Downy mildew of watermelon has been observed on watermelon in Knox County in southwestern Indiana. Downy mildew of cucurbits has also been reported in southwestern Michigan on the Indiana border and central Missouri. All cucurbit growers in Indiana should be scouting and managing for downy mildew.
The organism that causes downy mildew of cucurbits doesn’t overwinter in Indiana because it requires living plant tissues. That means that the fungus-like organism that causes downy mildew 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.
On pumpkin and cucumber, downy mildew causes angular yellow lesions on leaves (Figure 1). Lesions on cantaloupe and watermelon tend to be diffuse and amorphous. On any host, the lesions may coalesce, producing large areas of diseased tissue that may turn brown. On wet mornings or after a rain, downy mildew lesions on the undersides of leaves may be covered with a dark “fuzz”— the result of spore production (Figure 2). Downy mildew does not cause lesions on stems or fruit.
Figure 1. Downy mildew of (A) cucumber, (B) cantaloupe, (C) pumpkin, (D) watermelon.
Figure 2. Under moist conditions, downy mildew will cause a dark fuzzy growth on the bottom of leave, as seen here for watermelon.
Wind can easily spread downy mildew spores to other leaves, nearby plants, and more distant fields. The fungus- like organism can rapidly multiply and affect large areas of a field when conditions are favorable—100 percent humidity for at least six hours, with temperatures between 59°F and 68°F.
Many 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 decision of whether to apply fungicides for downy mildew of cucurbits depends on the value of the crop and how close the crop is to the final harvest. Since fruit are not directly affected, a crop that is within 2 weeks of harvest may not warrant a fungicide application. However, downy mildew can cause defoliation which in turn can decreased fruit size and quality in a crop that requires some time to mature.
The Midwest Vegetable Production Guide for Commercial Growers lists several products that will help to slow the progress of downy mildew of cucurbits. Among the products listed as likely to be effective against downy mildew include: Elumin®, Gavel®, Orondis Opti®, Orondis Ultra®, Ranman®, Zampro® and Zing®. 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.
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.
More information about downy mildew of cucurbits can be found at this link https://www.extension.purdue.edu/extmedia/BP/BP-140-W.pdf