Tree

Damaged bark on the trunk of an English walnut tree.

This far into spring, any tree that has not fully leafed out is probably dying or dead.

Drive around Walla Walla, and every few blocks you’ll notice one of these skeletal trees. Many of them are walnut trees. Some, judging from their size, are veterans of valley life.

Hungry Tree Surgeon Brad Manson has treated many dead and dying trees in Walla Walla in his 30-year career and has noticed an uptick in the number of walnut trees among his patients.

“There’s a lot of dead branches in the tops, sometimes half the tree,” he said. “It’s some type of insect or borer getting to them. If you neglect them, you’re going to have problems.”

An inquiry at Washington State University’s Walla Walla County extension office didn’t shed much light on why these trees seem to be dying at an unusual rate.

“It’s hard to tell what it is without taking samples and testing them,” said Extension Coordinator Becki Green. “We’ve seen maybe one or two in the office, but it has been at least three years since we’ve seen anything come in. We lost quite a few to that early frost three or four years ago.”

Maybe the local extension office hasn’t seen many cases because walnuts aren’t commercially produced on any scale in southeastern Washington. California, in contrast, produces almost all of the nation’s walnuts — and almost all of its walnut research.

Californian walnut growers have seen identical dieback symptoms as those observed in Walla Walla and attribute it to the thousand cankers disease.

Death by a thousand cankers

Thousand cankers disease develops from a relationship between a fungus, Geosmithia morbida, and the walnut twig beetle, which is not much bigger than a grain of sand.

The beetle tunnels under the bark of black and English walnut trees, where it lays its eggs and spends most of its life.

Fungal spores carried on the beetle’s body sprout in the tunnels and begin to kill small spots in the tree’s vascular system. As the beetle multiplies, these dead spots, or cankers, also multiply, eventually strangling main branches or even the whole tree.

Licensed pest control adviser John Christenson has been working with walnuts in California for about 30 years and knows plenty about the disease.

“It starts with a weak tree,” said Christenson. “A weak tree is vulnerable to the beetle. One beetle arrives, and then it releases a pheromone to bring others when it finds food.”

Once infested, a tree may live another few years before succumbing to the disease.

Entomologists believe that the walnut twig beetle is native to the southwest United States and has lived in balance with natural walnut trees in that region for eons.

But if the beetle is native to the United States, why has thousand cankers disease only become a problem in recent years?

Entomologist Dr. Steven Seybold of the USDA Forest Service, a respected expert on the walnut twig beetle and thousand cankers disease, attributes the current outbreaks in the northwest to the transportation of infested wood.

Dr. Seybold explained in an email that the beetles likely arrived in wood brought from California.

“We had been told that large Californian logs were moved to Portland, and if they were particularly good, then they were shipped up to Post Falls, Idaho, for veneer manufacture,” he said. “I’m suspicious that this type of activity played a major role in the spread of the beetle.”

Keeping the trees alive

Another arborist working in the Walla Walla Valley had additional thoughts — and tips.

“There are different reasons,” said Bradley Beckner.

He attributes some of the die off to a bad winter several years ago, probably referencing the same hard freeze described by Becki Green of WSU. That freeze was especially hard on English walnuts, which are less winter-hardy than black walnuts.

However, Beckner identified thousand cankers disease as an important culprit, especially among black walnuts.

“I called the WSU office, and they said there’s not much of a problem,” he said. “But I’ve been pulling out 100-year-old black walnuts everywhere.”

Beckner emphasized the importance of proper tree care to protect against the disease, echoing Christenson’s observation that weak trees are the main population at risk.

“Thousand cankers wouldn’t be a problem if people maintained their trees correctly. It’s the way they water their lawns. This is just a symptom, but we should be focusing on the underlying problem.”

Another part of the problem, he explained, is the way people continue spreading the beetles.

“We export a lot of walnut (wood),” said Beckner. “You’d be amazed at how far this wood can travel, and none of it has been kilned. I’m nervous about the way it’s being transferred.”

Other than keeping trees vigorous with proper maintenance, guidelines from the WSU extension office say there’s no known treatment or cure for the disease once walnut twig beetles have invaded the tree. Wood from infested trees should be chipped and dried or burned in place, and should not be transported for firewood or woodworking until the beetle has been positively eradicated.

For now, that’s about all that can be done.

Paul Froese is a Spanish-to-English scientific translator, agribusiness copywriter, and Walla Wallan.