Downy mildew has been observed in Adam County in northern Indiana on cucumber. In addition, downy mildew on cucumber has been reported in southern Michigan and northern Ohio. Growers in northern Indiana should manage for downy mildew on valuable cucurbit crops. Growers throughout the state should scout for the disease. Growers should assume that all cucurbit crops may be affected. The cool, foggy mornings we have had recently are conducive 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. Previcur Flex® is recommended for downy mildew, but is not labeled for Phytophthora blight. 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.
Note that downy mildew of soybeans is caused by a different pathogen.
To see a map of downy mildew of cucurbits online go to https://cdm.ipmpipe.org/.