Albiducins A and B, salicylaldehyde antibiotics from the ash tree-associated saprotrophic fungus Hymenoscyphus albidus

Since the early 1990s, European ash (Fraxinus excelsior) has been facing an onslaught of severe dieback in most of its natural distribution area in Europe. The causal agent of this epidemic disease is the invasive ascomycete Hymenoscyphus fraxineus (=Hymenoscyphus pseudoalbidus).^{1, 2, 3} This fungal pathogen most probably originates from East Asia and has almost replaced the European sister species Hymenoscyphus albidus.^{4, 5} The latter species, H. albidus, is known in Europe since 1851⁶ as a widespread, non-pathogenic decomposer of ash litter and produces numerous white-stalked apothecia on fallen, previous year’s rachises of F. excelsior.^{5, 7} Apparently, H. albidus is threatened with local extinction in regions colonized by H. fraxineus due to competitive exclusion from its ecological niche (fallen petioles of F. excelsior).⁴

Over the past years, H. fraxineus has been studied intensively for virulence factors ultimately responsible for the pathogenicity. The role of previously described secondary metabolites, like the tetramic acid antibiotic hymenosetin,⁸ during infection and necrosis formation has yet to be defined. Actually, the only known phytotoxins of H. fraxineus, viridiol and 3,4-dimethylpentan-4-olide, are, however, also produced by the non-pathogenic H. albidus.^{9, 10}

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