Thousand Canker fungus threatens Denver walnut trees
| July 21, 2009 | 7:02 PM
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A fungus-carrying beetle has Denver’s forestry department up a tree.
“Our biggest concern is that we could lose all of our walnut trees,” said Jude O’Connor, city forester. ”The bigger threat is that it will escape Colorado and spread in areas where walnuts grow naturally.”
The city has 500 walnut trees on public right-of-ways and another 200 in parks. But O’Connor estimates that another 4,000 to 5,000 black walnuts are on private property.
So far, 85 walnut trees in Denver – all privately owned — have been infected with geosmithia, a fungus that causes Thousand Cankers disease, so called because trees develop multiple cankers. Infected trees usually die within two to three years, after the fungus chokes off the trees’ circulatory systems.
“We can lose 6,000 trees of that species — that’s the potential,” O’Connor said.
Since last summer, Boulder has lost 70 percent of its black walnut trees to the disease, and the fungus-carrying walnut twig beetles have migrated to Broomfield, Westminster and Wheat Ridge before appearing in northwest Denver this year.
While the beetles can cause damage by tunneling into trees and laying eggs inside the bark, they aren’t killing the tree.
“It’s the fungus that’s killing the tree, but we’ve not figured out a way to get rid of the beetle,” O’Connor said. “There is no insecticide that can control that beetle.”
The beetles are tiny — about 1.7 millimeters long — and cannot be seen without a magnifying glass. They can fly or be carried by the wind. They also can be found in firewood, just underneath the bark.
Once they make their home in a tree, it may be two years before the tree shows any symptoms of the disease, which include the leaves drying out and turning brown and the branches starting to die.
By then, there’s not much to do except cut the tree down and dispose of the wood since there is no way to stop the disease. The tree can’t be used for firewood because of the risk of transporting the beetles to new locations.
O’Connor urges residents and property owners to contact her office if they have questions about the beetle, the fungus or the disease.
“We want them to call us to help them figure out if their trees are infected,” she said, adding that the city will send out a tree inspector if necessary.
Property owners could receive citations if they do not remove a diseased tree.
“The city does have an ordinance about communicable disease and other threats to the urban forest,” O’Connor said.
Denver will work with property owners on having diseased trees removed and setting up a payment plan for the service.
For more information, call the city forestry office at 720-913-0651.
black walnut, fungus, geosmithia, Jude O'Connor, Thousand Canker disease, trees
