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Thousand Cankers Walnut Disease
Linda Langelo, Horticultural Program Coordinator - Golden Plains
Area
Date: 6/23/2009
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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 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 University, 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 U.S. 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 if thousand cankers disease moves 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.
More Information:
Diagnosing Thousand
Canker Disease (406 kb)
Pest Alert - Thousand Canker
Disease (488 kb)
Thousand Canker Disease
Q&A (22 kb)
Walnut
Survey Q&A (15 kb)
Warning - Fresh Cut Walnut
Wood (287 kb)