Cucumber downy mildew disease confirmed in Michigan

First detection in 2021 made in Saginaw County.

Lesions and spots on cucumber leafs.
1, 2: Early symptoms of downy mildew on cucumber with the yellow-brown tissue bordered by the leaf veins. 3, 4: The dark spores of the cucumber downy mildew pathogen can be seen on the underside of the leaf. These spores move via air currents and infect unprotected plants. Photos by Mary Hausbeck, MSU.

On July 14, 2021, downy mildew affecting cucumber was verified on a farm in Saginaw County, which is in eastern Michigan approximately 80 miles northeast of Lansing, Michigan. This is the first confirmation of this devastating disease in the state for 2021. Our spore trapping network detected downy mildew spores a few times over the last few weeks. We did not have a spore trap in the field where downy mildew was confirmed.

The disease outbreak consisted of one plant with the characteristic “window-pane” yellow-brown lesion bordered on each side by the leaf veins (Photos 1-4). While other angular-shaped lesions were observed on many plants in this field, they were caused by a bacterium known to commonly occur on cucumbers. Our spore trap webpage is updated with the latest results on Wednesday afternoon each week and can be found on the Michigan State University Downy Mildew News website. We can distinguish between cucumber and hop downy mildew spores using molecular tools and both are reported.

The recent overcast and humid weather is optimal for downy mildew development; some growing areas are experiencing dews/rains that allow the pathogen to infect and cause disease.

It is very important cucumber growers use a proven downy mildew program with fungicides that have shown good activity in our yearly Michigan research plots, including:

  • Ranman plus chlorothalonil or mancozeb
  • Orondis Opti (chlorothalonil is part of the premix)
  • Zampro plus chlorothalonil or mancozeb

Previcur Flex is not recommended for downy mildew management in 2021 as it was not effective in the 2020 MSU research trials. It had been effective in the MSU research trials in 2018 and 2019.  Prior to that, it had not been effective. Since we cannot predict whether this year’s downy mildew population will be controlled by Previcur Flex, it has not been included in this year’s recommendations.

Alternate among these recommended fungicide treatments using a seven-day application interval. The pickling cucumber crops should be protected to prevent yield loss and a statewide outbreak. Delaying the use of fungicides until downy mildew is already well established in the field is not recommended because it can be too late to protect the crop and this delay contributes to the downy mildew pathogen developing resistance to our most important, effective fungicides. 

We are working with growers, scouts, consultants and Michigan State University Extension educators to get suspect samples submitted immediately for diagnosis. Click here for instructions on how to submit samples.  

Also, keeping an eye on the spore trap results across the state could be helpful in knowing when the cucumber downy mildew pathogen has been found in a particular production region. The cucumber downy mildew pathogen does not overwinter, and the spores move via air currents. Go to MSU Downy Mildew News for spore trap results and current downy mildew news.

Cantaloupe and watermelon crops are also at risk of becoming infected by the downy mildew pathogen and should be protected using the fungicides that have been proven to be effective in MSU research cucumber field studies.

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