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Ash dieback (Hymenoscyphus fraxineus)

Ash dieback

Ash dieback


Ash trees are a vital part of our landscape growing in woodlands, fields, hedgerows, parks, and gardens. They are the third most common tree in Britain and provide important habitat for almost 1000 species, some of which are wholly dependent on ash for habitat.  A guide to identifying ash trees can be found at Woodland Trust

What is ash dieback?

Ash dieback is a fungal disease affecting ash trees of all ages, leading to leaf loss and canopy decline. It is caused by a fungus named Hymenoscyphus fraxineus. The disease was first recorded in the United Kingdom in 2012 and is now found in most areas. It is expected that we will lose the majority, but not all ash trees, with a small proportion showing some genetic tolerance to the disease.

Symptoms of ash dieback

Trees affected by ash dieback exhibit a range of symptoms, therefore it is essential to speak to a suitably qualified consultant if uncertain.  Most affected ash trees will lose leaves at the top of the tree (the crown), as shoots and leaves dieback. In response to stress, new growth develops from previously dormant buds further down the branches and trunk, this is known as epicormic growth.

Other symptoms include:

  • Leaves develop dark patches in the summer.
  • Leaves wilt and discolour to black and may shed prematurely.
  • Lesions develop where branches meet the trunk. These are often diamond-shaped and dark brown. Especially evident in younger ash.
  • Basal lesions develop at the base of the trunk. These are often triangle shaped.
  • Other opportunistic fungus such as honey fungus (Armillaria) may be evident and cause the eventual death of the tree by accelerating wood decay and tree failure rather than ash dieback itself.


If an ash tree must be felled, it is important that replacement trees are planted wherever possible. Alternative species include alder, oak, lime, sycamore, rowan, field maple, aspen, and birch. The final choice depends on the site conditions, space available and the reason for planting be it, wildlife, aesthetics, or timber production.

Useful information on identifying ash dieback and managing ash tree and replanting can be found at The Tree Council and from Forest Research.

Last updated: 17 December 2021

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