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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.


Whilst there is no evidence of full resistance to the disease, research and experience in Europe indicates that up to 5% of the ash population may be genetically tolerant to ash dieback. This natural tolerance in some trees provides an opportunity to maintain ash in the UK because the tolerance may be inherited.

it is important to note that the poor condition of the canopy might not be a result of ash dieback. Other problems such as drought stress, waterlogging, root damage, soil compaction, or other pests and diseases can cause ash trees to decline. Look out for basal lesions, honey fungus (Armillaria spp.), shaggy bracket (Inonotus hispidus) or giant ash bracket (Perenniporia fraxinea), all of which have the potential to impair the structural integrity of ash trees.




For further technical information, and images, see

ash tree with ash dieback

Removal of an Ash tree with severe Ash dieback disease in Chorley

ash tree felling with ash dieback
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