Infection Imposition
Over the last decade the mortality of black walnut trees has increased in many states. Thousand Canker Disease (TCD) is the result of a tiny bark beetle which burrows in the bark and creates large and multitudinous galleries, making these trees susceptible to fungal disease and canker formation by creating an entry point in the bark. TCD earned its name from the huge number of cankers associated with dead limbs and leaders. The main culprits involved in the dieback of these trees are the walnut twig beetle (Pityophthorus juglandis) and a newly identified fungus Geosmithia morbida, both of which have only been found occurring on walnut species and a closely related species Pteryocarpa sp., the wingnut tree. Afflicted trees usually die within 3 years of infestation.
Thousand cankers disease has been reported in 9 western states, and in 2010 was found east of the great plains. The prognosis for the east is grim, with the expectation of finding both the beetle and the fungus anywhere there are walnut trees. Black walnut trees, specifically, are especially susceptible; they have a very broad distribution and both the vector and disease are highly adaptable.
Disease Diagnosis
There are 3 major symptoms characteristic of TCD: branch mortality, numerous small cankers apparent on branches and evidence of tiny bark beetles. The first blatantly visual symptom will be chlorotic foliage which will rapidly progress into brown wilted foliage and finally result in branch death. The fungus causes distinctive circular to oblong cankers in the phloem under the bark, these will eventually kill this tree tissue. Numerous, tiny entry holes may be seen on branches that are dying or dead and their galleries may be visible in the cankers. Cankers may be present with no visual symptom on the outer bark, or bark above a canker may be cracked or appear stained dark amber or black.
Adult walnut twig beetle, Pityophthorus juglandis. The walnut twig beetle is native to Arizona, California, and New Mexico. It has invaded Colorado, Idaho, Nevada, Oregon, Utah, and Washington where eastern black walnut has been widely planted. The beetle alone does not generally cause tree mortality.
Cankers in the phloem characteristic of thousand canker diseases. Other members of the genus Geosmithia have not been considered important pathogens in the past, but G. morbida appears to be an especially virulent species. Other than causing cankers though, this fungus is inconspicuous.
Afflicted Action
Currently visual inspection of walnut trees is the best monitoring method employed in the eastern states. Pheromone traps, installed near, but not in, walnut trees are also utilized for trapping and identification of beetles. Identification and monitoring are the extent of management at this point. If you suspect that any of your landscape trees or town trees have thousand cankers disease contact you arborist as well as getting in touch with your state’s branch of the National Plant Diagnostic Network (NPDN). The NPDN has a clinic in every state, more information on disease identification and control visit their website at www.npdn.org.
