Black walnuts in peril of canker disease

By Steven Olson
North Forty News

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The mountain pine beetle has everyone worried about their pine trees, but there are other bugs out there just as dangerous. The walnut twig beetle is one.

The beetle is an Arizona native, feasting on the Arizona walnut straddling the border between Arizona and New Mexico. It didn't do a lot of damage there because the trees were better able to resist their attacks.

Then the beetles came north and jumped to the black walnut, a native of the eastern United States that was planted in urban areas all along the Front Range because it's an attractive shade tree. It grows fairly slowly, can reach 40 feet in height and has a nice shape. The wood is pretty, somewhat durable and can be worked easily.

Tim Buchanan, Fort Collins city forester, said he doesn't have an accurate count of how many black walnut trees are in Fort Collins because the city doesn't keep those kinds of records. He estimates somewhere between 400 and 1,000 at the most.

The walnut twig beetle has the potential to change all of that, however. Buchanan said people in Boulder started noticing their black walnut trees dying in 2003. The tree would start "flagging," a word foresters use to describe the leaves on branches turning yellow or branches dying outright.

"If you stood below the tree and looked up in the center you would see all these dead branches," Buchanan said.

Things go downhill from there.

"Boulder and Colorado Springs lost a lot of them," said Dr. Ned Tisserat, an Extension specialist and professor of plant pathology who studies 1,000 canker disease. "Boulder lost about 800 in the past four years."

At first, people thought the trees were just dying from the effects of drought, then they realized that they were dealing with a disease. The cause was a fungus carried by the walnut twig beetle.

"We just discovered it in the spring of last year," said Tisserat. "The fungus is called Geosmithia."

Fungus-laden beetles burrow into the bark and get into the section of wood that carries nutrients from the roots to the leaves. Then the fungus kills a small area of wood in a formation called a canker. That wouldn't seem so bad, except that there are millions of the bugs, all burrowing into a tree, all carrying the fungus, all creating cankers. Eventually they create so many that the fungus girdles the tree, and the tree starves to death. That's where the disease gets its rather fanciful name--1,000 canker disease. It's akin to the Chinese torture called the death of a thousand cuts.

The beetles are really small and hard to detect. "The mountain pine beetle is a real monster compared to this guy," Tisserat said. "You really have to know where to look" on the tree to see if walnut twig beetles are in the wood.

Another problem in detection is that the disease can take about two years to kill a tree. "You can look at a tree and everything will look fine," Tisserat said. "But eventually the tree begins to collapse."

The disease has popped up in Colorado Springs, Boulder, Westminster and Longmont. Tisserat has not seen any confirmed cases in Denver. Nor has he seen any evidence the disease is in Loveland or Fort Collins, although he is analyzing a tree on the Colorado State University campus that may have it. Nevertheless, Tisserat thinks the disease will spread up and down the entire Front Range.

What worries him and people like Buchanan is that the disease will spread east of here into the black walnut's native range on the other side of the Mississippi River.

"That's what people are really worried about," said Buchanan. "Currently this thing is not in eastern North America. We're afraid somebody's going to cut down a dead tree and not take the bark off the logs. So, if I had to tell people something, it's 'Don't take the wood east, or don't ship the wood to someplace like Indiana.'"

"If you cut it down, take the bark off the logs," said Tisserat. "And dry the wood for about a year before you move it. We don't want this thing to move off the Front Range."