Management of Black Walnut Decline is too early to define

Last Updated: 
February 3, 2009

CORVALLIS, Ore. – A disease that infects and kills black walnut trees has been detected in Oregon, but effective ways to manage it are still unknown.

It's too early to recommend cutting down all trees infected, according to Jay Pscheidt, a plant pathology specialist with the Oregon State University Extension Service.

"Hard pruning might be all that is needed, but we won't know until we get more people doing observations and experiments," he said.

"Black walnut decline is a progressive dieback of the upper crown of mature black walnut trees," Pscheidt explained. "It was first detected in Oregon during the early 1990s but there were few complaints until the mid 2000s when dieback occurred in a wide variety of locations. Early investigations could not pinpoint any particular cause."

Dieback appears to start in the upper crown and progresses to lower branches in succeeding years. Some trees, however, experience dieback in their tops almost concurrently with spotty dieback of limbs at various places throughout the crown. Infected trees can die within two to five years.

Several theories and ideas have been proposed to explain causes of the black walnut decline in Oregon such as drought, genetic vulnerability, a newly identified fungus named Geosmithia and the walnut twig beetle, which was identified in Oregon in 2007 for the first time.

"Pathologists in Colorado have also observed a similar decline and dieback of black walnuts," Pscheidt said. They believe there is an association between the fungus and the walnut twig beetle, allowing spread of the disease. The working name for the disease in Colorado is "thousand canker disease."

"It appears to kill trees more quickly in Colorado than it does here," Pscheidt said.

Tree owners have spent a lot of money pruning dead branches only to have the tree continue to die back, Pscheidt said.

Pruning in less-affected trees may get below the diseased tissue and save the tree. But severely infected trees are not likely to recover, and Pscheidt advises not wasting money pruning the dead wood. Lumber value, however, in larger trees could help pay for tree removal.

"At this time there are no recommendations for use of insecticides or fungicides to treat infected trees," Pscheidt said.

Author: Judy Scott
Source: Jay Pscheidt