WSU Extension


Caption: Powdery mildew on leaves
Photo by: R.S. Byther
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Ornamental Pear : Powdery mildew
(revision date: 1/22/2016)

Powdery mildew is a fungal disease found on twigs, leaves, blossoms, and fruit. New growth is particularly susceptible. The entire terminal may become covered with powdery mildew. Leaves develop a characteristic gray-white powdery growth, usually on the underside. Severe infections may spread to the upper leaf surface, as well. Dark brown to black fruiting bodies of the fungus can be seen around midsummer, while the whitish mycelia turn brown at this time. Infected leaves are often curled and distorted, while other infected tissues may become brittle and die back. Fruit (if present) may show russet patches after infection. Unlike apple powdery mildew, pear powdery mildew has not been observed overwintering in buds. Powdery mildew development is favored by high humidity, warm days, and cool nights.
Management Options

Non-Chemical Management
  • Plant resistant varieties if possible.
  • Prune out and destroy severely infected shoots when practical.
  • Succulent new growth is often particularly susceptible to powdery mildew infection. Provide proper culture to prevent heavy flushes of new growth.
Select non-chemical management options as your first choice!

Chemical Management

Apply fungicides when leaves are separating, just exposing bud cluster. Make applications at intervals recommended on product labels until weather dries. When in blossom, wait until 3/4 of petals have fallen before making applications. Oils should not be applied within 2 weeks of any sulfur application or necrotic foliage may result. Do not use oils when temperatures are below 50 degrees F, above 90 degrees F, or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. 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
  • ferti-lome Horticultural Oil Spray RTSpray
    Active ingredient: oil/pet distillate  |  EPA reg no: 48813-1-7401
  • 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
  • This list may not include all products registered for this use.
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Caption: Powdery mildew on leaves
Photo by: R.S. Byther
Caption: Powdery mildew on leaf
Photo by: G.G. Grove