WSU Extension


Powdery mildew 
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Apple-and-thorn skeletonizer 
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San Jose scale 
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Caption: San Jose scale
Photo by: K. Grey
Crabapple : San Jose scale
(revision date: 4/11/2018)

San Jose scale is an armored (hard) scale found on many deciduous trees and shrubs including (but not limited to) apple, cherry, elm, maple, poplar, and willow. The scale insects are about 1/16" in diameter. The female is gray with a yellow spot in the center. The crawlers are yellow and are easily spread by wind, birds, or people. San Jose scale may be found on twigs, branches, leaves, and fruit. Heavily infested trees may wilt and appear water-stressed. Severe infestations may cause twigs and branches to die back. The scale overwinters as black immature scales.
Management Options

Non-Chemical Management
  • Provide proper plant care. Healthy plants are more tolerant of insect damage.
  • Prune and destroy localized infestations.
  • Natural predators and parasites help control scale populations, but may not prevent damage in severe infestations. Avoid use of broad-spectrum insecticides which kill beneficial insects.
  • By late April, begin monitoring for crawlers by using double-sided sticky tape.
Select non-chemical management options as your first choice!

Chemical Management

Pesticide recommendations are for flowering (non-edible) crabapples. Apply early spring to smother overwintering scale. Apply oils during delayed dormant period. 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.
  • All Seasons Horticultural & Dormant Spray Oil Conc [Organic]
    Active ingredient: oil/pet distillate  |  EPA reg no: 4-80
  • Hi-Yield Dormant Spray
    Active ingredient: oil/pet distillate  |  EPA reg no: 7401-428
  • Safer Brand Insect Killing Soap Conc II [Organic]
    Active ingredient: potassium laurate  |  EPA reg no: 42697-60
  • This list may not include all products registered for this use.

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Caption: San Jose scale
Photo by: K. Grey
Caption: San Jose scale damage on fruit
Photo by: A.L. Antonelli