Cucurbit Downy Mildew Update: July 2, 2025

On July 1, cucurbit downy mildew was confirmed on cucumber in Lancaster County, PA. Last week there were also confirmed reports from four counties in southern Michigan (see map below). This means that there are sources of the pathogen that can infect cucumber and cantaloupe coming from the west and southeast, depending on the forecasted weather conditions. It is highly recommended that all cucumber and cantaloupe crops be protected with fungicides, even those being grown in high tunnels. The high relative humidity in a high tunnel can be enough for downy mildew to develop. In order to be used in a high tunnel, the product must be labeled for greenhouse use. See Table 4.4 in the 2020-21 Mid-Atlantic Vegetable Production Recommendations  for a listing of products labeled for greenhouse production.

The closest report on a cucurbit crop other than cucumber and cantaloupe is in South Carolina on watermelon and acorn squash. The strain of the pathogen that infects pumpkin, squash, and watermelon is different from the one that infects cucumber and cantaloupe. This means that the downy mildew on your cucumbers will not infect your pumpkin or butternut squash.

It is important to be able to distinguish between downy mildew (oomycete pathogen – yellow/tan lesions on upper leaf surface and darker purplish sporulation on lower leaf surface), powdery mildew (fungal pathogen – white colonies on upper and lower leaf surface), and angular leaf spot (bacterial pathogen—no spores on the underside of the leaf). To further test suspicious leaves, place a few in a plastic bag with a moist paper towel and let them sit on the countertop overnight. The next day, observe leaf undersides for spores. A hand lens with at least 10x power is helpful when looking for spores. Although some protectant fungicides may help protect to varying degrees against these diseases, the recommended targeted products differ—especially for powdery and downy mildews.

Extension educators are visiting farms and scouting for disease. Please email Leah Fronk at lxf339@psu.edu to contribute to this report. The next report will be posted on July 9, 2025.