Thousand Cankers- Dying walnut branches
Thousand Cankers Disease- Walnut Branches Dying?
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 an easily recognizable sign of thousand cankers disease.
Upon closer examination, small dark spots (1/4″-1/2″ diameter) can be seen on the bark and bark crevices as a stain on 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.
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 sawdust-like frass created by tunneling beetles and will have turned black and become water-soaked. Many dead and often live beetles will be present. In the final stages of disease, trunk cankers (1-2 meters long and often 1/2 the trunk diameter) are formed from small, coalescing cankers of the Geosmithia morbida fungus.
Adults become active in April after overwintering in bark crevices. They tunnel into trees, introducing the fungal pathogen, and lay eggs which hatch and create galleries as larvae. The fungus, Geosmithia morbida, causes small cankers to form. There appears to be two generations of Walnut Twig Beetle each year.
Damage is caused by tunneling beetles and larvae. Despite their very small size, the combination of sheer numbers of beetles and the coalescing cankers caused by Geosmithia morbida quickly girdle smaller limbs and, within a few years, larger leaders and the main trunk. 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 imidacloprid 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 yet from the more common Black Walnut varieties. 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 imidacloprid 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.