildew in the sentinel plot.
The finding of downy mildew on cucumber indicates that this is Clade 2 of the pathogen which can infect cantaloupe, cucumber, and Buffalo gourd. There are some fungicide resistance concerns with this Clade 2. Please see the figure, below, for disease management guidance. I will keep an eye on our other cucurbits (Clade 1) as this group includes winter squash and pumpkins and these crops will likely see another month in the field and require foliar management.
Please contact me or our UW Plant Disease Diagnostic Clinic https://pddc.wisc.edu/ if you suspect cucurbit downy mildew. Confirmation of this disease is very important for appropriate management.