Thousand Cankers Disease of Walnut

Geosmithia morbida is a mitosporic ascomycete fungus belonging to the order Hypocreales and the family Bionectriaceae. This fungus is recognized as the causal agent of Thousand Cankers Disease (TCD), a significant threat to walnut trees. The disease derives its name from the multitude of coalescing cortical cankers that develop around the entrance holes created by its insect vector, the walnut twig beetle (Pityophthorus juglandis). The relationship between Geosmithia morbida and Pityophthorus juglandis is symbiotic, with the beetle carrying the fungal conidia on its body and introducing them to healthy walnut trees as it bores through the bark. While the greatest damage is observed on black walnuts (Juglans nigra).