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Thousand Cankers Disease

Thousand cankers disease is caused by a fungal pathogen and bark beetle combination that is fatal to black walnut (Juglans nigra). The disease was first observed in western states where infestations have led to extensive dieback and mortality of black walnut since the 1990’s. Although the disease appeared isolated to the western states there was concern that it could move east to the native range of black walnut, which extends across much of the eastern US. Unfortunately, in August 2010 the first occurrence of thousand cankers disease east of the Great Plains was reported in Knox County, Tennessee. The following year, thousand cankers disease was reported in central Virginia, then Fairfax and Prince William counties in 2012. Thousand cankers disease now threatens the health and survival of black walnut across its entire native range including here in Fairfax County.

Thousand cankers disease gets its name from the numerous areas of dead bark, called cankers, that eventually coalesce and block the transport of water and nutrients causing the tree to die. The fungal pathogen responsible for these cankers is Geosmithia morbida, which is spread to healthy black walnut trees by the walnut twig beetle (Pityophthorus juglandis). As the walnut twig beetle adults and larvae excavate galleries under the bark, they infect living tissues with the fungus and spread the canker disease throughout the tree. These two organisms form a symbiotic relationship in which the beetles are dependent on the fungus for food and the fungus is dependent on the beetle for finding new hosts.

Symptoms of thousand cankers disease are difficult to detect in the early stages of an infestation. The cankers are concealed under the bark and the beetle entrance and exit holes are very small. The most obvious symptoms of an infestation are sudden wilting of healthy leaves and gradual branch dieback from the top down. New branches may resprout from the base in response to crown dieback, but the sprouts will also become infected and killed. Black walnut trees are typically killed approximately 2 to 3 years after initial symptoms are detected in the crown of the tree.

There is no treatment currently that is effective against thousand cankers disease. However, black walnut trees that are properly maintained and protected against stresses such as drought and physical injury are less vulnerable to attack and damage from thousand canker disease. Visit Prevention and Treatment of Diseases and Infestations for more information on setting up your trees for success.

Everyone can help prevent the spread of thousand cankers disease if they follow Virginia’s quarantine restrictions on the movement of black walnut. These include restrictions on the movement of walnut logs, lumber, and firewood. Please find additional information about restrictions on walnut movement and the Virginia counties regulated under the quarantine in Chapter 318 of the Virginia Code.

Additional information about thousand cankers disease can found in the Virginia Cooperative Extension Walnut Thousand Cankers Disease Alert.

Signs and Symptoms of Thousand Cankers Disease

Click photo to enlarge

The walnut twig beetle is very small, 1.5 to 2.0 mm long, and reddish brown in color
Photo by: Whitney Cranshaw, Colorado State University, Bugwood.org

Black walnut have a compound leaf and deeply furrowed gray-brown bark. Their most distinctive feature is their large 1.5 to 2.5 inch yellow-green fruit that ripen in autumn.
Photo by: Jason Sharman, Vitalitree, Bugwood.org

A canker is visible where the fungus has stained the tissue just beneath the outer bark. Tiny beetle galleries and holes are often found within cankers.
Photo By: Troy Kimoto, Canadian Food Inspection Agency, Bugwood.org

Dead branches in the upper canopy of black walnut are a sign of TCD. Black walnut typically dies within 3 years of signs of crown dieback.
Photo By: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org

If you have seen black walnut trees with symptoms of thousand cankers disease, please contact the Forest Health Branch at 703-324-1770, TTY 711, or by email.

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