The Thousand Cankers Disease (TCD) is a destructive pest complex consisting of an insect pest, the Walnut Twig Beetle (Pityophthorus juglandis), which is a vector for the fungal pathogen, Geosmithia morbida sp. nov. The beetles feed on young branch tissue, preferably wood larger than 2cm, in the upper canopy early in the spring. Later in the summer, adults move into the lower trunk to overwinter. Adult bark beetles introduce fungal spores into the phloem when they construct galleries. The fungus causes small “cankers” by killing the inner bark around the galleries. These cankers may enlarge and coalesce to completely girdle the branch. The cumulative effect of “thousands” of coalescing cankers eventually kills the tree.  

Eastern black walnuts (Juglans nigra), butternut (Juglans cinerea), and other walnut species native to the western U.S. are susceptible to the disease and have sustained mortality. There appears to be a range in susceptibility of Juglans species to theGeosmithia fungus. Black walnut is a very susceptible host, with large cankers developing in response to inoculation. Other species, including Arizona walnut (Juglans major) and little walnut (Juglans microcarpa), species native to the western U.S., develop more restricted cankers following artificial inoculation.

signs and symptoms

The three major symptoms of this disease are:

• branch mortality
• numerous small cankers on branches and the bole
• evidence of tiny bark beetles

The dominant symptom is that branches fail to leaf out in the spring. Branches may leaf out very weakly in the spring or, more rarely, die anytime during the summer. The earliest symptom is yellowing foliage and thinning of the upper crown that progresses rapidly to brown wilted foliage, then finally branch mortality. Dieback always appears to start in the upper crown and progresses to lower branches in succeeding years.

The fungus causes distinctive circular to oblong cankers in the phloem under the bark, which eventually kills the cambium.  In comparison, cankers of the Butternut Canker Disease are elongated sunken cankers that tend to originate at leaf scars and buds. They have an inky black center with a whitish margin. 

For both diseases the bark surface may have no symptoms, or a dark amber stain or cracking of the bark may occur directly above a canker. To find cankers, carefully remove the bark from symptomatic limbs. When peeling the bark, do not cut too deeply as the beetle galleries and fungi of TCD are found in the living bark (cork cambium and not the wood cambium). Individual TCD cankers may at first be only a few millimeters in diameter, but ultimately can be 3 cm or greater (1” +) and have an elongated oval shape. Typically the appearance of a shallow tunnel produced by the walnut twig beetle will be present near the center of the canker. Many small, dark, dead areas (cankers) can be found under the bark of heavily infected trees.

Numerous pin-sized bark beetle entrance and exit holes are visible on dead and dying branches, and bark beetle galleries are often found within the cankers. In the final stages of the disease, even the main stem has beetle attacks and cankers.

See image slideshow above for signs and symptoms.