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Hydrangea : Powdery mildew
(revision date: 4/11/2018)


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

Non-Chemical Management
  • 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.
Select non-chemical management options as your first choice!

Chemical Management


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.
  • Bi-Carb Old-Fashioned Fungicide [Organic]
    Active ingredient: potassium bicarbonate  |  EPA reg no: 54705-10
  • Monterey Horticultural Oil [Organic]
    Active ingredient: mineral oil/pet distillate light  |  EPA reg no: 48813-1-54705
  • Spectracide IMMUNOX Multi-Purpose Fungicide Spray Conc
    Active ingredient: myclobutanil  |  EPA reg no: 9688-123-8845
  • This list may not include all products registered for this use.
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Caption: www.forestryimages.org
Photo by: Dept. of Plant Pathology, NCSU