A formal exterior quarantine for thousand cankers disease of walnut was signed into effect in August 2011. Fifteen 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), Tennessee. Since then, it has been detected in Virginia, Pennsylvania, North Carolina, and Ohio. The insect was trapped in Maryland in 2013 and testing for the disease is underway. Since the eastern black walnut's range covers 30 states including Minnesota (see Figure 1), folks are deeply concerned about TCD affecting their trees.
Figure 1: Map created by Erich Borchardt, MDA
Thousand cankers is a disease that affects several kinds of walnut trees (Juglans species) and is caused by a fungus carried by an insect. Smaller than a grain of rice, the walnut twig beetle (Pityophthorus juglandis) (Figure 2a) enters the bark, making a tiny hole. The fungus (Geosmithia morbida) is introduced to the walnut tree as the beetle feeds on and tunnels into the inner bark (Figure 2b). The bark and phloem are killed by the fungus, causing a canker which is visible only if the bark is removed (Figure 3a). With thousands of beetle attacks, the number of cankers increase, (Figure 3b) eventually girdling and killing the branch or trunk. Some walnut species are more susceptible than others, and death may occur in those that are susceptible.
At this time the disease is known to occur in western and southwestern United States, Virginia and Tennessee (map above). Southwest United States and Mexico are thought to be home of the original host, the Arizona walnut (Juglans major). While this walnut species suffers little damage from the disease, eastern black walnut (Juglans nigra) is very susceptible.
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. Trees on the northern edge of the native range tend to grow more slowly, resulting in wood qualities that increase its value compared to wood from more southern areas. A close relative to eastern black walnut, butternut (Juglans cinera), also occurs in Minnesota and can also be infected by TC
The MDA is working to protect the state’s black walnut resource by preventing the introduction of TCD into Minnesota by using regulation, outreach and early detection. 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.
We are collaborating with our partner agencies and doing outreach to our many stakeholders, including walnut growers, millers, loggers, nursery operators, woodlot owners and tree care companies. TCD is one of the tree diseases taught in the First Detector Training program.
In 2011 the MDA is collaborating with the U.S. Forest Service Northern Research Station and the Minnesota Department of Natural Resources to survey walnut trees in urban areas around the state and in forest and riparian areas in southern Minnesota. Beginning in mid-summer, visual assessment for signs and symptoms of TCD in walnut trees in urban areas around the state and of walnut trees in forest and riparian areas in southern Minnesota are performed. Beginning in 2012, we added trapping for the walnut twig beetle to the survey, using a funnel trap and a pheromone-specific lure. To date we have not identified the disease or the walnut twig beetle in Minnesota.
A tree can have TCD for six to eight years before symptoms appear in the crown. Early crown symptoms include thinning, small leaves, yellow, brown or wilting leaves.
Once a tree is actively wilting, you may see signs of the beetle and cankers under the bark of wilting branches as shown in Figures 2 and 3 above.
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.
Author's note: Information for this article was obtained from a variety of sources, including publications of and personal communication with numerous entomologists, plant pathologists and foresters with the U.S. Forest Service, Minnesota Department of Natural Resources, Colorado State University, and the University of Minnesota.