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Walnut trees dying on Colorado's Front Range

Walnut trees dying on Colorado's Front Range
by Carl Wilson

Photo courtesy of Jim LaBonte, Oregon Dept. of Agriculture

There’s a new beetle in town. Walnut twig beetles carry a fungus disease called thousand cankers that has produced decline and death of black walnuts.

Boulder has lost a large portion of its walnuts. Since 2002 the disease has moved southward through Arvada, Wheat Ridge and is now in Northwest Denver. It has crossed the Platte River and is threatening an estimated 5000 walnuts, about 1 percent of the City’s trees. Most of the walnuts in Colorado Springs are gone.

Photo courtesy of Carl Wilson

Tree decline begins with a general yellowing and thinning of the upper crown. In the second year branch dieback is common. By the third year rapid wilting and the death of larger branches is typical. Trees are often killed within three years of noting initial symptoms.

Colorado State University researchers first identified the cause of rising walnut mortality seen here in other states including Washington, Oregon, California, Idaho, Utah, Arizona and New Mexico.

While CSU researchers have made strides in learning the biology of the organisms involved, no methods to control or delay the course of tree decline have been found. Though not promising to date, it is hoped that further study will reveal some means for treatment.

In the meantime it is recommended that wood from tree removal be left in place. Beetles can survive in small chips and logs for up to two years. Make every effort to contain the spread of this devastating disease and its insect carrier.

Walnut is a native hardwood in the eastern U.S. and the fear is the disease could pose a threat to native trees if carried to states east of Colorado. Black walnut is an introduced tree in Colorado and western states.

See Colorado State University Extension Pest Alert, Walnut twig beetle and 1000 cankers disease of black walnut.

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Updated Saturday, September 25, 2010