Contributor: Dr. Ramesh R. Pokharel, Plant Disease Specialist, Maryland Department of Agriculture, Ramesh.firstname.lastname@example.org
ANNAPOLIS, MD (August, 2019) – Thousand cankers disease (TCD) is a complex problem in walnuts, Juglans spp., caused by the fungus Geosmithia morbida and transmitted by walnut twig beetle (WTB), Pityophthorus juglandis. The beetles tunnel under tree bark and introduce the fungus, causing cankers that eventually coalesce and girdle limbs and branches, resulting in tree death. Walnut twig beetle was first identified in 1928 in Arizona, in Arizona walnut, Juglans major. Slowly, the beetle has spread to other areas of the U.S., including several states in the Northeast. Black walnut, Juglans nigra, began to show widespread decline in the 1990’s. The first cases of black walnut mortality with bark cankers were found in Denver, CO in 2001, and the association of the Geosmithia morbida fungus with these cankers was identified around 2008. By then, nearly all black walnuts in the Denver area were affected. TCD has now been confirmed in MD, VA, PA, and several other neighboring states (Figure 1).
WTB feeds in the cambium and phloem of larger twigs, branches, and main stem of Juglans and Pterocarya spp. to create brood galleries beneath the bark. As WTB create galleries, they inoculate the phloem with Geosmithia morbida. The fungus colonizes and kills the surrounding cambium and phloem tissue. Cankers form around every beetle feeding site, which may coalesce to form larger ones, eventually leading to tree death. Generations of the beetle move to and from black walnut trees carrying the fungus to the next hole or tree as they create galleries.
Walnut twig beetle attacks all species of walnut and wingnut (Pterocarya). A considerable range of TCD susceptibility exists among various walnut species, with black walnut being particularly susceptible. As cankers grow within the phloem of black walnut, they reduce the tree’s ability to store and move nutrients. As TCD progresses, cankers coalesce to girdle branches. As the tree weakens, more bark beetles are attracted and more cankers are formed, eventually killing the tree. It may take several years of insect attack and fungal infection before symptoms other than minute exit holes of the beetle on the trunk are visible.
Walnut twig beetle is 1.5 to 2 millimeters long, has a relatively narrow body is reddish-brown to brown cuticle (Figure 2). It makes tunnels beneath tree bark. In such case, the outer bark can be peeled away to expose WTB galleries in the phloem, a key feature for diagnosing the TCD complex (Figure 3). Cankers become oval-shaped and inky black, reaching more than 3 cm in length. However, the bark remains firmly attached to the canker face, making necrotic areas very difficult to observe. Numerous cankers are present in a single tree due to multiple points of pathogen introduction. Small pin-sized holes are associated with each canker, usually denoting the entrance/exit hole for the WTB. Cankers often bleed, leaving dark ooze and staining on the outer bark surface.
Symptoms start with leaf yellowing and crown thinning of infected trees, which may initially be restricted to a single branch. As the disease progresses, foliage wilts, larger branches die, and eventually the tree dies. In susceptible hosts, trees are typically killed within 2–3 years after external symptoms of leaf yellowing are first observed. TCD was first detected in Maryland in October 2014 and a quarantine was ordered by the Maryland Department of Agriculture to minimize the risk of disease spreading from Cecil County. The beetles were collected from traps in Baltimore in August 2017. On May 1, 2019, a new quarantine order was issued to restrict the movement of infected materials in all of Baltimore City and part of Baltimore County (Figure 4). Introduction of TCD to a new area involves transportation of infected logs, trees, or other untreated wood products which may carry the beetle or fungus. Quarantines restricting the movement of these products help to limit long distance dispersal and manage this disease.
For more information:
Maryland Department of Agriculture, May 1 2019. Thousand Cankers Disease Detected in Baltimore City and County. https://news.maryland.gov/mda/press-release/2019/05/01/thousand-cankers-disease-detected-in-baltimore-city-and-county/
C. Nischwitz and M. Murray. 2011. Thousand cankers disease of walnut (Geosmithia morbida). https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1729&context=extension_curall
US Department of Agriculture: https://www.invasivespeciesinfo.gov/profile/thousand-cankers-black-walnut-disease
Maryland Department of Agriculture: https://mda.maryland.gov/plants-pests/Pages/tcd.aspx