Powdery mildew on rhododendrons is a fungal infection of the leaves. On rhododendrons, the disease often fails to develop the distinctive powdery white growth normally associated with powdery mildews. Leaves may display yellowish-green or purplish-brown spots or blotches on the upper surface. Lower leaf surfaces may show brown or purple areas of various shapes and sizes. This disease may cause extensive leaf drop and even death of very susceptible plants. Powdery mildew is more severe on shaded plants and is favored by the high humidity found in crowded plantings and damp locations.

Management Options

Non-Chemical Management

Rake and destroy fallen leaves year-round to reduce infection source. Do not compost diseased materials.
Do not over-water or over-fertilize plants, as the fungus favors succulent new growth.
Space plantings to allow good air circulation. Do not plant in extremely shaded or damp areas.
Plant resistant species or cultivars. A list should be available from your county extension agent.

Select non-chemical management options as your first choice!

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides

Begin multiple applications when you notice the disease on current-year leaves. Make certain to thoroughly cover undersides of leaves. Do not apply sulfur products when temperatures are greater than 85 F. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

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
Defend DF/Home & Garden Use/for Organic Gardening [Organic]
Active ingredient: sulfur | EPA reg no: 62562-8
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: Powdery mildew on rhododendron Photo by: C.R. Foss
Caption: Powdery mildew on underside of leaves Photo by: C.R. Foss
Caption: Powdery mildew on rhododendron Photo by: R.S. Byther
Caption: Powdery mildew on rhododendron Photo by: R.S. Byther
Caption: Powdery mildew on rhododendron Photo by: R.S. Byther
Caption: Powdery mildew on rhododendron Photo by: C.R. Foss

WSU Extension

Hortsense