Ash tree (Fraxinus excelsior) affected by ash dieback. Ash dieback is a chronic fungal disease of ash trees in Europe characterised by leaf loss and crown dieback. It is caused by the fungus Hymenoscyphus fraxineus which grows inside the tree, eventually blocking its water transport systems, causing it to die. Now widespread in Europe, the fungus is thought to have originated in Asia where the native trees are immune to the disease.