Thousand Cankers Disease threatens Black Walnut.. and some businesses

A species is at stake. Businesses are at stake. Both may survive but damage has already been done and the future is uncertain.




Black Walnut, Juglans nigra, is an important tree species in the eastern United States. In July of this year, Thousand Cankers Disease (TCD) was detected in Pennsylvania for the first time. We now join two other states east of the Mississippi with confirmed cases of a disease which is deadly to black walnut. A quarantine has been established by the Pennsylvania Department of Agriculture in an effort to slow the spread of the disease. The quarantine prohibits the movement of walnut out of Bucks County, the only county in Pennsylvania where detection has occurred. All firewood and wood chips are also subject to the quarantine since segregation of walnut from these potential infection sources cannot be assured.


Here are some questions and answers about Thousand Cankers Disease


Q. Where did it come from?


A. New Mexico, California, Colorado, Oregon, Washington and a few other western states began detecting the disease about ten years ago. In 2010 it was found in Tennessee... in 2011 Virginia and Pennsylvania. It is likely that movement of walnut logs or other walnut wood brought the infestation from the west to the native range of black walnut…. the eastern United States. Both the walnut twig beetle, Pityophthorus juglandis and the fungus Geosmithia morbida which are involved in the disease, are thought to be native to the southwestern United States where they originally infested/infected other Juglans species.


Q. How are black walnut trees affected?


A. A beetle about the size of a sesame seed, called the Walnut Twig Beetle, bores into trees. Actually thousands of beetles infest individual trees. The beetle’s larvae tunnel in the bark and at the same time infect the tree with a fungus which kills the plant tissue. These dead areas are called cankers. Many cankers combine to girdle and kill the stems. The numerous feeding sites and cankers give rise to the name Thousand Cankers Disease. Infested walnut trees exhibit yellowing, wilting and dieback in the crown or upper branches of the tree. Within 10 year of infestation, and three years from the time symptoms develop,  the tree dies. The best diagnostic sign will be numerous tiny (2 mm) holes in the bark of branches. Beneath the bark, darkly stained, cankered, wood indicates the activity of the fungus. Most Juglans species are susceptible to TCD but Black Walnut appears to be most severely affected.


Q. Are there treatments to cure affected trees or prevent infestation of healthy trees?


A. Not at this time. The nature and habits of the beetle present a great challenge to those who want to control this pest. For instance, adults are active from March through October and can fly 1-2 miles. Even if beetles are partially controlled, the fungus may still cause damage. Research is underway to better understand both the beetle and the fungus involved in the disease.


Q. Why has quarantine been established?


A. The quarantine is an attempt to slow the spread of the disease and preserve this important tree species, both within Pennsylvania and also in states that are currently uninfested.


Q. What businesses are affected by the quarantine?


A. It is unlawful to move any walnut wood (except kiln dried lumber or finished furniture) out of Bucks County. Since it is impractical to distinguish walnut from other wood in loads of firewood or wood chips, they are subject to the quarantine and may not be moved out of Bucks County. Walnut lumber is also quarantined unless it meets certain requirements including kiln drying.


Q. What is being done about the problem?


A. A task force involving regulatory and research experts is creating an action plan to manage TCD in Pennsylvania. Compliance agreements with those affected by the quarantine are being investigated in an effort to find a way to allow them to continue business activities without presenting a threat to walnut outside of Bucks County.


Q What do I do if I think my walnut trees have TCD?


A. Collect a sample of branches with wilting and dieback symptoms. If they exhibit many tiny holes in them, contact Pennsylvania Department of Agriculture 1-866-253-7189 or Penn State Extension.


Q. How can I learn more about the disease and its symptoms?


A. Call  (215-345-3283) for a fact sheet or see this website.


In summary, a new disease of Black Walnut threatens an important tree species. Sadly, we’ve seen situations like this before…. Chestnut Blight, Dutch Elm Disease (DED), Hemlock Woolly Adelgid. Perhaps Dutch Elm Disease is most similar because in both cases a beetle vectors a fungal pathogen. Decades after Dutch Elm Disease was introduced, we still have some elms, but the once loved American Elm has been seriously impacted. Many differences exist between DED and TCD but its probably a good place to start as we contemplate the effect of TCD in Pennsylvania. How the story of Thousand Cankers Disease of Black Walnut will play out remains to be seen.