Introduction of Mandshurian ash (Fraxinus mandshurica Rupr.) to Estonia: Is it related to the current epidemic on European ash (F-excelsior L.)?

Recent investigations in Japan have suggested that the causal organism of the ongoing epidemic affecting European ash (Fraxinus excelsior) in Europe, Hymenoscyphus pseudoalbidus, may originate in East Asia. The fungus may have been unintentionally carried to Europe during the introduction of Mandshurian ash (F. mandshurica), the host tree of the fungus in East Asia. Still unicentric emergence hypothesis is in force: An area in the eastern Poland or Baltic has been shown to be the presumed epicentre of the epidemic. Really, during the Soviet occupation, several consignments of F. mandshurica seeds and plants, originating directly from the natural range of F. mandshurica in East Asia (Russian Far East), reached Baltic areas. In this paper, an overview about the Mandshurian ash is presented, the history of introduction of F. mandshurica to Estonia is reviewed and colonization of F. excelsior in this country by H. pseudoalbidus is briefly discussed. At present, we could not find any evidence, spatial or temporal, for a direct connection of the disease emergence on native F. excelsior with the introduction of F. mandshurica. The pathogen first colonized northwest Estonia and moved southeast and not from south to north as would be expected according to the hitherto existing unicentric hypothesis. However, more information is needed from different regions before to pose a multicentric emergence hypothesis and to deepen more into the investigations of the environmental factors that affected the host and supported to the epidemic in different areas.