Hydrangea: Powdery mildew
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.
Use Integrated Pest Management (IPM) for successful plant problem 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.
IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides.
- 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
Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.