Thousand Cankers Disease (Geosmithia morbida sp. nov)
What does Thousand Cankers Disease look like? Reference the Thousand Cankers Disease Field Identification Guide for more information.
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 the Geosmithia 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.
The walnut twig beetle spends winter primarily, and possibly exclusively, in the adult stage sheltered within cavities excavated in the bark of the trunk. Adults resume activity by late-April and most fly to branches to mate and initiate new tunnels for egg galleries; some may remain in the trunk and expand overwintering tunnels. Beetle larvae feed for 4-6 weeks under the bark in meandering tunnels that run perpendicular to the egg gallery and pupate at the end of the tunnel.
Adults emerge to produce a second generation. Peak flight activity of adults occurs from mid-July through late August and declines by early fall as the beetles enter hibernation sites. A small number of beetles produced from eggs laid late in the season may not complete development until November.
Adult bark beetles carry fungal spores that are then introduced into the phloem when they construct egg galleries. Small cankers develop around the galleries; these cankers may enlarge and coalesce to completely girdle the branch. Trees die as a result of these canker infections at each of the “thousands of” beetle attack sites.
Identifying Walnut Trees
Black walnut and butternut trees can be identified by their:
- alternate, pinnately compound leaves;
- bark (black walnut is deeply furrowed and dark brown while butternut is light gray and almost smooth when young, but fissures into distinctly flat-topped ridges that are a shiny silvery color when mature); and
- chambered pith--somewhat like a honeycomb. Only black walnut and butternut have this type of pith.
Signs and Symptoms of Thousand Cankers Disease
The three major symptoms of this disease are:
- branch mortality;
- numerous small cankers on branches and the bole, and
- 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 kill 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 elongate 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 disease, even the main stem has beetle attacks and cankers.
View the most current distribution map of Thousand Cankers Disease in North America.