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Downy mildew

Michelle Grabowski

leaf with yellow splotches

M.Grabowski, UMN Extension

leaves covered almost entirely in yellow

M.Grabowski, UMN Extension

Downy Mildew, caused by Pseudoperonospora cubensis, is an oomycete that is not a true fungus and is often referred to as a water mold due to the fact that it thrives in wet or very humid conditions. Downy Mildew can infect all cucurbits including cucumber, melon, pumpkin and squash. Although it can be a problem in field, hoop house and greenhouse conditions, it has not yet been reported in Minnesota.


Important biology

Downy mildew does not create viable oospores (thick walled resting structures) that would allow it to survive in Minnesota's harsh winter. It cannot survive on plant debris and only grows on living plant tissue. This means that in order for downy mildew to occur in MN, spores must be blown in on air currents. In eastern states with similar conditions, downy mildew does not arrive until the end of the growing season, often in August. In recent years, however, downy mildew has been making its way to the Midwest earlier in the season. This early infection is possibly due to a change in the pathogen's biology or due to greenhouse production of cucurbits that is allowing the pathogen to overwinter on living plants.

Downy mildew can start an infection in a wide range of temperatures (41-86F) but is most severe from 59-68F. The pathogen needs moisture on the leaf surface in order to germinate and start a new infection. Under humid conditions downy mildew rapidly reproduces and spreads, resulting in severe crop damage. The pathogen can move on air currents, splashing water and on the tools and hands of workers.

There are several pathotypes or strains of downy mildew that attack different crops. It is not uncommon to see a healthy pumpkin field alongside a severely diseased cucumber field for this reason.


Commercial growers should read and follow all application instructions in the Midwest Vegetable Production Guide.

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