Downy mildew confirmed on cucumbers in four Michigan counties

Growers are urged to implement an aggressive fungicide program immediately

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

The downy mildew pathogen has had the upper hand in recent weeks with the overcast and humid weather that our production regions have experienced. With humid and overcast weather conditions likely today (July 23) and tomorrow over parts of Michigan, the risk of significant pathogen spread and crop infection greatly increases. While the first pickling cucumber crop of 2021 has been harvested with little disease issue, the subsequent pickle crops are going to be a challenge given the amount of downy mildew that has been reported in the state during the last few days.

The outbreak had been concentrated in pickling cucumber crops in Saginaw, Tuscola and Sanilac counties. However, last night, a positive sample came in from a homeowner’s garden in Ottawa County, indicating the pathogen is also on the west side of the state. In one case, the cotyledons of an emerging cucumber crop have downy mildew symptoms and abundant pathogen sporulation. Several years ago, our state experienced a similar situation which proved to be very challenging and expensive. The cucumber yield is most likely to be impacted when the plants become infected early and the pathogen is not controlled.

Our list of proven effective fungicides is shrinking as the downy mildew pathogen becomes resistant to those fungicides that had previously been highly effective. Each year, this situation becomes more worrisome. Our remaining downy mildew active ingredients must be used in a tank-mix with the protectant fungicides of either chlorothalonil (Bravo WeatherStik is an example) or mancozeb (Roper is an example). This year, we will likely need an aggressive spray program that alternates among our proven effective fungicides that include Orondis Opti, Ranman, Zampro, and Orbus 4F (a generic form of fluazinam). Please see below.

  • Ranman + chlorothalonil or mancozeb
  • Orondis Opti (chlorothalonil is part of the premix so add 1 pint of Bravo WeatherStik)
  • Zampro + chlorothalonil or mancozeb
  • Orbus 4F + chlorothalonil or mancozeb. Note: Orbus 4F has the same active ingredient (fluazinam) as Omega and has a seven-day preharvest interval. The 24 fluid ounce rate is recommended.

Due to the outbreak in the state and the weather that is favorable for disease development and pathogen spread, a spray interval of five days is recommended. This shorter interval should not be an issue as long as the recommended fungicides are used in alternation. Look at the fungicide labels carefully to ensure that you’re within the guidelines; the label is the law! The fluazinam active ingredient has proven effective in the MSU spray trials each year but was previously available only as Omega, which is too costly for use in pickling cucumbers. There are now generic products available that are less expensive and can be used in our cucumber downy mildew spray programs. Orbus 4F is one of these products and is available from the company Atticus, but there may be other generic products available in various regions that are not listed on the CDMS website of crop labels.

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 air samples in a particular production area. Go to MSU Downy Mildew News for spore trap results and current downy mildew news. Recently, the air samples from fields located in Bay, Saginaw and Monroe counties were all positive for the downy mildew pathogen and these results were posted on the lab’s website on Wednesday. On the west side of the state, the traps are placed in fields located in Muskegon, Allegan and Berrien counties; airborne downy mildew sporangia were not detected in these traps last week.

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