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Extension > Yard and Garden News > Powdery mildew covered shoots

Tuesday, May 23, 2017

Powdery mildew covered shoots

M.Grabowski, UMN Extension Educator

Ninebark shoot covered in powdery mildew
M. Grabowski, UMN Extension
As the buds of trees and shrubs open and young shoots begin to grow, some will emerge already infected with powdery mildew. Powdery mildew, a common fungal disease of many garden plants, is easily recognized by powdery white growth on the surface of infected leaves, shoots, and other plant parts. Gardeners often describe infected plants as dusted by flour or having cobweb like growth on leaves.

Some powdery mildew fungi survive Minnesota’s harsh winter by colonizing young plant tissue within dormant buds. When the buds open in spring and the new shoot emerges, it will be covered by the characteristic white growth of powdery mildew. From that one severely infected shoot, spores are blown throughout the plant canopy starting new infections on healthy leaves from neighboring shoots.

Powdery mildew leaf spots that
originated from windblown spores
M. Grabowski, UMN Extension 
This type of winter survival is common on woody ornamental plants like ninebark, hawthorn, currant, and rose. Gardeners should carefully inspect young shoots of woody shrubs. If a powdery mildew infected shoot is found, it should be pruned out, and buried in the compost pile promptly.  Although spores can be blown in from other locations later in the season, removing a concentrated source from right within the plant canopy will help to delay the start of disease and slow the progression of symptoms.

Powdery mildew fungi steal nutrients from their host plants, but rarely cause significant damage to the health of the plant. By delaying the start of disease, gardeners can reduce this stress even further. 

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