Powdery mildew

Clemson University - USDA Cooperative extension, Bugwood.org

Published March 31, 2022

The powdery mildew fungus is easily spread through the air and, unlike many other foliar diseases, does not require wet conditions for infection to occur. Powdery mildews thrive when the temperature is cool and relative humidity is high. Severely infected leaves become brittle and wither and die.

Many types of powdery mildew fungi have extremely narrow host ranges. They have absolute specificity in what they can attack.

One aspect that lends these pathogens to be extremely virulent is the fact that the spores are very easily spread by wind. The spores also can overwinter and cause infections the following spring.

Clemson University - USDA Cooperative extension,
Bugwood.org

the facts about powdery mildew

Hosts: Many popular ornamentals, notably roses, hydrangeas, dogwood, rhododendron, viburnum and azalea, as well as many herbaceous perennials like dahlia, phlox and zinnia.

Appearance: You can recognize this disease by the white, powdery mycelial and spore growth that forms on leaf surfaces and shoots and sometimes on flowers and fruits. Powdery mildews may infect new or old foliage. This disease can be serious on woody species such as rose, crape myrtle and sycamore where it attacks new growth including buds, shoots, flowers and leaves. New growth may be dwarfed, distorted, and covered with a white, powdery growth. Infected leaves generally die and drop from the plant earlier than healthy leaves.

Spread: Wind carries powdery mildew spores to new hosts. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, water on plant surfaces for extended periods inhibits germination and kills the spores of most powdery mildew fungi. Moderate temperatures of 60°-80°F and shady conditions generally are the most favorable for powdery mildew development. Powdery mildew spores and mycelium are sensitive to extreme heat and sunlight, and leaf temperatures above 95°F may kill the fungus.

Cultural control: Don’t overcrowd pots. By creating a spacing scenario in which airflow is strong between the plants, you can limit the incidents of sporulation and inoculation on the plants with powdery mildew.

Chemical control: Most fungicides are not curative. Growers should scout for diseases, especially when conditions are conducive, spot it as early as possible, and apply fungicide at the first sign of disease to prevent any further impact. Watch for the telltale leaf spots. Rotate the mode of action at every other application to manage resistance potential.

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