Contacts:
Dave Clement, University of Maryland Extension | clement@umd.edu
Karen Rane, University of Maryland Plant Diagnostic Lab | rane@umd.edu
ANNAPOLIS, MD (December 10, 2014) – Thousand cankers disease (TCD) is a disease complex first discovered in the western United States that primarily affects black walnut (Juglans nigra). This disease is the result of the combined activity of a fungus (Geosmithia morbida) and the walnut twig beetle (Pityophthorus juglandis). Because the insect and fungus were confirmed for the first time on a site in Cecil County, Maryland in the fall of 2014, MISC has chosen thousand cankers disease as the December Invader of the Month.
Spores of the fungus (Geosmithia morbida) are carried and introduced into the tree by adult walnut twig beetles (Pityophthorus juglandis) during egg gallery construction. The fungus kills the tissues around the beetle galleries, causing shallow dark cankers in the inner tree bark. The beetles attack trees in large numbers, resulting in numerous cankers that girdle branches and disrupt the flow of nutrients throughout the tree. Widespread mortality of black walnut since 2001 in Colorado led to the recognition and description of the disease. The list of confirmed cases of TCD in eastern states has been expanding and currently includes Pennsylvania, Virginia, Tennessee and North Carolina.
Black walnut is an important part of the forests of Maryland. Not a common tree, black walnut is found on moist to mesic sites with best growth in full sun. Black walnut is often found in forest buffers and is widely planted for conservation purposes. The wood is prized for its beauty and used in furniture as veneer and in solid wood construction. It is also a valuable wildlife food with the nuts enjoyed by humans as well.
A black walnut tree can be infected with TCD for many years before showing symptoms, but once branch dieback appears the tree begins to decline and eventually dies. The first symptom to appear by mid-summer is leaf flagging, which includes leaf wilting and yellowing, followed by thinning of the canopy from twig and branch dieback. Eventually the whole tree dies as “thousands of cankers” girdle branches and the trunk.
This disease is very different from Dutch elm disease or oak wilt. Those other insect-vectored fungal diseases only require one introduction of the fungus into the tree, and from there the fungus spreads through the water conducting system of the entire tree. TCD requires multiple introductions of the fungus by the walnut twig beetle to cause dieback.
Currently, there are no effective methods for saving trees with TCD. Prevention starts with stopping the spread of the walnut twig beetle into new areas where it does not yet occur. Keeping a close watch on your walnut trees is crucial. Inspect walnuts regularly during June through early August for symptoms of TCD including:
- Sparse foliage or thinning of the canopy
- Leaf yellowing or wilting or branch dieback
- Presence of tiny beetle holes in bark or galleries in branches or the trunk
- Presence of brown to black tissue surrounding beetle galleries inside the bark
- Epicormic shoot development
Dead and dying branches should be pruned from the tree and the wood destroyed by burning. The beetle spreads to new locations primarily by movement of infested wood materials such as logs, firewood, lumber and wood chips. Movement of nuts is not a concern since TCD is not systemic in infected trees and does not enter the nuts. Trees killed by TCD support development of large numbers of walnut twig beetles. Moving a single log with live beetles can start an outbreak in a new location. There are regulations and quarantines in states with confirmed occurrences of this disease – contact your state Department of Agriculture for information on local regulations.
Additional Resources
Thousand Cankers Disease. Invasive Species of Concern in Maryland
Thousand Cankers Disease (Pest Alert). USDA Forest Service
For more information about other Invasive Species of Concern in Maryland, visit the Maryland Invasive Species Council.
photos available electronically on request.