Have a problem with your tree, or trees? Private landowners should bring a sample of the problem and photos (if available) to their local UI County Extension Office. Many offices have a Master Gardener Plant Clinic, where problem diagnosis and control recommendations are given for problems on all types of  plants, including trees and woody shrubs. If the local office cannot make a diagnosis, the sample will be sent to the  Extension Forestry Tree Clinic at the UI Campus at Moscow.

Pest Alert  - 1000 Canker Disease of walnut: a serious new problem on black walnut.

The saga of 1000 canker disease on walnut begins in the summer of 2005, in Meridian, Emmett, and Boise, ID with walnut trees starting to die. No one know what was happening, but mature black walnut trees that were healthy on July 1 were completely dead on August 1. We initially thought it was drought stress killing trees, but the ‘problem’ was spreading from west to east and was occurring on both irrigated and un-irrigated sites. The leaves also looked ‘wilted’ when they were dying, indicating the possible presence of a wilt disease.

After collecting samples, Dr. Steve Cook, UI Insect Ecologist and Frank Merickel, curator of the Entomology Museum at the UI College of Agricultural and Life Sciences, identified the insect found in the samples as walnut twig beetle (Pityophthorus juglandis).

I then ‘Googled’ walnut twig beetle and came across some folks at Colorado State University, Whitney Cranshaw and Ned Tissant, who were working on the same mystery. They had identified the twig beetle and were linking it and drought stress, with perhaps a pathogen, as well. Work continued. We were all pretty sure by now that there was a fungal associate that produced the wilt-like symptoms and rapid death. In an email dated May 9, 2008 Ned wrote:

“We are calling this disease thousand cankers because trees literally die from beetle attacks and the formation of thousands of cankers on the tree. In my opinion this canker/pest complex poses a major threat to black walnut should it be introduced into its native range.” 

Thanks to Ned, we now had a name to go by. Ned was also successful in isolating a fungus, Geosmithia spp. New samples were collected from Boise in March 2008 and sent them to Colorado. Confirmation of the presence of the Geosmithia sp. fungus was confirmed by Ned Tissant, as was the association of this fungus with beetle galleries.

1000 cankers disease on walnut is now entered as a new state record for Geosmithia sp. in the National Agricultural Pest Information System (NAPIS). We have a name and we know what is happening. Sanitation (pruning out affected branches and removing them from the site) and deep watering, especially during hot weather could help, but to date we have no recommendations for control.