Thousand Cankers Disease: Virulence of <i>Geosmithia morbida</i> isolates and potential alternative vectors of the fungus

The fungal pathogen, Geosmithia morbida, along with its vector, walnut twig beetle (WTB), Pityophthorus juglandis is the causal agent of the Thousand Cankers Disease complex (TCD) in Juglans spp. and Pterocarya spp. Disease outbreaks have been reported in the eastern and western United States (U.S.). Since TCD was detected in the western U.S. in early 2000s, the disease complex has spread to the eastern U.S. and Italy, Europe. In our preliminary data, G. morbida isolates from eastern and western states have been placed in five distinct genetic clusters, though geography was not correlated with origins of isolates of each cluster. In the eastern U.S., fungal pathogen had been detected on other insect species besides WTB. These insect species included two ambrosia beetles, Xyleborinus saxesenii and Xylosandrus crassiusculus, and a bark weevil, Stenomimus pallidus. Since TCD incidence and severity have been reported to be higher in the western U.S. states when compared to the eastern states, we hypothesized that differences in disease severity and incidence are due to variations in the fungal virulence at different geographical locations. To test our hypothesis, the proposed study had two specific objectives: 1) to determine if other wood boring beetle species could be potential vectors of G. morbida in Tennessee, and 2) to evaluate the level of virulence of five isolates from each of the five genetically distinct clusters of G. morbida. Our results indicated that eleven species of bark and ambrosia beetles were found to carry G. morbida. This finding raised concerns about the role these potential vector species play in the dissemination of the pathogen to healthy walnut trees. We also found that canker area was not correlated with geography or genetic clusters. The genetic Cluster 1 produced significantly larger cankers with the mean canker area 271 mm² (millimeter square) than isolates from the four other clusters. In addition, isolates from the same genetic cluster exhibited significant variation in virulence. In conclusion, results from this study present potential implications to phytosanitary and quarantine efforts of TCD.