Thermal plasticity of virulence differs between two latitudinally distant populations of the ash dieback pathogen

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 resistant to the disease, which may have imposed novel selective pressures on H. fraxineus at the epidemic front. Virulence is a key trait that is expected to evolve during epidemics and to exhibit plasticity in response to environmental variation. We investigated whether the plasticity of virulence in response to temperature and ash species has evolved in H. fraxineus. Using leaf inoculations, we characterized individual reaction norms for virulence in a long-established Lithuanian population and a recently established Italian population of H. fraxineus. At moderate temperature, the two H. fraxineus populations showed similar levels of virulence on F. excelsior. Virulence decreased to a similar extent on F. ornus in both pathogen populations, indicating similar plasticity of their virulence in relation to ash species. Interestingly, increasing temperature reduced virulence in the Lithuanian population but did not significantly change the virulence expressed by the Italian population, which remained stable across temperatures. Although these findings warrant further investigation, they suggest that ash populations in southern Europe may be more vulnerable to ash dieback under warm conditions than previously anticipated.