Downy mildew outbreak on cucumbers in southwest Michigan

Immediate action against downy mildew in cucumbers across the entire state is recommended.

Downy mildew on cucumbers was reported in Berrien County in the southwest portion of Michigan and confirmed by the Hausbeck lab this afternoon (July 24). View the Michigan downy mildew map. This most recent report is a severe outbreak with 30 percent of a 30-acre cucumber field infected. Heavy sporulation was noted and the infected plants show obvious blighting.

With a recent downy mildew outbreak reported in northeast Michigan, it is clear that all Michigan cucurbit growers are now at risk and should not delay in moving forward with preventive fungicide sprays. Cucumber growers are most likely to suffer losses from downy mildew as this crop is highly susceptible to the disease. Growers of watermelon and cantaloupe are also at risk as these crops are also highly susceptible. In past years, summer and winter squashes and pumpkins have had a few scattered reports of downy mildew in the eastern United States, but these crops appear less susceptible to the disease than the cucumbers, watermelon and cantaloupe. See Photos 1-4 for symptoms of downy mildew on leaves of melons and squash.

Downy mildew on cucumber
Photo 1. Downy mildew on cucumber.

Downy mildew on cantaloupe
Photo 2. Downy mildew on cantaloupe.

Downy mildew on watermelon
Photo 3. Downy mildew on watermelon.

Downy mildew on yellow squash
Photo 4. Downy mildew on yellow squash.

Since 2005, the Hausbeck lab has tracked the downy mildew outbreaks in Michigan at www.veggies.msu.edu. It is important to the cucurbit vegetable industry in the state that we monitor and report all downy mildew outbreaks in both commercial fields and home gardens. It is also important that the commercial growers strictly adhere to the recommended fungicides that have been tested repeatedly in Michigan field trials. See tables for recommended fungicides for cucumber and other vine crops.

A misstep in the fungicide program through the use of an ineffective downy mildew product could mean crop loss. Each year, my program field tests dozens of products. The products that work are listed and haven’t changed too much over the last few years. Remember the old adage, if something, such as a “new” fungicide, sounds too good to be true, it probably is too good to be true. Also, remember that EPA does not require a company to prove that their product works before making claims on that product’s label.

Michigan growers are not novices at managing this disease and recognize that early action, effective fungicides and short spray intervals can win the day and protect the crop. Homeowners should rely on fungicides for cucurbit vegetables that contain chlorothalonil as an active ingredient. Organic growers could use an approved formulation of a copper-based material to help slow the progression of the downy mildew.

For more information, go to www.veggies.msu.edu and view instructions for reporting downy mildew.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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