Black walnut trees in the U.S. are facing a very serious new threat called thousand cankers disease according to Colorado State University researchers. This recently recognized problem has already devastated black walnut trees west of the Rocky Mountains over the past 10-15 years.
"Thousand cankers disease is caused by a newly discovered fungus that is carried to trees by a tiny bark beetle," said Ned Tisserat, a plant pathologist at Colorado State, who first identified the fungus last summer. "The fungus then colonizes and kills a small area of the bark surrounding beetle galleries. The number of beetle galleries and associated dead bark, called cankers, in a tree is enormous. Cankers eventually fuse and girdle limbs and the trunk so that nutrients can no longer move in the tree."
Trees typically die within a couple of years after they first show symptoms of leaf yellowing and branch dieback.
The walnut twig beetle, a native insect of the Southwest, is usually associated with the Arizona walnut tree, to which it is not harmful. It is, however, harmful to the black walnut tree which is highly valued for its wood and nuts. Although it is native to the East, black walnut has been planted extensively throughout the West.
"The walnut twig beetle has shown a fantastic spread through the western United States within the past 25 years," says Whitney Cranshaw, an entomologist at Colorado State, who is working with Tisserat. "In recent years we have seen new records for this insect in Colorado, northern New Mexico, Utah, Idaho, Oregon and Washington. And everywhere we are finding the beetle, all the black walnuts are dying."
The bigger concern is thousand cankers disease moving east where black walnut is a common forest species.
"I think thousand cankers disease has the potential to devastate black walnut just as Dutch elm disease nearly wiped out American elm and chestnut blight eliminated American chestnut," said Cranshaw. "Right now it is contained in the West but all it would take is one careless individual moving a walnut log with the beetles and we could have an outbreak that could quickly spiral out of control."
Tisserat and Cranshaw emphasize the importance of foresters, arborists, woodworkers and lumber mills to recognize this new threat. They say no walnut logs with bark intact should be allowed to move further east than where the disease is currently known. Walnut wood and well-dried logs without bark likely pose little threat of carrying the fungus-carrying beetles.
For more information, contact Ned Tisserat at (970) 491-6527 or Ned.Tisserat@colostate.edu or visit www.ext.colostate.edu/pubs/insect/0812_alert.pdf.