Jul 13, 2011

Downy mildew outbreak in southeast Michigan cucumbers

Downy mildew on cucumbers was reported in Carleton in southeast Michigan July 7. Click here to see the Michigan downy mildew map. It is not clear as to the extent of this outbreak or its severity. A picture associated with a news article about this outbreak would indicate that the infection is advanced. Regardless, with recent downy mildew outbreaks reported in Ontario, Ohio and Michigan, it is clear that Michigan cucurbit growers are 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 squash 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.

Since 2005, my lab has tracked the downy mildew outbreaks in Michigan on my webpage. 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 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 the 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 http://veggies.msu.edu/.

Photo 1. Downy mildew on cucumber. Photo credit: Mary Hausbeck, MSUE.

Photo 2. Downy mildew on cantaloupe. Photo credit: Mary Hausbeck, MSUE.

Photo 3. Downy mildew on watermelon. Photo credit: Mary Hausbeck, MSUE.

Photo 4. Downy mildew on yellow squash. Photo credit: Mary Hausbeck, MSUE.

By Mary Hausbeck, Michigan State University Extension