Slowing the Spread of Thousand Cankers Disease

Heat treatment schedule found effective for treating walnut logs and firewood

Forest Service entomologists Bud Mayfield (SRS, left) and Paul Merten (FHP, right) examine the bark of a black walnut branch for evidence of the walnut twig beetle, the vector of the fungus that causes thousand cankers disease. Photo by Adam Taylor, University of Tennessee.

U.S. Forest Service researchers have confirmed the efficacy of a heat treatment schedule that eliminates the insect and fungus that cause thousand cankers disease from black walnut logs.

Information included in the recently published research article by Forest Service Southern Research Station research entomologist Bud Mayfield and fellow authors provides managers and the public with a tool to help slow the spread of the disease in eastern black walnut, one of the most economically important tree species in the eastern U.S.

Thousand cankers disease is caused a fungus carried by the walnut twig beetle. Native to the U.S. Southwest, where it occurs on Arizona walnut without causing harm, the walnut twig beetle began moving out of its native range 20 years ago and started infesting eastern black walnut trees planted in the West. Some of these trees developed thousand cankers disease and began dying.

Starting in 2010, thousand cankers disease was discovered in the native range of eastern black walnut in Tennessee, Virginia, Pennsylvania, and Ohio. The fungal pathogen was recently isolated from symptomatic trees in North Carolina.

Black walnut is prized for its timber and nuts, as wildlife food, and for use as high-grade veneer. Black walnut growing stock in the eastern U.S. was recently valued at more than half a trillion dollars. If thousand cankers disease continues to spread through the native range of eastern black walnut, the negative impacts on lumber, veneer, and other wood-related industries will be substantial.   

A black walnut tree in Knoxville, Tennessee, exhibiting crown dieback caused by thousand cankers disease. Photo by Bud Mayfield.

Activity of the walnut twig beetle and of the fungus it carries is restricted to the phloem, the innermost layer of tree bark. Sixteen states in the eastern U.S. now have quarantines on the movement of walnut firewood to slow the spread of thousand cankers disease. Regulations do not include wood that is square-edged, kiln dried, and free of bark.

“The insect and pathogen can be easily moved to new locations in walnut logs, firewood, and other unprocessed walnut products,” says Mayfield, lead author of the article published in the Journal of Economic Entomology. “Treatments for walnut wood products with bark, including logs and firewood, are highly desired by industry to allow safe movement of these products as well.”

The researchers took logs from trees infected with thousand canker disease and tested debarking and three different heat treatments, looking for the survival of either the insects or the pathogens after treatment. Their results showed that heating black walnut logs to a minimum outer sapwood temperature of 56°C (132.8°F, measured at least 1 cm below cambium) for 40 minutes is an effective treatment for eliminating the thousand cankers disease vector and pathogen, and that the heat schedule used to treat logs and firewood for the invasive emerald ash borer—a minimum temperature of 60°C (140°F) at the wood center for 60 minutes—is more than adequate.

“Because many states are regulating all hardwood firewood in response to one or both of these pest problems, a single firewood heat treatment standard that is effective against both pests would be beneficial from a regulatory standpoint,” says Mayfield. “We therefore recommend the treatment schedule for the emerald ash borer as the logical, conservative heat treatment schedule for walnut firewood coming from areas affected by thousand cankers disease.”

The project is a result of cooperative research and funding by SRS, Forest Service Forest Health Protection-Region 8, the University of Tennessee, and the USDA Animal and Plant Health Inspection Service.

Read the full text of the article.

For more information, email Bud Mayfield at amayfield02@fs.fed.us .

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