A formal exterior quarantine for thousand cankers disease of walnut was signed into effect August 8, 2011. A few revisions were made to the emergency exterior quarantine that was issued in February 2011 based on public comments. Minnesota joins several other states within the native range of easter black walnut that have similar exterior quarantines in place (see map).

Thousand cankers disease (TCD) occurs on walnut trees and is caused by a fungus carried by an insect. TCD has been reported in western and southwestern United States for several years. In 2010 it was detected in Tennessee and in 2011 it was detected in Virginia and Pennsylvania. As the disease spreads, it threatens eastern black walnut (Juglans nigra) in its native range, which covers all or parts of over 30 states, including southeastern Minnesota (map below).

 
Map created by: Alison Rossow and Erich Borchardt, MDA, August 2011 

What is thousand cankers disease?

Figure 2a. Walnut twig beetle (Pityophthorus juglandis) Whitney Cranshaw, Colorado State University	Figure 2b. Entry/exit holes and tunneling of the walnut twig beetle, Whitney Cranshaw, Colorado State University
Figure 3a. Canker surrounding beetle tunnel. Ned Tisserat Colorado State University	Figure 3b. Cankers coalesce to create large dead areas visible under the bark, Ned Tisserat Colorado State University

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. Death may occur in susceptible walnut species.

Where is thousand cankers disease?

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 of this insect/fungus complex, the Arizona walnut (Juglans major). While this species suffers little damage from the disease, eastern black walnut (Juglans nigra) is very susceptible.

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. It is also an important tree along rivers and streams. 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. Walnut trees in landscapes grow as far north in Minnesota as Hibbing.

While we do have black walnut trees planted in urban settings as far north as Lake of the Woods County, we are most concerned about the potential threat to eastern black walnut that occur naturally and in plantations in southern Minnesota. Our state has over 6,000,000 eastern black walnut trees, with one to two million board feet of walnut wood harvested annually. Being on the northern edge of the native range, our trees tend to grow more, 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, but the butternut’s susceptibility to TCD is not known.

What are we doing about thousand cankers disease?

MDA is working to protect the state’s black walnut resource by preventing the introduction of TCD into Minnesota using regulation, outreach and early detection. The highest risk pathway for TCD movement is in walnut wood with bark on.

On August 8, 2011, Minnesota Agriculture Commissioner Dave Frederickson signed a permanent exterior quarantine restricting movement 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 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 aroudn the state and in forest and riparian areas in southern Minnesota. For the second consecutive year we are also collaborating in a targeted survey to locate walnut trees in Minnesota that are showing unexplained dieback. In addition to serving as an early detection tool now, we hope the survey identifies sites for future trapping once a lure for the beetle is developed. Our emerald ash borer firewood inspectors have added walnut to their list of targeted species.

How can I tell if a walnut tree has thousand cankers disease?

Black walnut tree with crown symptoms of TCD. Whitney Cranshaw, Colorado State University, Bugwood.org

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.

If you see a tree or stand of trees that you think might have thousand cankers disease, contact Kathy Kromroy at the Minnesota Department of Agriculture. Phone 651-201-6343. Email Kathryn.kromroy@state.mn.us

Authors 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 US Forest Service, Minnesota Department of Natural Resources, Colorado State University, and the University of Minnesota.