Thousand Cankers Disease of eastern black walnut

Thousand Cankers Disease Complex: A Forest Health Issue that Threatens Juglans Species across the U.S.

Canker of Thousand Cankers Disease on Black Walnut — Photo: Canker caused by Thousand Cankers Disease (*Geosmithia morbida*) found on Black walnut *(Juglans nigra*) in Tennessee.

A review of research relating to Thousand Cankers Disease (TCD) complex, in which of a fungus (Geosmithia morbida) is vectored by the walnut twig beetle (WTB, Pityophthorus juglandis). The disease causes mortality primarily of eastern black walnut (Juglans nigra), although other walnut and wingnut (Pterocarya) species are also susceptible. Trees infected with TCD exhibit drought-like symptoms, making field detection difficult without molecular and/or morphological methods. The recently sequenced G. morbida genome will provide valuable research tools focused on understanding gene interactions between organisms involved in TCD and mechanisms of pathogenicity. With no chemical treatments available, quarantine and sanitation are preeminent options for slowing the spread of TCD, although biological control agents have been discovered. High levels of black walnut mortality due to TCD will have far-reaching implications for both eastern and western states.

Citation: Daniels, D.A., Nix, K.A., Wadl, P.A., Vito, L.M., Wiggins, G.J., Windham, M.T., Ownley, B.H., Lambdin, P.L., Grant, J.F., Merten, P., Klingeman, W.E., and Hadziabdic, D. 2016. Thousand Cankers Disease Complex: A Forest Health Issue that Threatens Juglans Species across the U.S. Forests 7(11): 260. doi:10.3390/f7110260.

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Undergraduate Research:
To the vector go the spoils: Identifying fungal pathogens associated with walnut twig beetle, a vector of Thousand Cankers Disease complex

The fungus, Geosmithia morbida, vectored by the walnut twig beetle (WTB), Pityophthorus juglandis, causes mortality in black walnut (Juglans nigra) known as Thousand Cankers Disease (TCD). Infected trees exhibit symptoms similar to drought, making disease identification difficult. Early detection and confirmation of the pathogen is feasible using molecular markers. In 2010, TCD was discovered in Tennessee and has since been detected in four more eastern U.S. states within the native range of black walnut. Our objective was to identify fungal pathogens associated with WTB infestation of black walnut in Tennessee. A total of 180 WTB were collected from two symptomatic trees in Knoxville, TN. WTB were separated by sex, then divided into 3 treatment groups (decapitation, washing, or direct plating), and placed onto an antibiotic amended medium. After a week, fungal isolates were identified based on morphology and confirmed by sequencing ITS1 and ITS4 regions. Additionally, microsatellite loci were used to detect the presence of G. morbida. Statistically significant associations between pathogen presence and direct plating method (P=0.026), and between male WTB and G. morbida (P=0.044) were determined. Although four species of Fusarium, including F. solani, were isolated from both female and male WTB, the findings were not significant with respect to sex. It is vitally important to fully understand TCD complex and the role of secondary pathogens in TCD pathogenicity.

Citation: Daniels, D.A.; Lord, R.J.; Wadl, P.A.; Nix, K.A.; Vito, L.M.; Merten, P.R.; Windham, M.T.; Lambdin, P.L.; Grant, J.F.; Wiggins, G.J. To the vector go the spoils: Identifying fungal pathogens associated with walnut twig beetle, a vector of thousand cankers disease complex. In Proceedings of the Phytopathology; AMER PHYTOPATHOLOGICAL SOC 3340 PILOT KNOB ROAD, ST PAUL, MN 55121 USA, 2014; Vol. 104, pp. 30–30.