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Caption: Walnut blight
Photo by: R.S. Byther
  
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Walnut : Blight
(revision date: 4/11/2018)


Biology
Walnut blight is a bacterial disease affecting leaves, nuts, and other new growth including flowers. Bacteria are spread to new growth in the spring by rainy weather during the bloom season. Leaves develop reddish-brown spots with a yellow halo. Leaves affected during expansion are typically distorted due to these lesions. Twigs develop small black cankers and may be girdled and killed. Young nuts show a dark lesion at the blossom end and drop from the tree. Nuts infected later in development have small, water-soaked spots on the husk which develop into sunken, cracked, black lesions. Nut kernels may be shriveled or of low quality. The bacteria overwinter in buds and twig cankers and are spread by water, infecting through natural openings in plant tissues.
Management Options

Non-Chemical Management
  • Plant disease-resistant varieties such as 'Howe' and 'Spurgeon'. The variety 'Franquette' is highly susceptible to blight.
  • Prune out and destroy infected tissues, when practical.
  • Avoid overhead watering.
  • Space trees and prune to provide good air circulation.
Select non-chemical management options as your first choice!

Chemical Management


Make fungicide applications when catkins begin to enlarge, when shoots begin to expand, and early postbloom. 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.
  • Bonide Liquid Copper Fungicide Conc/Organic Gardening
    Active ingredient: copper octanoate  |  EPA reg no: 67702-2-4
  • Monterey Liqui-Cop Copper Fungicidal Garden Spray
    Active ingredient: copper-ammonia complex  |  EPA reg no: 54705-7
  • Soap-Shield Flowable Liquid Copper Fungicide [Organic]
    Active ingredient: copper octanoate  |  EPA reg no: 67702-2-56872
  • This list may not include all products registered for this use.
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Caption: Walnut blight
Photo by: R.S. Byther