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Ash dieback is a highly destructive disease of ash trees (Fraxinus species), especially the United Kingdom's native ash species, common ash (Fraxinus excelsior). It is caused by a fungus named Hymenoscyphus fraxineus (H. fraxineus), which is of eastern Asian origin. 

Identification and symptoms

Among the first symptoms that an ash tree might be infected with H. fraxineus is blackening and wilting of leaves and shoots (top picture) in mid- to late summer (July to September). These months are the best time of year to survey ash trees for chalara symptoms in the foliage. This is because once autumn begins in late September or October, the normal seasonal change in the colour of the leaves can be mistaken for symptoms of the disease.Most infected leaves are shed prematurely by the tree, but in some cases the infection progresses from the leaves and into the twigs, branches and eventually the trunk, causing dark lesions, or cankers, to form in the bark. These often have a characteristic elongated-diamond shape (pictured above) centred on the joints between branches, or where branches join the trunk. The lesions typically, but not always, spread upwards and downwards from the joint as the infection spreads in both directions. They can eventually girdle the whole trunk, cutting off the tree's supply of fluid and nutrients from the roots.