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Eastern black walnut faces disease threat

Thousand cankers disease, thought to be caused by walnut twig beetles, is currently confined to the West, but spreading

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The list of North American wood species affected by disease continues to grow. In the 20th century, there was chestnut and elm, which were later joined by butternut, ash, several Western pines, hemlock, and now Eastern black walnut. Currently confined to the West, thousand cankers disease — apparently caused by the walnut twig beetle — is threatening the nation’s Eastern black walnut (Juglans nigra) population.

The disease is also present in California in native Northern California walnut (Juglans hindsii) and Southern California walnut (Juglans californica). Although Eastern black walnut is not native to the West, it was introduced by pioneers in municipal areas, mainly as street trees.

“About 2001, 2002, a forester in Boulder [Colo.] began to notice a dieback and decline in walnut and, previous to that, there had been reports of a decline in Eastern black walnut in New Mexico near Santa Fe,” explains Ned Tisserat, a professor of plant pathology at Colorado State University in Fort Collins, Colo. “At that time, entomologists found this insect called the walnut twig beetle. The assumption was that sometimes during that period there was a drought, the trees went under drought stress, this little twig beetle attacked these drought-stressed trees and the combination of the twig beetle and the drought killed the trees.”

At that point, nothing more was made of the situation. But then reports started coming in from Washington, Oregon, California, Utah and Idaho that a similar decline in the Eastern black walnut population was occurring. So, as with any other tree disease, the main question is what can be done to prevent its progression?

“We hypothesize that the beetle came from the Southwestern United States … somehow that the beetle jumped its native distribution and wound up farther north, but how that happened is still a mystery. In addition, with this beetle came a fungus called geosmithia, and this fungus had never been reported as a pathogen on trees, so this is a brand-new fungus. Whether it has always been an associate with the beetle in its native range, or is some new combination, we still don’t know … but when it jumped host from Arizona walnut to Eastern black walnut, and other native species of walnut, that’s when we began to see damage.”

According to Tisserat, the eastern-most boundary of the walnut twig beetle is in Colorado in the Denver metro area, Boulder and Colorado Springs. Eastern black walnut, although not native, has prospered along Colorado’s Front Range. However, the beetle is extremely aggressive and the fear is it will continue its journey eastward.

“Every time a beetle attacks the bark, there’s a canker or a dead area that surrounds that beetle gallery. It takes a lot of these cankers to coalesce and that eventually girdles the branch. At that point, the branch dies and that’s why we named this Thousand Cankers because it is the action of multiple beetle hits and multiple cankers that eventually kills the tree, so it’s like death by a thousand cuts.”
For people in the Midwest, where walnut is prevalent, the fear is the disease will continue heading eastward.

“I lived for 20 years in eastern Kansas and I had beautiful black walnut on my property and I just cringe at the thought about what could happen,” Tisserat says. “In my opinion, we better stop this in the West and, if we don’t stop this, it has the potential to be catastrophic. … I don’t mean to be Chicken Little here, but we really need to be heads-up on this one. We have to be very careful about movement of wood, lumber that hasn’t been cured properly. If the bark is still attached, it’s a huge threat. The main thing that we are really harping on is if you save any wood, you have to debark it.

“I don’t want to be overly pessimistic, but I have seen butternut canker and I’ve had oak wilt on my property, and I’ve seen these other catastrophic problems. This one potentially could match those, [and] go toe-to-toe in terms of damage.”

This article originally appeared in the July 2009 issue.

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