Thousand Cankers Disease Threatens Walnuts

This black walnut limb shows multiple cankers and beetle galleries.

This black walnut limb shows multiple cankers and beetle galleries.

Thousand cankers of walnut has emerged as a disease of significant concern in the western U.S. The disease, caused by a newly described fungus Geosmithia morbida, is spread by a small bark beetle. Disease symptoms are most apparent in late summer and early fall. Thinning and yellowing of the upper crown is followed by larger branch dieback and eventual tree death. Dark staining on the bark, especially in July and August, is characteristic and is associated with pin-sized entry or exit holes of the beetle.

Removing the outer bark will reveal cankers surrounding beetle holes and galleries. The beetles first attack the upper branches, typically on limbs about hot dog size. Eventually they infest major limbs and the trunk, leading to tree death within two to three years of initial symptoms.

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What Is Thousand Cankers Disease?
Thousand cankers is an insect-vectored disease spread by the tiny walnut twig beetle (Pityophthorus juglandis). The beetle carries spores of the fungus Geosmithia and infects the tree as it tunnels into the bark. The fungus then colonizes and kills the phloem tissue and forms cankers around beetle galleries. There are usually numerous entry holes and galleries, each surrounded by a canker, hence the name “thousand cankers.” The cankers eventually coalesce, girdling branches and stems.

Colorado State University plant pathologist Ned Tisserat and entomologist Whitney Cranshaw first identified the pathogen and characterized its association with the beetle in rapidly declining eastern black walnut (J. nigra) landscape trees in central Colorado in 2004.

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Where Has Thousand Cankers Disease Been Found?
In addition to the initial observations in Colorado, the disease has been identified in both eastern and native black walnuts in California, New Mexico, Idaho, Utah, Arizona, Oregon, and Washington during the last decade. The first report east of the Rockies occurred in July, 2010, in Tennessee in eastern black walnut. Walnut twig beetle appears to attack all species of walnut (Juglans) and the disease is only known to occur in walnut. Thousand cankers disease has killed primarily black walnut species, with eastern black walnut thought to be the most susceptible. Persian or English walnut (J. regia) has been less affected so far.

In California, thousand cankers was first observed in June 2008 in declining northern California black walnut trees (J. hindsii) near Davis. Since then, it has been found statewide in both our native species — northern and southern California black walnut. This is a commercial concern because northern California black walnut seed is used to produce both the black and Paradox hybrid (J. hindsii x J. regia) rootstocks used in walnut orchards and thousand cankers disease also has been confirmed in Paradox rootstock. Fortunately for the walnut industry, thus far the occurrences of thousand cankers disease in English walnut trees scattered across California are primarily in trees having low vigor or other problems.

What Is The Beetle Distribution And Its Activity?
The walnut twig beetle, thought to be native to northern Mexico, New Mexico, Arizona, and California, was first described in 1928 in New Mexico and was first collected in California in 1959 in Los Angeles County. Utah’s records date to 1988 and the insect was first confirmed in Idaho in 2003.

Walnut twig beetles are active from April through October with the highest trap catches, using Yellow Pherocon AM traps, occurring during the summer. There appear to be two or more generations annually. Adults and larvae can overwinter in California in black walnut trunks.

How Can I Manage It?
There is considerable concern about the spread of the beetle and the disease into the native range of J. nigra in the eastern U.S. There are presently no effective controls. Rapid detection is the primary management option. In Colorado, infected trees are cut as soon as possible and all wood material is chipped. Chipping does not kill the beetles, but interferes with their breeding. Infected wood should not be sold as firewood in order to limit the spread of the beetle and disease; it should be burned in accordance with local air quality restrictions. Lumber for woodworking should be de-barked, dried, and moved only with extreme caution as it may harbor beetles if not handled properly. Quarantines enacted by Midwestern and eastern states to protect their black walnut forests need to be respected.

Research is underway to assess the relative disease susceptibility of various walnut species and the host preferences of the beetle. In California we are assessing disease distribution, testing insecticides and a plant stimulant for use in preventing infection, and the USDA Forest Service and University of California-Davis are developing a pheromone for use in walnut twig beetle detection and research.

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