University of Minnesota Extension
www.extension.umn.edu
612-624-1222
Menu Menu

Extension > Garden > Yard and Garden > Diseases > Powdery Mildew

Powdery Mildew

Rebecca Koetter and Michelle Grabowski

Importance

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.

Pathogen and susceptible plants

There are many species of powdery mildew fungi and they affect thousands of plant species. The fungi that cause powdery mildew are generally host-specific and only infect plants from the same genus or family. Therefore the fungus on your ash tree will not infect your viburnum bush. Only a few species of powdery mildew fungi are capable of causing disease in multiple trees and shrubs. In many cases, however, one species of tree or shrub is susceptible to several different species of powdery mildew fungi.

Table 1. Trees and shrubs on which powdery mildew has been identified in Minnesota


Fruit tree

M. Grabowski, UMN Extension

Powdery mildew infection on the lower leaves of a young oak

Identification

M. Grabowski, UMN Extension

This single ninebark shoot is completely infected with powdery mildew, and likely grew out of a bud infected by the fungus.

M. Grabowski, UMN Extension

Dogwood leaves discolored and distorted by powdery mildew

M. Grabowski, UMN Extension

Round black chasmothecia within white powdery mildew spots on viburnum

Biology

Powdery mildew fungi survive winter as chasmothecium (previously known as cleistothecium), dark colored round fungal structures that contain spores and protect them through the winter. In spring, the chasmothecium rupture releasing spores that are spread by the wind. Some species of powdery mildew fungi survive the winter as a dormant fungus – or mycelium – in infected buds or shoot tips. In spring, the mycelium or spores start new infections on succulent, new growth. The fungus grows and becomes evident as the white powdery mat on the leaf surface. Throughout the growing season, additional spores are produced and spread by wind. These spores start new leaf spots on the plant and on neighboring plants.

Powdery mildew thrives in humid conditions but does not do well if leaves are wet from frequent rain or irrigation. Favorable conditions for powdery mildew commonly occur when cool nighttime temperatures are followed by warm day temperatures. Consequently, in Minnesota increased levels of powdery mildew occur in the spring and fall of the year.

Management

Fungicides

Fungicides can be used to protect highly susceptible and prized ornamental shrubs like roses or ninebarks. Fungicides prevent infection and some fungicides eradicate small infections that have started to form. For shrubs that have a history of disease, fungicides can be applied before disease symptoms appear. Alternatively, scout plants regularly. When a few powdery mildew leaf spots appear, apply fungicides to protect the remaining leaves. In both cases, fungicide sprays will need to be repeated according to label instructions to protect plants throughout the growing season.

Chemical treatments include:

Always completely read and follow all instructions on the fungicide label.

 

  • © 2014 Regents of the University of Minnesota. All rights reserved.
  • The University of Minnesota is an equal opportunity educator and employer. Privacy