Ash dieback (Hymenoscyphus fraxineus)

What is Ash Dieback?

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. 

The disease is also known as 'chalara', ash dieback, and chalara dieback of ash. Calling it 'chalara' ash dieback helps to distinguish it from dieback on ash trees caused by other agents.

The asexual phase of the fungus's life cycle was formerly known as Chalara fraxinea, hence the name of the disease, and the sexual phase was called Hymenoscyphus pseudoalbidus. Some older scientific, technical and policy documents which are still consulted use these earlier names. 

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