Downy mildew has been observed on cucumber in Berrien County in extreme southwestern Michigan and in Monroe County in extreme southeast Michigan. The downy mildew spores that cause disease on cucumber will cause disease on cantaloupe and may cause disease on other cucurbits such as pumpkin and watermelon. The forecast is for the spread of the disease to move in a east to southeasterly direction. Therefore, cucurbit growers in northern Indiana should scout and manage for downy mildew.
The organism that causes downy mildew of cucurbits doesn’t overwinter in Indiana. It has to be blown in every year. It is common for downy mildew to start the season in the Gulf States and migrate north with the cucurbit crops. Downy mildew apparently overwinters in northern Michigan/southern Ontario in greenhouses where cucumbers are grown year-round. Therefore, downy mildew is often found in Michigan before it is found in Indiana.
For infection to occur, free moisture must be present on leaves for at least 2 hours. The temperature optimum is from 59 to 68 degrees F, however, disease can occur in much warmer temperatures.
Some cucumber varieties have some resistance to downy mildew. For susceptible cucumber varieties or other types of cucurbits, specialized systemic fungicides will help to reduce the severity of downy mildew. Unfortunately, many of the most effective systemic fungicides for downy mildew are not effective on our more common cucurbit diseases. This is because the organism that causes downy mildew, Pseudoperonospora cubensis, is not really a fungus at all. P. cubensis is more closely related to a brown alga. This fungus-like organism is related to the organism that causes Phytophthora blight (Phytophthora capsici). Therefore, many of the same fungicides that are effective against downy mildew are also effective against Phytophthora blight.
The Midwest Vegetable Production Guide for Commercial Growers lists several products that will help to slow the progress of downy mildew of cucurbits. Contact fungicides such as those with the active ingredient chlorothalonil or mancozeb products may slow down the disease. Systemic products that are listed include: Elumin®, Forum®, Gavel®, Omega, Orondis Opti®, Orondis Gold®, Orondis Ultra®, Presidio, Ranman®, Zampro® and Zing®. Products with the active ingredient phosphite may be helpful. Be sure to check the label for the re-entry interval, the pre-harvest interval, the FRAC group and other important information. Always alternate FRAC groups.
To see forecasts of downy mildew of cucurbits online go to https://cdm.ipmpipe.org/.
One other item of interest: Downy mildew of cucurbits is not caused by the same organism which causes downy mildew of soybeans. Therefore, downy mildew of soybeans will not spread to the pumpkin field immediately adjacent.
Below, find a photo of downy mildew of cucumber (Figure 1). Note that the yellow lesions are scattered across the leaf, not concentrated on the edge of the leaf. Under moist conditions the underside of the lesions will have the dark, fuzzy growth of the fungus-like organism that causes downy mildew.
Here is a link to the original article from Michigan State Extension.
https://www.canr.msu.edu/news/cucumber-downy-mildew-outbreak-of-2020
Figure 1. Downy mildew of cucumber.