A new black walnut tree disease has been identified as a potential threat to Iowa's walnut trees. The recently-recognized disease called thousand cankers of black walnuts has not been found in Iowa. Thousand cankers disease is present in the western United States and remains a potential, serious threat to black walnut trees throughout the country.
What is it? Thousand cankers disease causes a rapid decline in walnut trees followed by tree death. It has been observed for little more than a decade in western states, mainly Utah, New Mexico, Idaho and Oregon, and since 2001 in Colorado. The fungus is vectored by the walnut twig beetle. The twig beetles carry the fungus to uninfected trees. As the fungus infects the bark cambium tissue, twigs, branches and eventually entire trees are killed by multiple infections and small dead areas called cankers, hence the name "thousand cankers disease". The walnut twig beetle is native to North America and has been found in the western, but not in the eastern United States.
What to look for? Symptoms in infected trees often progress from the top of the tree downward. The early symptoms may be wilting and yellowing of the foliage and thinning (dieback) of the top branches. In later stages of the disease dead branches in late spring or summer brown, dead leaves remain attached to the twigs. Finally, large areas of foliage quickly wilt and the tree dies within three years of initial infection. Small beetle entry holes in the bark will be seen but the cankers for which the disease is named cannot be observed until the outer bark is removed. Thousand cankers-infected trees will have numerous small or large cankers under the bark with beetle galleries (tunnels) in the middle of the cankers. Multiple cankers may be observed.
How does the disease spread? The United States Department of Agriculture reported potential pathways of infested wood movement to be: timber, firewood, wood packaging material, nursery stock, and scion wood for grafting. Nuts are not a concern since this pathogen is not systemic or seed-borne. The disease most likely would enter Iowa by human activity such as movement of firewood, untreated wood or logs. Natural spread (wind and water) is unlikely since this pathogen requires a vector and the Great Plains are considered by many to be a natural barrier due to limited host plant material.
Are there control measures? There is no feasible control of the fungus or insect vector. Rapid detection and prompt removal of infected trees is the best management practice. If the disease is detected in Iowa, owners would have to take specific measures necessary to prevent the spread of disease following USDA regulations and instructions for proper wood disposal. In most cases, wood will need to be chipped, and cannot be kept for firewood and/or woodworking.
How can individual homeowners help? Leaf yellowing/flagging of black walnut can be a symptom of several, less important problems. To accurately diagnose or rule out thousand cankers disease, submit a sample for diagnosis to the ISU Plant and Insect Diagnostic Clinic, 327 Bessey Hall, ISU, Ames, IA 50011. A sample should consist of 2 to 4 branch pieces with symptoms. Select samples about 2 feet long from branches or the tree with a diameter of 2 to 4 inches. Bag the samples and send in a box along with the Plant Disease Identification Form that can be secured from your local ISU Extension office. Be sure to include information on where the sample was collected.
Stop in our Extension Office at 209 Centennial Drive Monday thru Friday 8:00 to 4:30. We have staff that can assist you getting the correct diagnose for you