Cucurbit Downy Mildew

Downy mildew has been observed in LaPorte County in northern Indiana on cucumber. In addition, downy mildew on cucumber has been reported in southern Kentucky and on watermelon in the Kansas City area of Kansas. Growers in northern Indiana should manage for downy mildew on valuable cucurbit crops (Figure 1). Growers throughout the state should scout for the disease. Note that the Kansas report is directly west of us and could, eventually, blow this way. 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.

Downy mildew of watermelon

Figure 1. Downy mildew of watermelon causes chlorotic lesions that eventually turn necrotic.

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. Although it not listed in the Midwest Vegetable Production Guide, Previcur Flex is recommended this year by Michigan State University against downy mildew of cucumber; Previcur Flex 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.

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 cucurbit field immediately adjacent.

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