This is a disease that affects Walnuts (Juglans), especially Black Walnuts (Juglans nigra), named in 2009 by Ned Tisserat, professor of plant pathology at Colorado State University. It gets its name from the thousands of beetles, both the walnut twig beetle and in the latter stages of decline the ambrosia beetle, that infest inflicted trees and simultaneously vector fungal colonies into the bark causing decline and mortality. Although one would assume that with "twig" borer the beetles would be found in the smaller twigs of a tree, these borers are generally found in limbs 1" and larger as well as the trunks and large limbs as the trees decline.
The disease is typically discovered by flagging limbs that occur as the many cankers eventually girdle limbs and trunks. Flagging is a term used to describe branches that die during the growing season, wilting the leaves but not dropping them, even as the remaining live leaves drop in the autumn. They eventually turn brown and are a easily recognizable sign of thousand cankers disease.
With a closer look small dark spots (1/4"-1/2" diameter) can be seen on the bark and bark crevices as a stain on the the trunks. With an even closer look, perhaps with the aid of a hand lens or magnifying glass, the tiny entrance holes can be found in the low areas of bark crevices. They are about .25mm in diameter, the size of the smallest mechanical pencil lead or a safety pin. These holes are more readily seen on the smooth bark of the smaller limbs (limbs 1" and larger in diameter). They appear to start with a few holes and then rapidly (within 2-3 years) increase in numbers as more and more generations are produced and re-infest the tree.
Thousand Cankers Disease | Twig Beetle Holes
As a tree is overcome with the beetles the thicker bark will become loose and be easily peeled from the trunk. The underside of the peeled bark will show hundreds of tiny tunnels and frass created by tunneling beetles and will have turned black and become water-soaked. Many dead and often live beetles will be apparent. In the final stages of disease, trunk cankers (1-2 meters long and often 1/2 the trunk diameter) are formed from coalescing small cankers of the Geosmithia morbida .
The life cycle of the walnut twig beetle begins as the egg hatches into a larva just under the bark surface in a tunnel carved out by the adult. The larvae hatch from the egg then tunnel and pupate under the bark emerging as adults. The beetle flies and overwinters as an adult in bark crevices usually on the trunk. Beginning in April they tunnel into the bark introducing the Geosmithia morbida fungus which moves beyond the tunneling into bark (phloem).
They appear to have at least 2 generations per year and are active from April through October.
Damage is caused by tunneling beetles and larvae. Despite their very small size the combination of the sheer numbers of beetles and the Geosmithia morbida fungi quickly girdle smaller limbs and within a few years larger leaders and the main trunk become hosts. Girdling is the physical damage caused by tunneling, to the bark and cambium, 360 degrees around the limb or trunk. The cambium is the essential mover of water and nutrients within a tree system.
The saddest part of this disease is the lack of effective control or protection that has yet been developed. Thousand Cankers Disease is being compared to Dutch Elm Disease and the American Chestnut Blight both of which devastated the American Elms and American Chestnuts during the twentieth century.
The best results seem to be from treatments of imidicloprid (Merit) that were being applied systemically to Walnuts for aphid control prior to wide discovery (or naming) of Thousand Cankers Diseases. The presence of this systemic pesticide seems to have deterred Thousand Cankers Disease either partially or completely in some cases. It could be coincidental that these treatments have been helpful in protecting the trees in that there may be resistant varieties of Black Walnut which have not been differentiated as of yet from the more common Black Walnut species.
For now the best practices are keeping trees healthy through proper watering, fertilizing and pruning out infected limbs.
In addition it is a good idea to do an annual imidicloprid systemic treatment and a late season (August) trunk and limb spray of Permethrin or Carbaryl. The immediate removal of flagging limbs and other tree parts where the beetle can be identified is essential in any attempt to save a walnut tree from death.