Variation in aggressiveness plasticity between northern and southern European populations of Hymenoscyphus fraxineus, an invasive fungal pathogen of ash

Hymenoscyphus fraxineus is an invasive fungal pathogen responsible for the ash dieback epidemic, which continues to cause severe mortality of common ash (Fraxinus excelsior L.) across Europe. Following its likely introduction in northeastern Europe, the pathogen rapidly colonized most regions where common ash is present. As it spread southward, H. fraxineus encountered warmer climates and a higher occurrence of Fraxinus ornus L., a species largely resistant to the disease. Despite this environmental heterogeneity, which likely imposed adaptive challenges on H. fraxineus at the epidemic front, ash dieback continues to expand in southern Europe. Aggressiveness is a key life-history trait that is expected to evolve during epidemics and to exhibit plasticity in response to environmental variation. We investigated whether the plasticity of aggressiveness in response to temperature and ash species has evolved in H. fraxineus during its propagation towards southern Europe. Using a synchronic approach based on leaf inoculations, we characterized individual reaction norms for aggressiveness in a long-established Lithuanian population and a recently established Italian population of H. fraxineus. The Italian H. fraxineus population is exposed to warmer summers than the Lithuanian population, while F. ornus is present in Italy but absent in Lithuania. We observed no difference in the aggressiveness expressed on F. excelsior under moderate temperature between the two H. fraxineus populations. However, the ability of Italian isolates to cause severe leaf symptoms was less negatively affected by increasing temperature and host species change than that of Lithuanian isolates, suggesting local adaptation of H. fraxineus during its spread toward southern Europe. Our findings highlight the importance of considering the evolution of adaptive traits and their plasticity in fungal pathogens when anticipating disease risk. They also suggest that ash trees in southern Europe may be slightly more vulnerable to ash dieback than previously anticipated.