WSU Extension

Hortsense

Arborvitae : Leaf blight
(revision date: 4/11/2018)


Biology
Leaf blight is a fungal disease. The symptoms typically appear first in late spring as bleached spots followed by brown or black cushion-like fungal fruiting bodies. Infected foliage may be anywhere on the plant. Individual leaves are killed by the fungus and turn a light tan to gray color. As the fruiting bodies drop out of the leaves, deep pits are left in the dead tissue. The infected tissues often have a "scorched" appearance. Infected leaves often drop in the fall. Western red cedar (Thuja plicata) is most commonly infected, particularly the cultivars 'Atrovirens' and 'Excelsa'. A similar disease attacks junipers.
Management Options

Non-Chemical Management
  • Plant resistant cultivars.
  • Avoid overhead watering. Leaf blight is favored by moist foliage conditions.
  • Space plants and prune to allow good air circulation.
Select non-chemical management options as your first choice!

Chemical Management

Make applications in spring and early summer. 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.
  • 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.
Images
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Caption: Arborvitae Didymascella leaf blight
Photo by: R.S. Byther
Caption: Arborvitae Didymascella leaf blight close-up
Photo by: R.S. Byther