Photographer: Whitney Cranshaw, Colorado State University Source: Bugwood.org Copyright: CC BY 3.0
This disease is complex between the fungus Geosmithia morbida, and the walnut twig beetle Pityophthorus juglandis. Alone, this very small (1/16 inch) reddish brown beetle from the family Scolytidae, does not cause significant mortality and it is characterized by its 4-6 concentric ridges on the upper surface of the pronotum. The fungus is newly discovered and initially members of the genus Geosmithia were not considered to be important plant pathogens. Overall the fungus is subtle and inconspicuous until the cankers start to form on the branches. The first symptom is leaves wilting and yellowing in mid-summer which is then followed by thinning of the canopy and eventually the whole tree dies as thousands of cankers girdle branches and the trunk. For now, only DNA analysis or agar culturing can confirm the identity of the fungus.
Host Plant: Black Walnut (Juglans nigra)
This disease complex kills trees within 3 years of initial symptoms, killing off black walnut trees and a closely related tree called wingnut. Thankfully it is not a danger to the Arizona walnut tree. However, with walnuts and their lumber being a cash crop this disease can severely impact the walnut tree nut, lumber, furniture and veneer industries. Also, wildlife may be negatively impacted by the loss of J. nigra trees if TCD spread throughout the Eastern United States.
Fungal spores are carried to the tree by adult walnut twig beetles. The cankers develop in the inner bark and disrupt the flow of nutrients throughout the tree. For each beetle that bores into the bark, another small canker is formed. Even though the cankers are small the repeated feeding and egg laying by the walnut twig beetles leads to repeated fungal introductions within the tree eventually killing of the branches and then the whole tree due to lack of nutrition circulating.
Aid for the Identification of the Walnut Twig Beetle click here
The walnut twig beetle is native to Arizona, California and New Mexico. It has invaded Colorado, Idaho, Nevada, Oregon, Utah and Washington. Also, since 2010, established beetles populations are found in Pennsylvania, Tennessee, and Virginia. All of which are states that now have TCD. This disease complex first discovered in Colorado in 2003, and has spread to the other states in just a few years. Since the black walnut tree can be found in North and Eastern Texas; TCD is an eminent threat to Texas and 21 other states within the tree’s native range.
Beetles are native to AZ, CA and NM; the fungus genus Geosmithia is worldwide
U.S. Habitat: Anywhere that J. nigra, the beetle Pityophthorus juglandis and the fungus, Geosmithia morbida, appear
U.S. Present: AZ, CA, CO, ID, NM, NV, OR, PA, TN, UT, VA, WA
TCD can be spread by moving walnut wood by the nuts are not a concern since TCS is not spread by the seeds. In the Eastern United States, the best way to detect the disease is to visually inspect walnut trees for dieback. Also, like with other insect surveys, a pheromone-baited trap is also available. Currently there are no known insecticide sprays that reliably control this disease. Contacting local or state entities is always helpful in tracking any invasive disease; but do not mail samples to diagnostic laboratories without contacting them fist to prevent beetle emergence during shipment.
Kolarík, M.; Freeland, E.; Utley, C.; Tisserat, N. 2011. Geosmithia morbida sp. nov., a new phytopathogenic species living in symbiosis with the walnut twig beetle (Pityophthorus juglandis) on Juglans in USA. Mycologia 103(2):325–332.
Tisserat N, Cranshaw W, Leatherman D, Utley C, Alexander K. 2009. Black walnut mortality in Colorado caused by the walnut twig beetle and thousand cankers disease. Plant Health Progress Published, 11.
Wood SL, Bright DE. 1992. A catalog of Scolytidae and Platypodidae (Coleoptera) 2. Taxonomic index. Great Basin Nat Mem 13:1–1533.