Thousand Cankers Disease: Scolytine beetles and fungal pathogens associated with symptomatic eastern black walnut

Thousand cankers disease (TCD) is a pest complex formed by the association between the walnut twig beetle (WTB), Pityophthorus juglandis (Coleoptera: Curculionidae: Scolytinae), and the fungal pathogen Geosmithea morbida. TCD has caused the widespread death of walnut trees throughout the West. Currently, TCD has been confirmed in seven eastern states within the native range of black walnut (Juglans nigra), including Ohio. In 2014, we emerged beetles from stem and branch sections of four TCD-symptomatic trees growing in Butler Co., Ohio to determine the extent to which other insects might vector the pathogen. We reared eight predominant beetle taxa and isolated G. morbida and other fungal pathogens, including Fusarium solani, from bark and ambrosia beetles emerged from symptomatic trees. We also isolated three known fungal pathogens of J. nigra (G. morbida, F. solani and Botryosphaeria dothidea) from branch and stem cankers. In summary, a suite of insect species colonizes black walnut as TCD develops; however, WTB is considered the primary colonizer. Moreover, other phytopathogenic fungi besides G. morbida appear to contribute to canker development and branch death on TCD trees, but their roles relative to G. morbida warrant further investigation. Characterizing the assemblage of scolytine beetles associated with eastern black walnut and assessing the interaction of G. morbida and other fungal pathogens in affecting tree health is essential for understanding the etiology of this disease complex.