Thousand cankers disease

Thousand cankers disease (TCD) is a recently recognized disease of certain walnuts (Juglans spp.). The disease results from the combined activity of the walnut twig beetle (Pityophthorus juglandis) and a canker producing fungus, Geosmithia morbida. Until July 2010 the disease was only known to the western United States where over the past decade it has been involved in several large scale die-offs of walnut, particularly black walnut, Juglans nigra. However, in late July 2010 a well-established outbreak of the disease was found in the Knoxville, Tennessee area. This new finding is the first locating it within the native range of its susceptible host, black walnut. In 2013, an outbreak was found in the Veneto region of Italy, where the disease has been found on both black walnut and English walnut. Thousand cankers disease (TCD) is a recently recognized disease of certain walnuts (Juglans spp.). The disease results from the combined activity of the walnut twig beetle (Pityophthorus juglandis) and a canker producing fungus, Geosmithia morbida. Until July 2010 the disease was only known to the western United States where over the past decade it has been involved in several large scale die-offs of walnut, particularly black walnut, Juglans nigra. However, in late July 2010 a well-established outbreak of the disease was found in the Knoxville, Tennessee area. This new finding is the first locating it within the native range of its susceptible host, black walnut. In 2013, an outbreak was found in the Veneto region of Italy, where the disease has been found on both black walnut and English walnut. Adult walnut twig beetles carry spores of the Geosmithia morbida fungus, which grows profusely around the pupal chamber of the beetles. Following emergence from trees the beetles subsequently tunnel into branches and trunks of walnut for production of egg galleries or overwintering shelters. The fungus is introduced into the tree during this wounding where it subsequently germinates and grows. The fungal mycelium initially colonize tissue immediately surrounding the beetle galleries. However, in less than a month black, oval-shaped, inky cankers extend considerably beyond the galleries and may reach more than 3 cm in length in susceptible hosts (e.g., black walnut). In the beginning these cankers develop in phloem and tissues formed by the cork cambium. The affected area is very shallow and never show the ‘open-faced’, perennial, target-shape typical of many canker diseases of trees (e.g., Nectria canker). Instead, in TCD the bark remains firmly attached to the canker face making the necrotic areas very difficult to observe. Branch cankers usually are not visible until the outer bark is shaved to expose the beetle tunnels, although during late stages of the disease a dark amber stain may form on the bark surface in association with the cankers. Each time a beetle tunnels into a tree a canker is initiated. Cankers also may continue to expand and penetrate into the cambium of the tree. Each such injury destroys the phloem and robs the tree of its ability to store and move nutrients. As TCD progresses cankers coalesce to further girdle branches greatly restricting nutrient movement. As the tree declines, more bark beetles are attracted and more cankers are formed. Eventually the enormous number of beetle attacks and subsequent canker formation overwhelms and kills the tree. Thousand cankers is a progressive disease and its effects result from the culmination of a large number of relatively small cankers over a period of time. Just as a thousand cuts was once used as a form of human execution in Imperial China, black walnuts are subjected to death by thousands of branch and trunk cankers produced by infection from the Geosmithia fungus. In end stages of the disease external symptoms become visible. Leaf yellowing on the exterior of the crown is often the first symptom and may originally be restricted to a single branch. However, as the cumulative effects of the girdling progress increasingly large areas of the tree are affected. Sudden leaf wilting, ultimately involving large limbs, characterizes end stage thousand cankers disease. In susceptible hosts, trees are almost always killed within 2–3 years after external symptoms of leaf yellowing are first observed. The progress of thousand cankers will vary due to several factors, notably the susceptibility of the host. There appears to be a considerable range of TCD susceptibility among various Juglans species with Juglans nigra (black walnut) being particularly susceptible. Conversely, Arizona walnut (Juglans major) appears to be quite resistant to the disease, with bark beetle attacks largely limited to small diameter branches, the fungus growing to a very limited extent, and effects of the disease rarely, if ever, progressing to involve large areas of the tree. Similarly southern California walnut (Juglans californica) and little walnut (Juglans microcarpa) may show fairly high resistance. Northern California walnut (Juglans hindsii) and the commercial nut-producing Persian (English) walnut (Juglans regia) apparently show various degrees of intermediate TCD susceptibility. Thousand cankers is a recently described disease and its involvement in tree mortality apparently is also recent. The first published note involving black walnut die-offs that likely can be contributed to TCD occurred in the Espanola Valley of northern New Mexico in 2001. Walnut twig beetles were associated with this unusual cluster of walnut mortality, but drought was originally thought to be the cause. A parallel situation occurred in eastern Colorado during the early 2000s where unexplained black walnut deaths were observed in several sites (Colorado Springs, Boulder, Westminster) and originally thought to be associated with drought. Walnut twig beetles were subsequently recovered in 2004, which constituted a new state record for the species. This appears to have been preceded by unusual black walnut mortality during the mid-1990s of undetermined cause that were noted in the vicinity of Logan, Utah. Collections of walnut twig beetles were made in 1996 in association with this outbreak. A 1988 collection of walnut twig beetle from the Provo area, collected incidentally when sweeping vegetation, appears to be a precedent first collection of this species in the state.