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Downy mildew of cucurbits

Quick facts

  • Downy mildew can infect all cucurbits including cucumber, melon, pumpkin and squash.
  • It thrives in wet or very humid conditions as a water mold.
  • Pale green to yellow spots form on upper surface of leaves, and later turn brown.
  • The pathogen can move on air currents, splashing water and on the tools and hands of workers.
  • Plant resistant varieties when available.
  • Fungicides are effective if applied before disease becomes severe.
Green and yellow spots from downy mildew on a plant leaf

Downy Mildew, caused by Pseudoperonospora cubensis, is an oomycete that is not a true fungus. It thrives in wet or very humid conditions as a water mold. 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, there have not been any reports of it in Minnesota.

Identifying downy mildew symptoms

  • 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 develops rapidly. Leaf spots grow together and entire leaves turn brown. The leaves appear as if frost had killed them.
  • On watermelons, an exaggerated upward leaf curling is common.
Green and yellow spots from downy mildew on plant leaves

What causes downy mildew

Downy mildew does not create viable oospores (thick-walled resting structures) that would allow it to survive Minnesota's harsh winter. It cannot survive on plant debris and only grows on living plant tissue. For downy mildew to occur in Minnesota, air currents must blow the spores.

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 arrived in 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 allows the pathogen to overwinter on living plants.

Downy mildew can start an infection in a wide range of temperatures (41-86° F) but is most severe from 59-68° F. 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 possible to see a healthy pumpkin field alongside a severely diseased cucumber field for this reason.

Preventing and managing the disease

  • Plant resistant varieties when available.
    • Cucumber varieties with moderate to high level of resistance are available.
    • Moderate to low levels of resistance are available for melons.
    • Some resistant varieties are available for pumpkins and winter squash, but this is usually specific to certain pathotypes of downy mildew and may not be effective in all cases.
  • Use drip irrigation and wide row spacing to dry the leaves and encourage good air movement around the plants.
  • Monitor plants for symptoms of disease, especially from August through harvest.
  • If found in a home garden, remove and destroy diseased plants immediately to prevent spreading to other plants.
  • Fungicides are effective if applied before disease becomes severe.
    • Both contact and systemic fungicides are available for use against downy mildew.
    • Systemics are more effective if weather conditions are conducive to disease and the host is very vulnerable.
    • 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.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Michelle Grabowski

Reviewed in 2018

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