Jul 1, 2020
Cucumber downy mildew moves across Michigan

Downy mildew on cucumber plants has been verified on two farms in Monroe County, which is located in southeast Michigan. This is the first confirmation of this devastating disease on the east side of the state for 2020.

Our spore trapping network detected downy mildew spores from our two spore traps on this east side of the state for several days last week but we did not find diseased plants in nearby cucumber fields until the afternoon of June 29.

The disease outbreak consisted of several plants with the characteristic “window-pane” yellow-brown lesion bordered on each side by the leaf veins (Photos 1-4).

Our spore trap webpage is updated with the latest results on Wednesday morning of each week. The counts of the downy mildew sporangia from a field are always a few days behind due to processing of the spore tapes. We are able to distinguish between cucumber and hop downy mildew spores using molecular tools and both are reported.

While the current hot and sunny weather is not optimal for downy mildew development, some growing areas are experiencing dews and light rainfall that can 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)
  • Elumin plus chlorothalonil or mancozeb
  • Zampro plus chlorothalonil or mancozeb
  • Previcur Flex plus chlorothalonil or mancozeb*

*Note: Previcur Flex is back in the downy mildew fungicide recommendations for 2020 as it has been effective in the Michigan State University research trials in 2018 and 2019. Prior to that, it had not been effective. Since we cannot yet predict whether this year’s downy mildew population will be controlled by Previcur Flex, be sure to use it as a tank mix with either chlorothalonil or mancozeb.

Alternate among these recommended fungicide treatments using a seven-day application interval. Growers across the state should consider using a seven-day application interval for their fungicide sprays. Many of the pickling cucumber crops are still quite young and should be protected as a downy mildew infection at this stage would likely hurt yields significantly. In previous years, cucumber seedlings have become infected with downy mildew. Delaying 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 extension educators to make sure that we get suspect samples immediately and make a diagnosis. See 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. See spore trap results and current downy mildew news.

Note: 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.

, Department of Plant, Soil and Microbial Sciences 

Photos 1-2: Early symptoms of downy mildew on cucumber with the yellow-brown tissue bordered by the leaf veins. Photos 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: David Perla/MSU

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