Hydrangea: Powdery mildew
Photo by: Dept. of Plant Pathology, NCSU
Biology
Powdery mildew is a fungal disease affecting leaves, young shoots, and flowers. White to grayish, powdery fungal growth develops on the top of the leaf and may also appear on tender shoots, buds, and flowers.
Leaves may also develop yellowish blotches or turn purplish brown, with a white powdery growth on the underside. Affected plant parts may be stunted and deformed. Severely infected leaves often drop from the plant, and shoots may die back.
Unlike other fungal diseases, powdery mildew does not require moist leaf surfaces for infection and is typically most prevalent during dry weather with warm days and cool nights. It thrives in conditions of high humidity, such as on plants in heavily shaded areas.
While powdery mildew may occur on other species of hydrangea, bigleaf or florist’s hydrangea (Hydrangea macrophylla) is most commonly affected.
Management Options
See "Using Integrated Pest Management (IPM) in the Landscape" for successful problem management techniques.
Non-chemical Management
Select non-chemical management options as your first choice!
- Avoid overfertilizing, which encourages susceptible new growth. If fertilization is necessary, use a slow-release or lower-nitrogen formula.
- Gather and destroy all fallen leaves.
- Pick off infected leaves and prune severely infected shoots to prevent spread of disease.
- Space plantings and prune to provide good air circulation.
- The cultivar ‘Veitchii’ is resistant to powdery mildew, while ‘Nikko Blue’ is susceptible. ‘Madame Emile Mouilliere’, ‘Forever Pink’, ‘Lilacina’ and ‘Holstein’ show intermediate susceptibility.
Chemical Management
IMPORTANT: Always read and follow label directions when working with pesticides.
For more information on using pesticides visit :
- Begin applications when conditions are favorable for disease.
- If necessary, repeat applications at weekly intervals or as label permits.
- Complete coverage including both upper and lower leaf surfaces is important, especially when using bicarbonates
Approved Pesticides
If you choose to use a pesticide, some examples of products that are legal in Washington are listed below. Some products are labeled for just INDOOR or just OUTDOOR use, or may allow both uses. Be sure to choose a product appropriate for your situation. Always read and follow all label directions.
WSU Extension programs and employment are available to all without discrimination.
Evidence of noncompliance may be reported through your local Extension office.