Green Horizons

Walnut twig beetle. (Pencil added to show scale.) Their small size makes diagnosis extremely difficult. The relationship between the beetle and the fungus associated with them is still not understood.

The American chestnut tree is gone, we've lostmost of our urban elm and many of our forest butternut trees, ash is now in rapid decline, and we stand to lose our black walnut trees. 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, which infested the trees by 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 the University of Colorado, "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.

"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 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. Woodworkers, lumber yards, tree removal services and firewood distributors are among the key groups that need to be provided information on this new disease."

The beetles are tiny - about 1/16 of an inch - smaller than a grain of rice. In late April and early May, they fly to walnut twigs and tunnel under bark where they mate and lay eggs. The larvae tunnel through the tree and chew their way out as adults. Researchers have found as many as 20,000 beetles in a four-foot 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.

Map showing areas of known canker infestation (dark), and the native range of black walnut (light). So far, it appears the disease has only affected areas where walnut has been planted outside its native range, but the nearly 100 percent mortality and rapid spread make it critical not to allow the beetle and fungus into the native range

The first physical symptom of the disease is a tiny entry hole in the outer bark of a branch or twig. Unfortunately, both the beetle and the entry wound are too small to be detected without a magnifying lens. 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. Mortality rate is nearly 100 percent.

There are no known means of controlling the spread of the disease. Application of insecticides and fungicides do not appear to be effective. Further frustrating attempts is the fact that an outbreak would not likely be detected for several years after the initial infestation, giving the beetle and fungus plenty of time to settle in and spread to other trees. This means that a quarantine or destruction of infected trees would likely be ineffective.

The walnut twig beetles eat through the cambium layer of the stem and branches, blocking the flow of nutrients - in essence, the tree starves to death.

Walnut tree dying of the Thousand Canker Disease. In the initial observed stages, the branch tips die back. By the time this occurs, there is little to no chance the tree will survive.

Dead walnut twig showing the exit holes of the mature walnut twig beetle. A single 4-foot block of walnut was found to contain more than 20,000 beetles.

At this time, there are no known cases of the disease east of Colorado. To infect trees in the main part of the black walnut range, it would have to cross the Great Plains. The beetle and fungus could hitchhike across on a logging truck, hidden under the bark of a log or of a slab of walnut sold to an unsuspecting customer or moved into new areas by campers taking firewood with them. There are steps you can take to help stop the spread of this disease to black walnut's native range. Prevent untreated wood cut in infected areas - Colorado and areas west - from moving east. 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. If you live in an unaffected area, verify the sources of any walnut logs or slabs before buying them. As people salvage dead or dying walnut, it may be tempting to purchase it. As long as the wood is kiln dried, or consists only of heartwood (with NO bark), it poses no threat. It has been found that the beetle survives in walnut chips, so movement of walnut mulch into unaffected areas must also be avoided.

The U.S. Department of Agriculture does not consider the TCD to be an exotic disease, and therefore left it up to the states to deal with. The best line of defense is information. You can help by letting buyers and sellers know that walnut logs or lumber containing bark should not be shipped east from infested areas. If you have walnut trees, contact your state's department of agriculture or your state forestry agency for assistance with diagnosing any tree problem.

So, why is this important? 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 emotionally devastating this would be for them.

Spread the word - not the disease.

Photo credits: Disease and black walnut range maps adapted from U.S. Dept. of Agriculture maps. Other photos courtesy of Whitney Cranshaw, Colorado State University. This is an edited version of an article that appeared in the March 2010 issue of Sawmill & Woodlot Management magazine.