By Mark Gleason
Extension Plant Pathologist
Iowa State University
Eastern black walnut (Juglans nigra) is one of the most beloved native trees in Iowa. Its nuts are eaten by both people and wildlife, and its wood is a favorite for making high-quality furniture and housewares. On top of its everyday uses, black walnut is a stately, long-loved shade tree that graces thousands of Iowa yards and a key part of our forest ecosystems.
Although black walnut has earned a reputation as a trouble-free species, a disturbing cloud has appeared on the western horizon. A newly recognized malady called thousand cankers disease has devastated black walnut trees in many towns in the western United States, from New Mexico to Idaho. What happened?
The story begins with an insect and a different walnut species living together in the desert Southwest. Although the insect – the walnut twig beetle, Pityophthorus juglandis – bored holes in the Arizona walnut (Juglans major), the damage it caused was usually minor, because the beetle and the tree had evolved together over many millennia.
Then the plot thickened. In the 19th and 20th centuries, settlers of the Southwest often brought along their favorite trees, including eastern black walnut, that were not natives. Black walnut, a native of the eastern United States, seemingly adapted well to its new environment, and soon was widely planted in towns throughout the Southwest and Far West.
At some point during the past few decades, the unexpected happened. As far as the experts can tell, the walnut twig beetle made a host shift – to black walnut. Beetle and tree had never met before, but it was a disastrous encounter for the tree.
It turned out that the walnut twig beetle carries around a fungus called Geosmithia that does a number on black walnut. The fungus follows the beetle as it bores through the bark, then invades the tree through the tunnels bored by the beetle. The result is numerous small cankers (areas of dead bark and wood) on twigs, branches, and trunks. The outer surface of the bark over the canker may develop a dark amber-colored stain. Under the bark, the damage is much more obvious: dark brown, discolored sapwood, up to six inches across, centered on beetle tunnels.
By the time cankers are visible, branches and even entire trees may be dying back. The number of cankers on a single tree can be huge; this is how thousand canker disease got its name. The course of thousand canker disease can take years, but the result is always the same: dead black walnuts. Tens of thousands of black walnut shade trees have died throughout the West – as close to Iowa as Denver, Colo.
No useful control measures have been developed yet, and none appear to be likely. The only strategy is to cut down the diseased trees and chip up the bark and outer sapwood, in order to slow the buildup of walnut twig beetle populations.
The fight against this disease appears to be over in the West, and the black walnut lost. But what about Iowa, not to mention the rest of the black walnut’s native range in the eastern states? Unfortunately, we are likely to face a high risk of thousand canker disease.
The walnut twig beetle does not fly very far by itself, so left to its own devices it might not venture from Colorado across the wheat fields and pastures of the Great Plains to Iowa. That’s where people enter the story again.
What happens to all those dead walnut trees in the West? Many are promptly chipped up. But others may fall into the hands of Uncle Mort and Aunt Elsie from Denver. Mort and Elsie just happen to be driving to Iowa to camp in our beautiful state parks, with a load of Colorado black walnut firewood in their trailer. They arrive at the campground after the 700-mile trip. The trailer is popped open, and thousands of walnut twig beetles from Colorado fly out into the Iowa forest, carrying payloads of Geosmithia fungus. You don’t need to have special superpowers to predict the outcome of this story.
To add more risk, some individuals in the West are selling black walnut lumber slabs on the Internet, to anyone who will buy it. Unfortunately, these slabs are likely to be riddled with walnut twig beetles unless the bark and outer sapwood are removed. If a woodworker in Ankeny ends up with a garage fully of beetle-infested Colorado black walnut lumber, it’s the Mort and Elsie scenario all over again. Ironically, no U.S. or state agency has yet stepped in to try to block interstate movement of black walnut or lumber from the West.
The take-home message from this sad and disturbing tale is that Iowans need to do everything we can to keep western black walnut out of the state. Please do not buy any black walnut lumber from the West, and tell Mort and Elsie to leave their Colorado firewood in Colorado.
If you suspect that a tree has thousand canker disease, please remove a cankered limb and send a segment to the ISU Plant and Insect Diagnostic Clinic, 351 Bessey Hall, ISU, Ames, IA 50011. This step would help to alert state officials about the presence of the disease in our state.
As far as we know, Iowa has not yet been visited by this plague. In the meantime, enjoy your beautiful, healthy black walnuts while you can.