Thousand Cankers Disease and Black Walnut in Indiana
Matthew Ginzel and Bridget Blood, Purdue University
Thousand cankers disease (TCD) is a pest complex that was first detected in New Mexico in 2001 and has caused the widespread death of walnut trees (Juglans spp.) throughout the western United States. This disease is caused by a fungus, Geosmithia morbida, vectored by a bark beetle, the walnut twig beetle (WTB), Pityophthorus juglandis. The pest complex has been detected in Tennessee, Virginia, Pennsylvania, North Carolina, Ohio and Maryland, and threatens black walnut (Juglans nigra) in its native range throughout the midwestern and eastern U.S. Although WTB and G. morbida have been individually detected in Indiana, neither has been detected in a black walnut tree and no symptomatic tree has been detected and confirmed with TCD in the state. Nevertheless, this disease rapidly spread throughout the western U.S. and its widespread establishment within the native range of black walnut would have serious environmental and economic consequences.
Black walnut plays significant ecological roles in the Eastern deciduous forest as wildlife feed upon the nutrient-rich nutmeat of the walnuts, and the polyphenol-rich leaves serve as a controlling ecological force within soil ecosystems. Black walnut is also valued for timber, veneer, nuts, nursery stock production and ecosystem services. There is approximately 3.4 billion cubic feet of black walnut growing on timber land in the eastern US, with an estimated value of over a half trillion dollars. In Indiana alone, nearly 31.5 million walnuts provide ~17.7 million board feet of lumber and veneer each year at a value of $21.4 million.
The walnut twig beetle is the primary vector of G. morbida and TCD affects all members of the genus Juglans, but black walnut is considered especially susceptible. This pathogen is not systemic and requires a vector to become established. It appears that a high population density of the vector is necessary for G. morbida to kill its hosts. In fact, trees often succumb to the disease only after thousands of beetles have colonized them. In the early stages of the disease, small cankers develop around the galleries of colonizing beetles. There are few outward signs of infection apart from the small entrance holes made as colonizing beetles bore through the bark. At this stage, the fungus is often restricted to the cambium, but as the disease progresses, cankers expand into the phloem and outer bark. In the more advanced stages, cankers become more diffuse, causing the tissues to become dark-colored and macerated (Fig. 1). These thousands of cankers under the bark destroy the tree’s ability to transport water and nutrients; gradually killing the tree. Trees infected with TCD show signs of general decline including yellowing, wilted or tufted leaves and crown dieback.
In the West, black walnuts are typically killed within two to three years after symptoms (e.g., yellowing and thinning of the leaves) appear, and smaller trees and those growing on poor sites decline more rapidly. Although TCD rapidly spread throughout the West, it is unclear how this disease will affect forest health in the East. Observations of TCD-affected trees within the native range of black walnut suggest that there may be regional differences in the progression and severity of the disease. In fact, a three-year study conducted in Tennessee and Virginia found that some TCD-symptomatic trees could remain in a quiescent state and even recover from the disease. The progression of the disease appears to be closely linked with environmental stressors; in drought years TCD-symptomatic trees showed symptoms of decline, whereas in years of high precipitation these same trees had new foliage and growth. Purdue University and U. S. Forest Service researchers are currently further assessing the etiology and progression of this disease within the native range of black walnut.
Detection efforts of TCD in the eastern U.S. primarily rely on visual surveys of symptomatic trees and trapping for WTB using a commercially available pheromone lure. In 2011, the U.S. Forest Service, in cooperation with scientists from the University of Missouri and Purdue University, conducted a trap tree survey in Missouri and Indiana. Although no WTB were recovered from Indiana, 435 adults of the weevil, Stenomimus pallidus (Fig. 2), were obtained from the main stem samples from 12 sites in Indiana. G. morbida was recovered from three individual S. pallidus that emerged from two trees growing in a black walnut plantation at Yellowwood State Forest in Brown County, Indiana. The fungus was not detected on any other wood-boring beetles reared from this site or on any beetles from other sites in Indiana. This is the first report of G. morbida from Indiana, and the first report of the fungus from an insect other than WTB. Although the pathogen responsible for TCD is present at the site, the trees are asymptomatic at this time. Moreover, the low frequency of occurrence of G. morbida on S. pallidus suggests at least a very casual relationship between the fungus and this beetle. Moreover, the low population density of S. pallidus suggests that it may not be capable of vectoring enough of the pathogen to affect tree health.
In response to these findings, the DNR Division of Entomology & Plant Pathology intensified surveillance for WTB and G. morbida across Indiana and in counties bordering Butler County, Ohio, where TCD has been recently detected. As a result of this effort, WTB was detected in a pheromone-baited trap placed at a sawmill in Franklin County. Additional adult beetles were also discovered upon further inspection of walnut logs and lumber at the site. It is important to note, however, that there are no symptomatic or infested walnut trees at the sawmill or anywhere in the county. Walnut material on the property was destroyed to prevent any movement of WTB from the site, and State Entomologist Phil Marshall ordered the mill to be quarantined. Franklin County is not under quarantine and additional surveys around the sawmill and county are ongoing this summer. Also, movement of walnut logs, lumber and other walnut material within Indiana is not restricted; however, there is an external quarantine in place that restricts the transportation of black walnut material into and out of Indiana.
Forest landowners should not harvest their black walnut trees as a result of this detection or the detection of G. morbida in Brown County. If you notice a suspicious decline in black walnut trees or otherwise suspect an infestation of TCD, call the DNR toll-free at 1-866-663-9684. If approached by someone offering to remove black walnut trees because of the disease, notify the DNR or a consulting forester to have the tree evaluated.