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Walnut trees face possible extinction

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The chestnut tree is gone, we've lost most of our elm, ash and many of our butternut trees, and now we stand to lose our walnut trees as well.

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Just two years ago, researchers discovered that a sudden decline in black walnut (Juglans nigra) in Colorado was due to a combination of the walnut twig beetle and a previously unknown fungus that infested the trees by the hundreds of thousands, causing cankers and cutting off the flow of nutrients. With a mortality rate near 100 percent, what is the prognosis if the disease moves into black walnut's native range?

According to Whitney Cranshaw, professor of bioagriculture science and pest management at Colorado State University: "Based on the patterns seen in the West, such a colonization could very possibly develop into an uncontrollable outbreak. This may ultimately have the potential to destroy black walnut in its native range.

The walnut twig beetles eat through the cambium layer of the stem and branches, blocking the flow of nutrients.

"It is critically important that fresh cut logs from walnut harvested in the Western states never be allowed to move outside the area where thousand cankers [disease] currently is present. Movement of a single log with live beetles can be the initial source of an outbreak that could ultimately devastate black walnut in uninfested areas."

Tiny but deadly

The beetles are tiny - about 1/16" (smaller than a grain of rice) - but they make up for their size with numbers. Researchers have found as many as 20,000 beetles in a 4' section of a small walnut log. By themselves, the beetles cause only minor damage to the walnut trees. The fungus they bring with them infects the tunnels, killing the cambium layer of the tree and cutting off the food supply. The tree literally starves to death. The dead cambium forms cankers, which gives the disease its name. The fungus is so deadly to black walnut trees that it has been named Geosmithia morbida.

After the first year of infection, some of the foliage in the upper branches turns yellow at the tips and thins out. By the time these symptoms appear, the disease has progressed to the point where the tree cannot be saved. As the disease progresses, larger branches die. The tree dies within three years of the first visible symptoms. Once infected, there is no effective treatment.

The beetle's small size makes diagnosis extremely difficult.

In July, the U.S. Department of Agriculture announced confirmation of thousand cankers disease in Knoxville, Tenn. - the first occurrence in the native range of black walnut. Although other states quickly imposed bans on walnut from Tennessee, researchers estimate that the disease has been in the area for eight years. It is impossible to know how far it has spread in that time and in what direction.

Stop the spread

There are steps you can take to help stop the spread of this disease to black walnut's native range. Do not sell or transport walnut logs, slabs or firewood (any walnut with bark attached) from areas of known or suspected infestation into unaffected areas. Kiln-dry walnut lumber, however, poses no threat. If you suspect that you have walnut trees that are affected, call your area forester or the USDA. Do not send out a sample without specific instructions because the beetle can chew its way through plastic and cardboard packaging.

Walnut trees and their nuts play a vital role in the ecology of many of our forests. Many livelihoods depend on walnut trees - woodworkers, loggers, log buyers, sawmillers, the edible nut industry, furniture makers, carvers and makers of many specialty walnut products. Harlan Palm, president of the Missouri Walnut Council, estimates that the loss of walnut trees in Missouri alone would amount to roughly a half-billion dollars and would wreak financial havoc on thousands of individuals. Serious tree farmers have been tending walnut plantations for decades to provide retirement income or to leave something of value for their grandchildren. It's hard to describe how devastating this would be for them.

This map shows the areas of known canker infestation in red and the native range of black walnut in green.

Spread the word, not the disease.

This article originally appeared in the March 2010 issue of Sawmill & Woodlot Management magazine.

This article originally appeared in the October 2010 issue.


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