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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.
- Pale green to yellow spots form on upper surface of leaves, and later turn brown.
- Leaf spots are angular bounded by leaf veins. This is most distinct in cucumber.
- Dark purplish grey fuzz forms on underside of the leaf in high humidity.
- In wet or very humid conditions, disease progresses rapidly. Leaf spots grow together and entire leaves turn brown. Often appearing as if they were killed by frost.
- On watermelons, an exaggerated upward leaf curling is common.
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.
- Plant resistant varieties when available. Cucumber varieties with moderate to high level of resistance are available, moderate to low levels of resistance is available for melons. Some resistance available in pumpkins and winter squash but this tends to be specific for certain pathotypes of Downy Mildew and may not be effective in all cases.
- Use drip irrigation and wide row spacing to promote leaf drying and encourage good air movement around the plants.
- Monitor plants for symptoms of disease especially from August through harvest.
- If the disease is found in a home garden, plants should be immediately removed and destroyed to prevent the spread to other plants.
- Fungicides are effective if applied before disease becomes severe. Both contact and systemic fungicides are registered for use against downy mildew. Systemics are more effective if weather conditions are conducive to disease and the host is very susceptible. However using the same systemic fungicide repeatedly can result in fungicide resistance. It is important to rotate systemic fungicides or tank mix with a contact to avoid this.
Commercial growers should read and follow all application instructions in the Midwest Vegetable Production Guide.