Downy Mildew of Cucurbits in Michigan

Cucurbit downy mildew has been observed on cucumber in the southwest corner of Michigan, just across the border from La Porte County and LaGrange Counties, Indiana. All cucurbit growers in Indiana should be scouting for downy mildew. Cucurbit growers in northern Indiana should be managing 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. In most years, the downy mildew fungus will blow from southern Michigan to Ohio before it tracks south.

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 algae. 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. Among the products listed as likely to be effective against downy mildew include: Elumin®, Forum®, Gavel®, Orondis Opti®, Orondis Gold®, 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.

An extension bulletin that describes downy mildew in more detail can be found at https://www.extension.purdue.edu/extmedia/BP/BP-140-W.pdf

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. Note that the yellow lesions are not concentrated on the edge of the leaf. (Figure 1) Under moist conditions the underside of the lesions will have the dark, fuzzy growth of the fungus-like organism that causes downy mildew (Figure 2).

Downy mildew of cucumber can be recognized by the yellow angular lesions on the top of the leaf.

Figure 1. Downy mildew of cucumber can be recognized by the yellow angular lesions on the top of the leaf.

Downy mildew of cucumber causes a fungal-like growth on the underside of the leaves in moist conditions.

Figure 2. Downy mildew of cucumber causes a fungal-like growth on the underside of the leaves in moist conditions. (Photo by L. Maynard).

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