Observations on the non-native thousand cankers disease of walnut in Europe's southernmost outbreak

Abstract

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.