Powdery Mildew

Powdery mildew is common on a wide range of plants in western Washington, including annuals, perennials, food crops and ornamental shrubs.

Our cool, humid weather creates ideal conditions for the disease. Powdery mildew does best when damp nights are followed by warm, sunny days, such as in late summer and fall.

On most landscape plants, powdery mildew looks like a fuzzy, whitish coating that resembles a thick coating of powder. On rhododendron, symptoms include spotted leaves, with brownishpurple or yellow spots on the underside and sometimes ring-shaped spots on the upper leaf surface.

What you can do

• Choose disease-resistant plants or varieties. Many plants have cultivars bred to resist powdery mildew.
• Prune out severely affected leaves when symptoms are present.
• Prune your plants to increase air circulation.
• Begin monitoring for powdery mildew symptoms
in mid-summer. Call us if you are concerned.
• Be careful about where your plants are situated. Plants that are next to the house may get powdery mildew even if they are not prone to the disease.
• Ask us for our Sanitation fact sheet.

Resistant rhododendron varieties
American Rhododendron Society,
www.rhododendron.org/performers2.htm

"Rhododendron Powdery Mildew," Mary Robson, WSU Cooperative Extension,
http://gardening.wsu.edu/column/06-11-00.htm