WSU Extension

Hortsense

Azalea : Powdery mildew
(revision date: 3/10/2017)


Biology
Powdery mildew on azalea forms a characteristic fuzzy white growth on upper or lower leaf surfaces. Entire leaves can be covered. In late summer and fall, small black specks may be found in the white areas. Powdery mildew is more severe on shaded plants. It is favored by the high humidity found in crowded plantings and damp locations.
Management Options

Non-Chemical Management
  • Prune and space plantings to allow good air circulation. Do not plant in extremely shaded or damp areas.
  • Hand-pick and destroy mildewed leaves to control small amounts of infection.
  • Rake and destroy fallen leaves year-round to reduce infection source. Do not compost diseased materials.
  • Do not overwater or overfertilize plants, as the fungus prefers succulent new growth.
  • Hose diseased plants with water when practical. This can help remove fungus and prevent new infections.
Select non-chemical management options as your first choice!

Chemical Management

Begin applications when you first notice the disease on current-year leaves. If disease appears late summer, applications are not necessary on deciduous azaleas. Do not apply sulfur products when temperature is over 85F or within a few weeks of an oil spray.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • Bayer Advanced Natria Neem Oil Conc [Organic]
    Active ingredient: clarified hydrophobic extract of neem oil  |  EPA reg no: 70051-2-72155
  • Bi-Carb Old-Fashioned Fungicide [Organic]
    Active ingredient: potassium bicarbonate  |  EPA reg no: 54705-10
  • Hi-Yield Snake Eyes Dusting Wettable Sulfur
    Active ingredient: sulfur  |  EPA reg no: 7401-188-34911
  • Lilly Miller Sulfur Dust Fungicide/Insecticide Dust or Spray
    Active ingredient: sulfur  |  EPA reg no: 802-16
  • Monterey Horticultural Oil [Organic]
    Active ingredient: mineral oil/pet distillate light  |  EPA reg no: 48813-1-54705
  • Safer Brand Garden Fungicide/Flowers, Fruit & Vegetables Conc
    Active ingredient: sulfur  |  EPA reg no: 42697-37
  • 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.
Images
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Caption: Azalea powdery mildew
Photo by: R.S. Byther
Caption: Azalea powdery mildew fruiting bodies
Photo by: R.S. Byther
Caption: Azalea powdery mildew: early signs
Photo by: R.S. Byther