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Integrated Pest Management · Agriculture and Natural Resources

University of California

Thousand cankers disease
and walnut twig beetles in California

Branch with thousand cankers disease.

Walnut branch with thousand cankers disease, surface removed to show beetle galleries.

Thousand cankers disease, caused by the fungus Geosmithia morbida, is killing walnut trees in California and threatens wildland and landscape trees as well as commercial walnuts.

The fungus is spread by a tiny bark beetle, the walnut twig beetle, Pityophthorus juglandis. The fungus enters the tree through the feeding or reproductive activities of the beetle, and colonizes and kills the phloem and cambium of the branches and main stem. As the beetles and pathogen spread, small cankers form and coalesce, girdling branches. Thousand cankers disease gets its name from the large number of dark cankers that rapidly develop on affected branches.

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