Hymenoscyphus pseudoalbidus is a 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 was only the asexual (anamorphic) stage of a fungus that was subsequently named Hymenoscyphus pseudoalbidus.
Initially, small necrotic spots appear on stems and branches. These then enlarge resulting in wilting, dieback of branches and particularly in the death of the top of the crown. The disease is often chronic but can be lethal. It is particularly destructive of young ash plants, killing them within one growing season of symptoms becoming visible. Older trees can survive initial attacks, but tend to succumb eventually after several seasons of infection.
The disease can be confused with another disorder known as “ash dieback” which has been known for many years and is widespread in the UK. The disorder causes crown dieback in mature trees.