Thousand Canker Disease of Walnut
Thousand Cankers poses a serious problem to the health of the black walnut tree. Walnut trees are important because of their nut crop and the desired wood for various wood products. The Kansas Department of Agriculture, Kansas Forest Service, and the Kansas State University Extension Service are asking for your help in educating clients, stopping introduction, and limiting the spread of this disease into Kansas with early detection. We are deeply concerned if the disease reaches the native range of black walnuts in central and eastern Kansas that we may lose this tree to both our urban and native forests.
The disease is known currently in the nearby states of Colorado and New Mexico. Colorado scientists believe that it is the move-ment of infected wood either as firewood or for woodworking that has introduced the disease into urban areas of Colorado. Wood, bark, and chips with beetles and cankers are highly contagious and should not be moved off of a site for three years. Do not bring in walnut wood from out of state sources.
What to Look For in a Tree
Thousand Cankers disease of walnut is a progressive disease that kills a tree within two to three years after initial infection.
The disease causing fungus, Geosmithia sp., is transmitted by a small twig beetle. Branches and trunk tissue are killed by repeated infections by the fungus, as beetles carry the fungus into new bark cambium tissue, repeating the infection. That’s why it is called Thousand Cankers disease.
Here are several key points to remember when surveying and sampling for Thousand Cankers. Dead trees require careful scrutiny of the localized area.
- Look for declining trees. Initial symptoms are yellowing and thinning followed by death in two to three years. This is early symptom development.
- Trees with dead leaves are highly suspect and an advanced symptom. Branches collapse in late spring and summer, and leaves die and remain attached to the branch. This flagging symptom is similar to Dutch Elm Disease.
- In Colorado, twig beetles are attracted to branches with southern and western exposure. Samples should come from this area of the tree, if possible.
- Collect a sample from branches 2 to 4 inches in diameter. Cut the branch down.
What to Look For in a Branch
Take a strong bladed knife or drawknife, and cut or scrape away the bark. Now take the knife and carefully slice the tissue directly under the bark parallel to the surface, peeling away the layers. If dieback is caused by Thousand Cankers, you will see:
- Black cankers about the size of a dime or larger.
- Beetle galleries in the centers of the cankers.
You may also see:
- Beetles about the size of a pencil lead.
A gray spot/mass in some beetle galleries. This is a fungus colony. - Small beetle entry holes in the bark above the cankers.
Twig Beetle Entry Holes.
Photo by M. Kennelly, KSU
Additional Information is available from KDA by clicking on the links below:
Thousand Canker Pest Alert
