Disease that attacks black walnut trees sweeps across US

By Laura Crawford

Following on the heels of an Emerald Ash Borer infestation that wiped out millions of ash trees in the Great Lakes region, a new disease, dubbed Thousand Cankers Disease (TCD), has been decimating black walnuts in the western U.S. — and now it’s spreading east.

It’s already been detected in one Great Lakes state, Pennsylvania.  Three others – Indiana, Michigan and Wisconsin – have quarantines on black walnut entering them.

Black walnuts are common in suburban lawns and also are grown commercially. The slow-maturing black walnut can fetch high prices for its prized wood. Some farmers consider it “the 401K on the stump,” cashing in on the wood for their kid’s college education or their retirement, said Bruce Moltzan, Ph.D., national program leader for forest pathology with the U.S. Department of Agriculture (USDA) Forest Service.

But the lucrative trees face a new threat.

TCD is unique because it’s not caused by a new invasive species like many other tree infestations, and because it’s the result of a combined insect and a fungal attack.

The walnut twig beetle was first associated with black walnut deaths in Colorado in 2001, and further investigation found that it carried a partner in crime – a newly identified fungus, geosmithia morbida.

The twig beetle penetrates the bark of the tree.. The fungus then infects the tree’s living tissue with each twig beetle bite, leaving a canker.  After thousands of bites, sometimes taking years, the tree is destroyed.

TCD has been detected in 12 states, which all now have quarantines to prevent the transporting of infected wood outside their borders.  Other states including Missouri, Kansas, Michigan, Nebraska, Oklahoma, Indiana, Minnesota, North Carolina and Wisconsin, now have quarantines preventing imports of the wood from infected states.

“These are hard knocks for hardwoods,” Moltzan said.

“At first this disease seemed like just a native species gone awry, but once we looked closer, TCD was very distinctive,” Moltzan said. “These are native species, but the result could be as damaging as any invasive species.”

TCD is especially deadly to the Eastern Black Walnut, which is the native species in the central and eastern parts of the U.S. and southern Ontario. Eastern Black Walnuts transplanted into western states have been decimated by TCD.

“While TCD is not a true invasive because it’s not coming from outside the country, it can be considered a biological invasive,” said Dennis Fulbright, a professor of plant pathology at Michigan State University. Fulbright often advises members of the Michigan Nut Growers Association.

“But in this case, we may have taken the host to the disease,” he said.  “If we keep moving wood, firewood or otherwise, it (TCD) will definitely come to our region.”

“The Emerald Ash Borer was a capitalist disease,” said Fulbright, who believes that EAB entered Michigan from Asia, probably because of lax inspections. “People didn’t check, didn’t follow rules and didn’t care. These seem like boring regulations but they need to be done.”

Climate change could drive the disease north to Great Lakes states as well.

“One of the most important questions is ‘why now?’” Moltzan said. “The beetle is native to Arizona; the fungus also prefers warm temperatures. Surveys of insects and walnuts in northern regions haven’t found the twig beetle before.”

“It’s speculation, but it could be spreading north because of temperature,” Fulbright said.

Walnut is a valuable commodity, Moltzan said. That’s one reason why Great Lakes tree farmers are worried about the disease.

Des Jones, a certified tree farmer in Augusta, Mich., harvests black walnuts and other hardwoods from his 86-acre forest. He’s attended workshops that describe how to identify TCD.

“I look for tell-tale signs; if the tree were dying back at the top, then I’d look for small holes in the bark,” Jones said. “If I found those, I’d peel a small section of the outer layer to check for tunnels in the wood.”

So far his trees look healthy.

“As a walnut grower, I’m very concerned,” Jones said. “We’ve had a lot of trouble controlling other diseases like the Emerald Ash Borer, and I’m afraid it (TCD) will sweep through the states.”

One of the goals of the USDA Forest Service, Moltzan said, is to better coordinate state regulations and improve detection and control of TCD. He hopes that more researchers will investigate TCD’s causes and prevention.

“We’ve seen it cause significant damage,” he said. “But we’re just in the infancy of understanding this disease.”