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The ascomycete fungus Geosmithia morbida (GM) and the walnut twig beetle (WTB), Pityophthorus juglandis, constitute a recently discovered fungal–insect complex responsible for the harmful Thousand Cankers Disease (TCD) in walnut trees. Key aspects of the epidemiology and ecology of the fungus and its insect vector were investigated in a disease outbreak found in central Italy, currently the southernmost introduction area of TCD on the Old Continent. Walnut trees with symptoms of crown dieback and epicormic branching with beetle galleries were sampled for pathogen isolation, while funnel traps were used to catch beetles and obtain adult flight curves. Growth-temperature relationships were determined for the fungus; optimum growth temperature for GM was 25°C, but it thrived up to 37°C. The phloem-boring bark beetle showed a longer flight period than in northern Italy. The beetle haplotype identified in Tuscany (H1) differed from haplotypes H2 and H21 found in northern Italy, suggesting possible multiple anthropogenic introductions. The substantial high temperature tolerance shown by the two organisms suggests they will not be constrained by the high temperatures of warmer areas. As a consequence, the impact of TCD in southern Europe, where the native Juglans regia is widely cultivated for its nuts and wood, would potentially be disastrous to local economies. In fact, although J. regia is not considered as susceptible as J. nigra, it can nevertheless be attacked in conditions of high bark beetle population density, a harbinger of high fungus’ propagule pressure.

Successful infection by a tree pathogen depends both on species-specific traits, related to its infection biology, and on external factors, i.e. the biotic and abiotic components of the ecosystem. Among species-specific traits, virulence, biomass (propagule) production and dispersal ability all contribute to pathogen establishment and spread. Among the external factors, two crucial roles are played by an environment conducive to disease and by vector occurrence. We investigated the endophytic fungi and xylophagous beetles associated with declining oaks in a natural stand of central Italy, particularly focusing on the fungi isolated from insects’ body. This paper extends and reinterprets the outcome of a previous study, which looked into the whole mycota (pathogenic/neutral/beneficial) found on these oaks. A rich assemblage of fungi was found on declining trees on which a number of wood-boring beetles (Buprestidae and Cerambycidae) also occurred. In the laboratory two-thirds of the insects proved to convey propagules of prominent fungal pathogens (Botryosphaeriaceae, Pestalotiopsidaceae, Plectosphaerellaceae, Pleosporaceae); this confirms pathogen pervasiveness in oak stands is enhanced by their association with xylophagous insects. The outcome of this research will benefit forest manager and other stakeholder efforts to manage stands and species invasions, also in the light of climate change. Keywords: Oak decline, Endophytic fungi, Xylophagous beetles