'Thousand Cankers' Rapidly Kills Walnuts

Yet another deadly tree disease is spreading in North America. Dubbed "thousand cankers" disease, this one affects black walnuts. There are early hints it could also affect English (or Persian) and California walnuts, which are valued for their agricultural production.

So far, thousand cankers disease -- which appears to be caused by the combined effects of a bark beetle and a fungus -- has been documented in 8 states (AZ, CA, CO, ID, NM, OR, UT, and WA), and in Chihuahua, Mexico. It was first confirmed in 2001, though it may have been the culprit in a die-off of some Utah walnuts in the early 1990s. It has now been confirmed on the eastern side of the Rocky Mountains in Colorado. It is spreading primarily from South to North, and a little bit from West to East. Some entomologists are very concerned that it could soon jump to native walnut stands in the eastern half of the country, posing a substantial threat to the trees themselves and the ecosystems they occupy.

The bark beetle has been known for at least 80 years, and seemed to do little significant harm. The aggressive fungus is a newly-identified co-culprit that remains undescribed. The combined presence of the two pests can lead to tree death within about three years. It appears that pesticides are of little help. The only effective prevention so far appears to be fast recognition and removal of infected trees, and preventing transportation of infected wood or trees to unaffected areas.

BACKGROUND INFORMATION:

• "Diagnosing Thousand Cankers Disease of Walnut," Colorado State University, June 15, 2009.
• "Pest Alert: Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut," Colorado State University (via PurdueUniversity).
• "Beetle and Fungus One-Two Punch Threatens Black Walnut Trees, Scientists Warn," Univ. of California, Davis, Dept. of Entomology, July 2, 2009.

Another indication of the disease's importance is its inclusion on the Dept. of Agriculture's current watchlist:

• "Additional pests of concern for fiscal year 2010," April 10, 2009.

Many jurisdictions (state, county, city) have a forester who may be a source for this story. If you can't find one, you might check with the National Association of State Foresters.

• NASF Membership Directory.

Entomologists, forestry officials, arborists, nursery personnel, and other tree and plant disease experts may be able to add to your story by providing context on some of the many other tree diseases that have been of substantial concern recently, such as sudden oak death, emerald ash borer, pine bark beetle, spruce budworm, Dutch elm disease, and many others.