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Walnut twig beetles boring under bark bring a fungus that causes these dark cankers in black walnut trees.
Credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Brown, collapsed leaves on a walnut afflicted with thousand cankers disease.
Published: Tuesday, Aug. 31, 2010
Hank Stelzer, 573-882-4444
ALBANY, Mo. – First it was the American chestnut, then it was American elm. Get ready to add eastern black walnut to the threatened and endangered species list, said a University of Missouri Extension state forestry specialist. A fungal infection called thousand cankers disease (TCD) has ravaged black walnut trees in a handful of western states over the last decade. In July, foresters discovered infected trees in Tennessee.
Unlike the western states, Missouri and Tennessee are native homes to the eastern black walnut tree, supporting extensive natural populations that could fuel an uncontrolled TCD outbreak in the eastern U.S., said Hank Stelzer, who discussed the disease at a recent field day at the MU Hundley-Whaley Research Center in Albany.
“Missouri has more black walnut trees than any other state within the species’ natural range,” he said.
According to Missouri Department of Agriculture estimates, the disease could cost the state more than $850 million over a 20-year period due to losses in the wood products industry and nut production as well as costs associated with the removal and replacement of urban trees.
Before the discovery of infected walnut trees in Knoxville, Tenn., Colorado was the easternmost state with reported cases of TCD. The big jump could be the result of people transporting firewood or hobbyists buying wood on eBay, Stelzer said.
Researchers and foresters from MU, the Missouri Department of Conservation and the Missouri Department of Agriculture are working with the USDA Forest Service and scientists from those western states where TCD originated to develop an effective monitoring program before next year’s growing season.
”Finding TCD this late in the growing season is a mixed blessing,” Stelzer said. “Since the obvious symptoms of wilting, yellowing or collapsed brown leaves still attached to the branches are best observed in June and July, we have some time to put together a good monitoring program and educate the natural resource professionals and the general public.” However, he added, “It gives the disease more time, too.”
Stelzer notes that TCD should not be confused with walnut anthracnose, a very common foliar disease on walnut this time of year. “Anthracnose-infected leaves are yellow and have black spots all over the leaflets,” he said. “The compound leaves also lose their leaflets from the bottom up.”
TCD is a bit different. “Entire leaves remain intact. They have a collapsed appearance and remain attached to the branch. Leaf color can range from yellow to entirely brown and they lack the visible black spots associated with anthracnose.”
TCD is caused by a fungus that attaches to walnut twig beetles. These tiny bark beetles inflict very little damage themselves, but the fungus creates small patches of dead tissue under the bark. As these cankers grow and merge, nutrients can no longer move through the tree. By the time the first symptoms appear, the damage has been done and the tree dies.
“All walnut plants and plant parts, as well as all hardwood firewood from TCD-infected states, are now prohibited from entering Missouri,” Stelzer said. “This includes nursery stock, bud wood, scion wood, green lumber and other material living, dead, cut or fallen, including stumps, roots, branches and composted and uncomposted chips. Exceptions are nuts, nutmeats, hulls and processed lumber. Processed lumber means 100 percent bark-free, kiln-dried with squared edges.”
If you suspect that your black walnut has TCD, contact either your county MU Extension center or your local Missouri Department of Conservation office, Stelzer said.
“TCD has not been detected in Missouri yet,” he said. “But we do need to be aware of this serious threat to our most valuable hardwood species.”
More information on the disease is available from the Missouri Department of Agriculture at http://mda.mo.gov/plants/pests/thousandcankers.php.
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