From the Alps to the Apennines: possible spread of ash dieback in Mediterranean areas

The causal agent of ash dieback, the ascomycetous fungus Hymenoscyphus fraxineus, was isolated from Fraxinus excelsior in the Apennines Mountains in central Italy. Infected trees showed severe symptoms of ash dieback such as dieback of twigs and branches, and characteristic necroses on shoots, leaves, rachises and petioles. Typical H. fraxineus colonies were obtained from symptomatic tissues and identified from morphological and cultural traits as well as by means of molecular analyses. The pathogen was isolated both from potted plants in nurseries and from trees in nearby forests, which confirms that the spread of H. fraxineus is largely human-mediated. This is currently the southernmost record of the pathogen in the Italian peninsula and in Europe, indicating that the fungus is able to survive and infect F. excelsior in those parts of the geographic range that border areas with Mediterranean climate. H. fraxineus has not been isolated so far from F. ornus despite the symptoms of decline observed on this species in the study area. However, as suggested by recent studies, F. ornus and other species of Oleaceae might be suitable for growth of H. fraxineus and might be colonised by the fungus when growing in stands or communities bordering F. excelsior stands Such species might contribute, if infected, to increase the reservoir of inoculum of the pathogen in these areas. This newly updated southernmost limit of the distribution of H. fraxineus might serve as a possible bridge for disease outbreak in other ash species or for host jump to other species of Oleaceae that are characteristic of the Mediterranean landscape.