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Powdery mildew on trees and shrubs

Quick facts

  • Powdery mildew appears on plant surfaces as white to gray powdery spots, blotches or felt-like mats on leaves, stems and buds.
  • It can reduce photosynthesis and cause yellowing of leaves.
  • Disease is often most severe on young leaves, water sprouts and green shoots.
  • Powdery mildew thrives in humid conditions but does not do well if leaves are wet from frequent rain or irrigation.
  • It does not cause significant damage and does not require management.

Powdery mildew is a very recognizable and common fungal disease found on several plants in Minnesota.

On trees and shrubs, powdery mildew rarely causes serious damage to its host. The disease can significantly reduce the ornamental value of plants grown for their appearance, like roses and purple leafed ninebark shrubs.

Symptoms

Dogwood leaves discolored and distorted by powdery mildew
Dogwood leaves discolored and distorted by powdery mildew

Powdery mildew appears as superficial growth on plant surfaces and is seen as white to gray powdery spots, blotches or felt-like mats on leaves, stems and buds.

  • Infected plants may appear to be sprinkled with baby powder or covered in cobwebs.
  • Disease is often most severe on young leaves, water sprouts and green shoots.
  • Once 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/early fall, tiny round orange to black balls form within white fungal mats.
  • Most prevalent when outdoor conditions consist of cool temperatures with high humidity. But it can be seen in warm, dry conditions as well.
  • Disease is most severe on plants or plant parts in shaded areas with poor air movement (interior or lower branches).

Managing powdery mildew

A single ninebark shoot completely covered by powdery mildew.
Ninebark shoot infected with powdery mildew

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

  • Photosynthesis can be reduced in heavily colonized leaves.
  • Powdery mildew resistant cultivars are available for many ornamental shrubs. Choose disease resistant cultivars for new plantings or as replacement plants.
  • Do not overcrowd plants – use size at maturity as a spacing guide when planting.
  • Prune the tree or shrub to increase light penetration and improve air circulation throughout the canopy.
  • Many plant species are most susceptible when they produce new, succulent shoots and leaves.
  • Pruning and fertilizing can stimulate growth and trigger or prolong powdery mildew.
    • Do not fertilize trees and shrubs suffering from powdery mildew infections unless it is recommended by a soil test to correct a nutrient deficiency.
    • Remove only severely infected and damaged shoots in the summer to reduce spread and overwintering of the fungi within the canopy.
    • Avoid excessive pruning of infected plants.
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Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2018

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