Thousand Cankers Disease Found Closer to Missouri

Published: November 1, 2010

Thousand cankers disease (TCD) is one of the newer pest problems that causes walnut tree decline and eventually kills trees. The disease was first identified in Denver, Colorado and was later found in many western states. However, in July 2010, TCD was confirmed on several street trees in Knoxville, Tennessee. Since then, TCD was found at three other sites in Knoxville, as well as Knox County. It is thought that it takes eight to ten years from the time the insects infest the tree to tree mortality. Researchers estimate that TCD has been in Knox County for as long as twenty years. Unfortunately, entomologists believe that it is too late to implement an eradication program in Tennessee.

Thousand cankers disease is an insect-fungal complex that is lethal to walnut trees. The walnut twig beetle (Pityophthorus juglandis) and the fungus, Geosmithia morbida sp. nov. are associated with the disease. This insect is a small brown bark beetle about the size of the L in LIBERTY printed on a dime. Adults overwinter in cavities excavated in the bark of the tree trunk. In the spring, mating occurs and new tunnels are formed for egg galleries. During tunneling, the Geosmithia fungus is introduced with the insect and subsequently spreads. Cankers with dark staining then develop around the tunnels of the twig beetle. Cankers form on the bark surface the tree but they may not be always be visible until the outer bark is removed to expose the tunnels around the galleries. The first symptoms of thousand cankers disease are leaf yellowing and branch dieback in the upper part of the tree. Limb mortality occurs and eventually, the whole tree dies. More information regarding thousand cankers disease can be found at http://mda.mo.gov/plants/pests/thousandcankers.php.

Many agencies including the University of Missouri, Missouri Department of Agriculture, and Missouri Department of Conservation are rapidly responding to the threat of this disease. Surveying and identifying this disease is very important to its containment. If you suspect that your trees may have this disease, these are key elements of diagnosis:

1. Confirm that the tree is actually a black walnut.
2. Dead leaves are still on the limbs during the growing season and do not drop.
3. Vigorous shoot sprouting below the dead area at the top of the tree or on large branches.
4. Usually multiple trees at a site are dying.

If trees have these symptoms, do not remove the tree. Take a photos of the whole tree, as well as close-ups of affected branches about the diameter of a bratwurst (˜ 2 inches). Record the location of the tree, the site conditions, and any other pests or problems that might affect tree growth. When collecting a sample, cut a branch section (diameter 2 to 4 inches, length 1 foot) that includes both infested and healthy tissue. Shave thin layers of bark off without cutting into the wood. Look for dark tissue with round holes that go into the wood. If this type of tissue is visible, wrap samples in a plastic bag and enclose it in another sealed bag and freeze it for at least 48 hours. Freezing should kill the insects and not harm fungi, which are needed for diagnosis. Contact your local extension office for information concerning diagnostic services, forms, and fees. Alternatively, e-mail photos and information to your local forester and they can submit samples for a diagnosis. A directory of local foresters is available at: http://mdc.mo.gov/regions under the section “who’s my local contact”. While state surveys will be conducted, local help in identifying affected walnut trees will facilitate the effort to limit any further distribution of thousand cankers disease.