College of Agricultural, Human, and Natural Resource Sciences

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Gwen-Alyn Hoheisel
Gwen-Alyn.Hoheisel@co.benton.wa.us

Fungus New to Washington Is Killing Walnut Trees

PROSSER, Wash. – A new disease is infecting and killing walnut trees in Washington. Symptoms of the disease include a yellowing of leaves, usually at the top of the tree, as well as subsequent die-back of the tree’s larger branches.

Thousand Canker Disease damage. Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Thousand Canker Disease damage. Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org. Click image for high-resolution version.

The disease, called Thousand Canker, was discovered in the Prosser area during the summer of 2008, according to Gwen-Alyn Hoheisel, Washington State University Extension Educator in Benton and Franklin counties.

Thousand Canker Disease is caused by a fungus called Geosmithia, which in recent years has spread throughout the Pacific Northwest. The fungus was first diagnosed in Oregon in 2007 and in Idaho in 2004, although walnut decline has been seen for many years.

The fungus is carried by a tiny beetle called the Walnut Twig Beetle. Walnut Twig Beetles burrow into a tree’s larger branches or trunk, forming galleries just beneath the bark. Feeding on the tree’s phloem, which circulates nutrients throughout the tree, the beetles spread the fungus. The result is the rapid decline and death of the tree in one to three years.

“Unfortunately,” said Hoheisel, “there is no treatment for this disease. The current management strategy is to quickly remove infected trees, chip them, and let the fungus and beetle desiccate. The wood cannot be saved for fired wood or crafts unless it is debarked.

“The best strategy for keeping walnut trees healthy is to not stress them,” she added, “meaning provide them with adequate nutrients and water. A stressed tree is more vulnerable to the Walnut Twig Beetle.”

Thousand Canker Disease damage. Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Thousand Canker Disease damage. Photo credit: Whitney Cranshaw, Colorado State University, Bugwood.org. Click image for high-resolution version.

Thousand Canker Disease can infect almost any type of walnut tree, including Black and English walnut. However, because the Walnut Twig Beetle appears to be specific to walnuts, threat of infection to other types of trees appears to be minimal.

Other control methods are being investigated by WSU pest-management experts in collaboration with scientists in Colorado who have been working on the disease for more than a decade, as well as local city and county agencies.

For more information, tree owners should contact their local county extension office. A map of Washington State University county Extension offices is available online at http://wsuwest.wsu.edu/map/; visitors should put a check in the box next to “WSU Extension County Offices.”

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