When Cankers Attack

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Keep an eye out for dark fungus-stained splotches like these seen on th branch of a black walnut.

Credit:  Ned Tisserat, Colorado State University

The beetle and fungus that cause Thousand Cankers Disease can kill a mature black walnut in a year.

What Does It Do to my Trees?

Thousand Cankers Disease is a team effort. The walnut twig beetle is dark brown and tiny, about 1/16th of an inch long. That's smaller than a sesame seed. The beetles carry the Geosmithia canker fungus. As they burrow, they spread the fungus through the tree.

Some of the symptoms of Thousand Cankers Disease include:

• Declining foliage. The leaves may wilt or turn yellow or brown. This usually starts at the top of the tree and moves down. The leaves often stay attached to dead branches.
• Cankers. The disease gets its name from the dark fungus-stained splotches on the wood under the bark. These can be seen when the bark is peeled back. They eventually spread to cover much of the wood.
• Exit holes. When the new adult beetles leave the tree, they do it through tiny exit holes. You may need a magnifying glass to see the holes.

If you think your trees might have Thousand Cankers Disease, a local extension agent or forester can help you figure out what's going on.

If you have Thousand Cankers Disease on your land, there are steps you can take. Read about what to do when you have an outbreak. [LINK TO outbreak page]