Thousand Cankers Disease Threatens Black Walnut Trees

First posted on 02-18-2010

Missouri’s black walnut trees could be in danger if a disease in western states continues moving east, said a University of Missouri Extension forestry specialist.

Thousand cankers disease has devastated black walnut trees in at least eight western states: Washington, Oregon, California, Idaho, Utah, Arizona, New Mexico and Colorado.

The disease is caused by a fungus that attaches to walnut twig beetles. These tiny bark beetles inflict very little damage themselves. The fungus, however, creates small patches of dead tissue under the bark. As these cankers grow and merge, nutrients can no longer move through the tree, killing it in one to three years.

The fungus poses a serious threat to black walnut trees in the eastern U.S., said Hank Stelzer.

In a typical survey in one western state, two 18-inch logs yielded more than 23,000 beetles. The sheer number of beetle attacks and the resulting cankers led to the name “thousand cankers disease.”

“To prevent such an outbreak, it is critical that foresters, arborists, woodworkers, mills and companies involved in lumber movement take every precaution to avoid transporting infected walnut logs, slabs, turning wood and firewood with intact bark to areas further east,” said Harlan Palm, president of the Missouri Walnut Council.

It could take just one careless person moving a walnut log with the beetle to cause an outbreak, he said.

No currently feasible treatment can prevent or cure the disease, researchers say. Rapid detection and removal of infected trees remain the primary means of managing the disease.

Symptoms include dark stains on the outer bark that extend beneath the surface to the wood; yellowing and thinning of the upper crown; wilting of leaves; and dead or dying branches.

The first Thousand Cankers of Black Walnut National Conference took place recently in St. Louis, drawing 145 participants from 24 states.

If you live in Missouri and suspect your walnut tree has thousand cankers disease, contact the Office of the State Entomologist, Missouri Department of Agriculture, at 573-751-5505.

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