Thousand canker disease
Thousand cankers disease of walnut (TCD) is caused by a fungal pathogen (Geosmithia morbida), which is introduced into the tree by the walnut twig beetle (Pityophthorus juglandis) which carries spores of the fungus on its body. The beetle is native to Arizona, New Mexico, Mexico and possibly to southern California. The origin of the fungus is not known - it was first detected in New Mexico in 2001. The geographic range of the pathogen-insect complex has expanded within the western United States, and it has recently been detected in the eastern U.S. - in the native range of black walnut (Juglans nigra). It was found in Tennessee in 2010, and since then was detected in Virginia, Pennsylvania, and North Carolina (pathogen only); in Ohio walnut twig beetle was found but without the pathogen. Widespread die-off of Juglans species is found throughout the range of the patho-insect complex. This disease is expected to spread through the range of black walnut, which is quite susceptible to the disease, posing a serious threat to black walnut in eastern Canada.
The adult walnut twig beetle, 1.5-2.0 mm long, tunnels into the inner bark of walnut branches (those 1.5 cm or more in diameter). Under the bark, the beetles mate and the female constructs galleries along which she lays her eggs. When a beetle enters a branch it carries the fungus with it, introducing the pathogen under the protective bark of the tree. The fungus colonizes the walls of the beetle's gallery as well as adjacent bark tissue. The growing fungus kills the affected bark tissue, causing cankers in susceptible trees. The cankers grow and eventually merge, girdling the branch and impairing its vascular tissue, causing mortality of the girdled branches. As the tree declines, the beetle will attack larger branches as well as the main trunk of the tree. Susceptible trees die within three years of developing symptoms of TCD.
Read more about thousand canker disease and its effect on walnut trees.
Trees at risk
This disease appears to affect only Juglans species. Black walnut (Juglans nigra) is very susceptible to TCD. Some Persian walnut (English walnut, Juglans regia), which is planted for ornamental and commercial purposes, are susceptible to the pathogen as well. There is evidence that butternut (Juglans cinerea), already under threat due to butternut canker, could be susceptible.
Currently, TCD is only known to be in the United States; infections in Pennsylvania, Ohio and Indiana are the closest to Canada.
See the most recent range map for TCD.
The first evidence of TCD is yellowing and wilting of the foliage, followed by branch die-back. This is usually noticeable in the spring when affected branches fail to leaf out, although sometimes occurs over the summer. Branch die-back usually starts in the upper crown and progresses to lower branches in subsequent years. Trees affected by the disease will develop cankers, however, they are under the bark and are not usually visible from the outside of the tree. Peel back the bark of dying, but not yet dead, branches to look for cankers. Galleries made by the feeding beetle and its larvae may be found in the cankers. In some cases, the outside of the bark may have brown or black stains around the beetle entrance hole, or the bark may crack over the canker making it visible. Starting the year after it is first infested, there will be tiny round exit holes on the dead and dying branches of the tree where the adult beetles exit the tree. The disease affects the branches first, but at later stages, cankers and exit holes are found on the main trunk of the tree as well.
Read more about detection of TCD
What you can do
TCD is not known to be in Canada yet. Early detection will allow a broader suite of management options. Monitor walnut trees for signs of the disease.
If you think you have found TCD, contact:
- In Ontario: Invading Species Hotline at 1-800-563-7711 or report online.