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Powdery mildew on trees and shrubs

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

Dogwood leaves discolored and distorted by powdery mildew
Dogwood leaves with powdery mildew
  • White to gray powdery spots, blotches or felt-like mats form on leaves, green stems and buds. Infected plants may appear to be sprinkled with baby powder or covered in cobwebs.
  • Young leaves, water sprouts and green shoots are often most severely affected.
  • Infected leaves may be cupped or twisted at the site of the infection.
  • When severely infected, leaves may turn yellow and fall prematurely during the growing season.
  • In some plants, leaves turn purple to red around the infection.
  • In late summer or early fall, tiny round orange to black balls form within white fungal mats.
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

  • Tolerate powdery mildew. Powdery mildew does not significantly affect the health of the tree or shrub and does not require management. 

  • Powdery mildew resistant varieties are available for many ornamental shrubs. Choose disease resistant varieties for new plantings or as replacement plants.

  • Do not overcrowd plants. Use size at maturity as a spacing guide when planting.

  • Do not fertilize infected trees and shrubs unless it is recommended by a soil test to correct a nutrient deficiency. Fertilizer will cause the tree to produce young shoots which are highly susceptible to powdery mildew.

Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2018

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