Walnut: Thousand cankers disease

Biology

Thousand cankers disease (TCD) of walnut is a fungal disease associated with and spread by walnut twig beetles (WTB). Beetles feed on both healthy and damaged trees, but injured or weakened trees are more susceptible to rapid decline. The beetles attack branches about 3⁄4″ and larger, feeding in the phloem (living bark tissue). Initial damage consists of numerous small, diffuse cankers that appear as brown to black stained areas associated with beetle tunnels. These small cankers are seldom visible on the surface of the twigs and branches. The earliest visible foliar symptoms may be yellowing leaves, thinning or flagging foliage in the crown, poor leaf-out in the spring, or limb dieback, but by this time the disease is well-advanced in the tree. Over time, the small cankers coalesce to form large, girdling cankers that kill twigs, branches, and eventually the entire tree. Large cankers (up to 6 feet in length) may develop on the trunk and can show visible brown or black staining. TCD also weakens trees and makes them more susceptible to other problems, including Armillaria root rot. Small cankers are most easily detected on smooth-barked branches about 3⁄4″ in diameter. Shaving the bark on these twigs should expose the galleries and staining associated with the beetles and the disease. Exit holes made by adult beetles may be observed, as well. Since the beetles and the fungus are both found on all infected trees, the presence of either one is considered diagnostic for TCD. The disease is eventually fatal to black walnuts (Juglans nigra). Black walnut hybrids appear to be somewhat resistant. English walnuts (J. regia) are considered fairly resistant, though not immune. See Walnut: Walnut twig beetle for more information.

Management Options

Use Integrated Pest Management (IPM) for successful plant problem management.

Non-chemical Management

Select non-chemical management options as your first choice!

• Do not plant susceptible species in areas where TCD is known to occur.
• Maintaining excellent tree health by providing adequate water and good nutrition may help slow progress of the disease, but will not prevent infection or eventual death of infected trees.
• In known TCD areas, do not move live plant material or raw wood (branches, chips, logs, firewood, stumps, etc.) from susceptible trees. Beetles are able to complete their lifecycle in cut wood and can also reinfest cut logs that retain any bark.
• Beetles have been observed to survive in large-diameter, freshly cut chips. However, chips will dry and become unsuitable host material for both beetles and fungus in a relatively short time, so chipping is considered one of the better ways to deal with infested materials.
• Wood from TCD-affected trees should be milled and used locally to prevent spread of WTB and TCD into new areas. Milled, bark-free lumber will not support WTB, but it is not known how long dried logs or slabs that retain bark can be reinfested.

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides.

• Currently there are no known effective controls for TCD once it has become established in a tree.
• There are no chemical options recommended for home use for either TCD or WTB.