Small beetle brings fungus to trees

A very small beetle and the fungus it carries are creating a very big problem for the survival of black walnut trees

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One of the four horsemen has come for the area’s black walnut trees, but instead of a white horse, this pestilence rides a beetle smaller than a piece of uncooked rice.

The first sign of trouble likely to be seen by a layman is an unexplained die-off of leaves and limbs, which occurs when a fungal infection called thousand cankers disease encircles the afflicted tree limb.

From that first sign to the end of the line is a short trip: Trees can die in two or three years from the infection, and there is no known treatment, except — perhaps — cutting down afflicted trees as soon as the problem comes to light.

A particularly notable example is the enormous black walnut that towers over Klicker’s Antique and Fruit Store at 3300 E. Isaacs Ave. Once a leafy oasis for picnickers and workers at neighboring businesses, the circa-1870 tree is nearly stripped bare on its upper portion.

Another of the more obvious casualties was a black walnut removed midmonth along Palouse Street by the YMCA. But once you start looking at black walnuts around town, the problem is hard to miss.

Whitman College arborist Larry Malott first noticed the problem a few years ago in a black walnut on Green Street but only recently discovered the cause.

Several trees on campus show signs of the ailment — and telltale drill holes from the beetles — as do numerous other trees on public and private property around the city.

Malott said simply cutting down ailing trees isn’t the whole answer to the problem.

"If you’re going to cut down your tree, don’t transport the wood or sell it to someone who’s going to transport it," he said.

The arborist has been in touch with researchers around the country, looking for a way to stem the tide of the disease without having to remove diseased trees.

But for now, that’s a stickier wicket than you might guess.

The fungus, which came to researchers’ attention in 2001, is carried by a small, native and previously irrelevant insect, the walnut twig beetle.

The beetles are known to live in much of the West and Southwest and as their name suggests, burrow into the bark of trees. They reach their targets from the air and are active from mid-spring to early fall, so surface insecticides likely are impractical.

In addition, even if the beetles can be killed once they arrive at a tree, the fungus — which is the real killer — can spread on its own, and researchers have yet to find a countermeasure for the disease.

Interestingly, while the beetle is native to the West, the black walnut is not, originally inhabiting an area roughly from east Texas to southern Minnesota, east to southern New England and the Florida Panhandle.

The black walnut is widespread as a shade and ornamental tree, however, and may act as a conduit for beetle and fungus to economically important species such as the California walnut and possibly pecans and hickories.

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