Ash dieback (Hymenoscyphus fraxineus) is a fungal pathogen of ash trees. It is a native of Europe.
Initial symptoms of infestation by this pathogen include small necrotic spots which appear on stems and branches. These necrotic lesions 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. Ash dieback has been observed in forest trees as well as nursery stock.