Cucumber downy mildew disease confirmed in research plot in southwest Michigan

This is the first confirmed report in Michigan in 2019. Time to switch to proven downy mildew fungicides.

Downy mildew symptoms on cucumber.
Downy mildew symptoms on cucumber. Photo by Mary Hausbeck, MSU.

Downy mildew on cucumber has developed in a small research plot at the Southwest Research and Extension Center in Benton Harbor, Michigan. This is the first confirmed report in Michigan in 2019. Disease symptoms developed over the last several days and this research plot had not been sprayed with any fungicides. There is not a spore trap at this site. There is a spore trap in Berrien County and that spore reel will be collected today (July 29) and analyzed. Results from Berrien County and others where we have placed spore traps will be posted on our Downy Mildew News page as soon as the spore reels have been processed.

The recent stretch of humid and wet weather favors downy mildew and the more moderate temperatures forecasted will be just what the pathogen prefers. Last week, I suggested growers could use broad-spectrum fungicides including Gavel, Zing!, chlorothalonil or mancozeb as downy mildew had not been detected in the state. It is time to move to a stronger downy mildew program with fungicides that have shown good activity in our yearly research plots including:

  • Ranman + chlorothalonil or mancozeb
  • Orondis Opti (chlorothalonil is part of the premix)
  • Elumin + chlorothalonil or mancozeb
  • Zampro + chlortothalonil or mancozeb

Alternate among these four fungicides using a seven-to-10 day application interval. Growers on the west side of the state should be using the seven-day application interval. Growers on the east side of the state could consider a 10-day application interval until the pathogen is found there. Remember, applying fungicides when downy mildew is already well established in the field is not recommended because it can be too late to protect the crop and contributes to the downy mildew pathogen developing resistance to our most important fungicides.

We will be watching all our production areas carefully for the movement of downy mildew into these regions. We are working with growers, scouts, consultants and MSU Extension educators to make sure we get suspect samples immediately and make a diagnosis. Click here for instructions on how to submit samples.

Also, keeping an eye on the spore traps totals across the state could be helpful in knowing when the cucumber downy mildew pathogen is in your production regions. The cucumber downy mildew pathogen does not overwinter and the spores move via air currents. Some years, we’ve had cucumber downy mildew disease in the state as early as mid-June. In many years, the first cucumber downy mildew symptoms were detected the week of July 4.

Hop downy mildew spores are being found in our spore traps but do not pose any risk to our cucumber industry. Unlike the cucumber downy mildew, the hop downy mildew overwinters in Michigan. It makes sense that we would find hop downy mildew spores early in spring and summer since it is present in the hop plants through winter. Previously, we would not have known whether these spores were cucumber or hop downy mildew as they look identical. Integrating molecular tools into our spore trapping efforts enable us to know if the spores that we are seeing via the microscope are the cucumber or hop downy mildew pathogen.


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