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Powdery mildew on trees and shrubs

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Quick facts

  • Powdery mildew is a common fungal disease of trees and shrubs in Minnesota.
  • White to gray, powdery spots, blotches or felt-like mats form on leaves, stems and buds of infected plants.
  • Disease is often most severe on young leaves and green shoots.
  • Powdery mildew does not cause significant damage to the health of the tree and does not require management.
  • The disease can significantly affect the look of ornamental plants like roses and purple-leaved ninebark shrubs.
  • Resistant varieties, cultural control practices and fungicides can be used to manage powdery mildew.

How to identify powdery mildew

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Round black chasmothecia within white powdery mildew spots on viburnum leaves.

How does  powdery mildew survive and spread?

  • Powdery mildew fungi create dark round resting structures that contain and protect spores through the winter.
  • In spring, these resting structures break open, releasing spores that are spread by the wind. These spores start new infections on succulent, new growth.
  • Some species of powdery mildew fungi survive the winter in infected buds.
    • In spring, the young shoots growing from infected buds are covered with velvet-like white growth of powdery mildew.
  • The powdery mildew fungus grows into the plant to steal nutrients.
  • Powdery spores are produced in leaf spots throughout the growing season.
    • Spores spread by wind and start new infections within the plant or in neighboring plants.
  • Powdery mildew needs humid conditions to start new infections.
  • Spread of the disease is reduced by rain or irrigation.
  • Water on the leaves prevents the light airy spores from moving on the wind.

How to manage powdery mildew

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Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2018

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