Thousand Cankers Disease of Walnut
A formal exterior quarantine for thousand cankers disease of walnut was signed into effect in August 2011. Seventeen other states have similar exterior quarantines in place (see Figure 1).
Mortality of black walnut trees had been observed in western and southwestern United States since the 1990s. In 2008 in Colorado an insect/fungus combination was identified as the cause of the mortality, which had come to be known as "thousand cankers disease (TCD) because of the numerous cankers on the stems and branches of the walnut trees. In 2010, TCD was detected for the first time within the native range of eastern black walnut (Juglans nigra), in Tennessee. Since the eastern black walnut's range covers 30 states including southeastern Minnesota, we are concerned about TCD.
What is Thousand Cankers Disease?
Thousand cankers is a disease that affects several kinds of walnut trees (Juglans species). It was originally thought to be caused by a fungus, Geosmithia moribida, carried by an insect, the walnut twig beetle (Pityophthorus juglandis). It is now accepted that TCD is caused by a complex relationship of organisms that scientists are still unraveling.
Where is Thousand Cankers Disease?
At this time the disease is known to occur in western and southwestern United States, and it has been confirmed in limited areas within Maryland, North Carolina, Ohio, Pennsylvania, Tennessee and Virginia (Figure 1).
Why Do We Care About Thousand Cankers Disease?
Eastern black walnut is highly valued for lumber and veneer, nut meat industries in some states, and wildlife food. The USDA Forest Service estimates that over 200 million eastern black walnut trees occur across its native range. Most eastern black walnut trees grow in natural stands of mixed hardwoods, with plantations accounting for only about 1% of the total volume. Black walnut trees are planted in urban settings as far north as Hibbing, Minnesota, though the greatest concern lies in the potential threat to eastern black walnut that occurs naturally and in plantations in southern Minnesota. Minnesota has over 6,000,000 eastern black walnut trees, with one to two million board feet of walnut wood harvested annually. A close relative to eastern black walnut, butternut (Juglans cinera), also occurs in Minnesota and can be infected by TCD.
What Are We Doing About Thousand Cankers Disease?
The MDA is working to protect the state’s black walnut resource by preventing the introduction of TCD into Minnesota using regulation and collaborating with our partner agencies in early detection surveys and outreach to our many stakeholders. The highest risk pathway for TCD movement is in walnut wood with its bark on.
The state exterior quarantine restricts movements of products that could be harboring TCD from those states known to have TCD and from other potentially infested areas into Minnesota. The list of walnut products covered by the quarantine includes: live walnut trees, walnut logs, walnut lumber, walnut nursery stock, wood chips and mulch made from walnut wood, walnut branches and roots, and packaging materials made from walnut wood. The quarantine also applies to all hardwood firewood. It does not apply to walnut nuts, nutmeat, walnut hulls, finished products made from walnut wood without bark, or processed lumber that is 100 percent bark-free, and kiln-dried with square edges.
How Can I Tell If a Walnut Tree Has Thousand Cankers Disease?
Several different forest health issues can cause symptoms similar to those of TCD. Early crown symptoms include thinning, small leaves, yellow, brown or wilting leaves. As the disease progresses you may see signs of cankers under the bark of wilting or recently killed branches.
What Can I Do?
Contact the Minnesota Department of Agriculture via Arrest the Pest if you think you have found a tree or stand of trees with thousand cankers disease.