Rapid invasion by an aggressive pathogenic fungus (Hymenoscyphus pseudoalbidus) replaces a native decomposer (Hymenoscyphus albidus): a case of local cryptic extinction?

Ash dieback caused by the infectious fungus Hymenoscyphus pseudoalbidus currently threatens the common ash, Fraxinus excelsior, in Europe. An intriguing aspect is the morphological and ecological similarity between H. pseudoalbidus and the native saprotroph Hymenoscyphus albidus. We revisited four localities where H. albidus apothecia were collected from 1989 to 2005 and established the current relationship of the species in these Danish ash stands based on microsatellites and differences in ITS sequences (used as CAPs marker). Scottish collections from 2010 supported the hypothesis that Danish herbarium samples prior to 2005 are identical to H. albidus still found in Scotland. The markers further revealed that herbarium samples from 1989 to 1994 were all H. albidus, while the latest collection (2005) was H. pseudoalbidus, which coincides with the first Danish symptom observations. Collections from 2010 were purely H. pseudoalbidus. We suggest that expanding H. pseudoalbidus excludes H. albidus from its niche resulting in H. albidus becoming a rare species in Denmark, which is perhaps already locally extinct.

► Expanding H. pseudoalbidus is turning H. albidus into a rare species in Denmark. ► Danish herbarium samples prior to 2005 corresponds to H. albidus from Scotland. ► Recent Danish collections at former H. albidus locations are all H. pseudoalbidus.