[From the December 2014 issue of the UC IPM Green Bulletin]
For at least a decade, thousand cankers disease (TCD) has been killing walnut trees across California wildlands and landscapes. The causative agent is a yeast-like fungus (Geosmithia morbida) spread by a tiny native beetle called the walnut twig beetle (Pityophthorus juglandis) (Figure 1). TCD has killed thousands of walnut trees and threatens not only ornamental and landscape trees but also English walnut trees in commercial orchards. Once a tree becomes infected no control is available; therefore, it is critically important that practicing arborists and landscapers be aware of this disease and not move any infested wood. The minute beetles can hide and survive beneath the bark in dry walnut wood; they may emerge later and transmit the disease to other walnut trees.
Figure 1. Walnut twig beetles are tiny, about 1.5 mm in length.
Known hosts include walnuts (Juglans species) and wingnuts (Pterocarya species). In California the disease has been observed on both native species of black walnut, Juglans californica and J. hindsii, and both are quite susceptible. English walnut, J. regia, is affected to a lesser degree but is typically grown grafted on a hybrid rootstock, which is susceptible, so symptoms of the disease may be observed in walnut orchards as well. The eastern black walnut, Juglans nigra, is highly susceptible. Although of limited importance and distribution in California, this species is grown in eastern North America for its excellent wood and in much of western North America as a shade tree. Therefore, TCD represents a threat throughout most of the United States. Eastern black walnut appears to have been the key host species that provided for the range expansion of the disease in most of the western states.Wingnut, a less frequently planted tree in California landscapes, is also susceptible to the disease.
Symptoms and Signs
On small branches (but not on twigs less than about 1 cm in diameter), a set of pinhole-sized beetle entrance and exit holes can be found. Each entrance hole is close to, or surrounded by, a dark wet or oozing canker (especially prominent on the smooth bark of the English walnut) (Figure 2). The holes are just large enough to accommodate the tip of a mechanical pencil. The attacks of many beetles result in hundreds of dark lesions so numerous that they coalesce, girdling and killing the branch, and giving rise to the disease name. If the bark is removed, small beetle galleries in necrotic (dead) areas of phloem comprise the cankers (Figure 3). From a distance, the affected tree will show dieback of branches beginning at the top and spreading downward (“branch flagging and crown dieback”). As the crown is gradually
Figure. 2. Dark staining of black walnut bark around the entrance and exit holes made by walnut twig beetle.
killed, epicormic branches may sprout from the trunk. Eventually, the walnut twig beetles attack the main stem and colonize the phloem. These attacks can go right to the soil line in the trunk.
Figure. 3. Dead phloem beneath the surface of black walnut bark.
How is It Controlled?
Currently, no insecticides or fungicides have been shown to save trees affected with TCD. Thus, it is important that infested wood is not moved off site. Infested trees should be taken down and ground or burned (where allowed) on site. Because the beetles are very small and difficult to detect, it is important that freshly cut walnut branches, logs, or burls not be moved or shipped from infested areas, not even for woodworking purposes. Seasoning wood on site for 2-3 years should allow walnut twig beetles and other woodborers time to emerge at the site of infestation, but it is prudent to have all wood inspected by a knowledgeable entomologist or cooperative extension specialist prior to movement of the material from the site, even when properly seasoned and de-barked.
For more information, visit the UC IPM web page on Thousand Cankers disease.