Hymenoscyphus fraxineus

Hymenoscyphus fraxineus is an ascomycete fungus that causes ash dieback, a chronic fungal disease of ash trees in Europe characterised by leaf loss and crown dieback in infected trees. The fungus was first scientifically described in 2006 under the name Chalara fraxinea. Four years later it was discovered that Chalara fraxinea is the asexual (anamorphic) stage of a fungus that was subsequently named Hymenoscyphus pseudoalbidus and then renamed as Hymenoscyphus fraxineus.

The first recognised report of this disease was in 1992 after a large number of ash trees had died in Poland. The disease is now widespread in Europe, with up to 85% mortality rates recorded in plantations and 69% in woodlands. It is closely related to a native fungus Hymenoscyphus albidus, which is harmless to European ash trees. A 2016 report published in the Journal of Ecology posited that a combination of H. fraxineus and emerald ash borer attacks could result in the extinction of European ash trees.