Downy mildew on cucumbers confirmed in eight Michigan counties

Farmers are urged to immediately implement an aggressive fungicide program

PUBLISHED ON July 22, 2024

EAST LANSING, Michigan – The downy mildew pathogen has gained the upper hand in our Michigan growing areas over the past few weeks due to cloudy, humid and wet weather. With more moderate temperatures forecast, the risk of significant spread of the pathogen and infection of crops increases significantly.

While the first cucumber crop of 2024 was harvested without significant disease problems, subsequent cucumber crops will be challenging given the amount of downy mildew reported in the state in recent days. The outbreak (Fig. 1) affects cucumber crops in Washtenaw, Saginaw, Bay, Tuscola, Iosco, Arenac, Muskegon and Ingham counties and has spread within a short period of time (16 days).

There are positive results from spore traps in the southwestern part of the state, suggesting that the pathogen is widespread. Even the cotyledons of a cucumber crop can become infected and show symptoms of downy mildew. A few years ago, our state experienced a similar situation that proved very difficult and costly. Cucumber yield is most likely to be affected if plants are infected early and the pathogen is not controlled.

The list of proven effective fungicides is getting shorter as the downy mildew pathogen becomes resistant to the fungicides that were previously very effective. This situation becomes more worrying every year. The remaining active ingredients against downy mildew must be used in a tank mix with the protective fungicides chlorothalonil (Bravo WeatherStik is an example) or mancozeb (Roper is an example). This year we will probably need an aggressive spray program that alternates between our proven effective fungicides.

It is important that cucumber growers use proven fungicides against downy mildew. These fungicides were effective on our research plots in 2023 and include:

• Elumin + chlorothalonil or mancozeb
• Omega (Orbus) + Chlorothalonil or Mancozeb
• *Orondis Opti (Chlorothalonil is part of the premix, additional chlorothalonil is recommended (maximum chlorothalonil amounts can be found on the label))
• Previcur Flex + Chlorothalonil or Mancozeb
• *Ranman + Chlorothalonil or Mancozeb
• Zampro + Chlorothalonil or Mancozeb

*Products considered particularly effective based on annual, season-long field trials in Michigan.

Due to the rapidly expanding outbreak in the state and weather that favors disease development and pathogen spread, the spray interval should not be extended beyond 7 days. This interval should not be a problem if the recommended fungicides are used in rotation. Look carefully at the fungicide labels to make sure you are following the guidelines; the label is the law!

By keeping an eye on the spore trap results, you can find out when and how frequently the pathogen that causes downy mildew on cucumbers is found in air samples in our main growing areas (Fig. 3). Check out the Downy Mildew News for spore trap results and current news on downy mildew.

This article was published by Michigan State University Extension.

—Mary Hausbeck and Evelyn Mongeon, Michigan State University Extension