Geosmithia species in southeastern USA and their affinity to beetle vectors and tree hosts

The fungal genus Geosmithia is best known due to one species, G. morbida, which is vectored by the walnut twig beetle (WTB) and contributes to Thousand Cankers Disease (TCD) on walnut and wingnut trees. However, the genus is globally very diverse and abundant, and dominates a ubiquitous but understudied niche - the twig-infesting, phloem-feeding bark beetle mycobiome. The Geosmithia community in North America is only now beginning to be described. Very limited information is available for the South East, despite the region's potential to be a Geosmithia diversity hotspot. To survey the Geosmithia community in the subtropical USA, to assess their beetle and tree associations, and to test for the presence of G. morbida, we systematically deployed branch sections of nine tree species, including three Juglandaceae, in North Florida. We recovered 55 Geosmithia isolates from 195 beetle specimens from 45 exposed branch units. Neither G. morbida nor its beetle vector were detected. We identified 14 Geosmithia species; those in the G. pallida species complex were the most prevalent. Four undescribed phylogenetic species were recovered, indicating that the Geosmithia diversity in North America remains under-documented. Analysis of the association of Geosmithia with beetles and trees suggested that most species are generalists, five display preference for certain tree species, and none is specific to any beetle species. (C) 2019 Elsevier Ltd and British Mycological Society. All rights reserved.