Ash dieback

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Ash dieback, also known as Chalara dieback of ash, is a serious disease that is killing ash across Europe. Ash is a very important tree in the UK both ecologically and culturally so this disease is causing great concern about the damage it will do.

What is ash dieback?

Ash dieback affects ash trees (Fraxinus excelsior) and is caused by the fungus Hymenoscyphus fraxineus (previously known by the names Chalara fraxinea and Hymenoschyphus pseudoalbidus). It blocks the water transport systems in trees causing leaf loss, lesions in the wood and on the bark and ultimately the dieback of the crown of the tree.

This disease was first described in Poland in 1992 and has since swept westwards throughout Europe. It was first identified in Britain in 2012 in nursery stock then in the wider environment in 2013 although it could have been in the country much longer.

The number of confirmed findings is continuing to increase and the distribution is reported by the Forestry Commission on a regular basis.

Young trees are particularly vulnerable and die quickly once they succumb. Older trees can be slowly killed by a yearly cycle of infection. Spread of the disease in the UK is most likely to be as a result of the planting of infected nursery stock and wood but wind borne distribution of the fungal spores also occurs.

There are several key signs to look out for on ash trees. All of these symptoms can also be caused by other problems, so final diagnosis should be made by an expert. Summer is a good time to look for symptoms as in autumn and winter, ash trees will naturally be shedding their leaves making it difficult to identify ash dieback.

Symptoms

  • Dark lesions – often long, thin and diamond-shaped – appear on the trunk at the base of dead side shoots
  • The tips of shoots become black and shrivelled
  • Blackened, dead leaves – may look a bit like frost damage
  • The veins and stalks of leaves, normally pale in colour, turn brown
  • Saplings have dead tops and side shoots
  • In mature trees, dieback of twigs and branches in the crown, often with bushy growth further down the branches where new shoots have been produced
  • In late summer and early autumn (July to October), small white fruiting bodies can be found on blackened leaf stalks.

Causes

  • The disease is spread by spores from the fruiting bodies of the fungus produced on fallen ash leaves. These airborne spores can disperse naturally via wind over tens of kilometres
  • Prior to the ban in October 2012 on the movement of ash trees, spread over longer distances was likely to have been via the movement of infected ash plants.

Outlook

  • The disease is spread by spores from the fruiting bodies of the fungus produced on fallen ash leaves. These airborne spores can disperse naturally via wind over tens of kilometres
  • Prior to the ban in October 2012 on the movement of ash trees, spread over longer distances was likely to have been via the movement of infected ash plants
  • We don’t know what the full impact of ash dieback will be. Evidence suggests young trees are killed quickly while many mature ash trees can resist infection for some time until eventually dying or becoming weakened and succumbing to attack from another pest or pathogen.
  • Scientists have developed techniques to identify individual trees that are less susceptible to ash dieback disease, this technique combined with resistance breeding trials, can be used to grow trees that are more likely to survive the disease.

What we are doing

We are at the forefront of the fight against ash dieback. From researching resistant strains to campaigning for better biosecurity, we are in a race against time. Find out how we are working in partnership with other organisations to safeguard ash populations for the future.

For more information

Read Forestry Commission Ash Dieback pest alert (PDF, 700 KB)

Download the Forestry Commission picture guide (PDF,0.3MB)

Find out more about how to identify ash trees and why it is so important to our woodland and wildlife from Malcolm Allen, Woodland Trust Site Manager for Cornwall and East Devon.

This video shows samples affected by ash dieback from The Food and Environment Research Agency.

Report it

If you think you’ve spotted this disease please inform the Forestry Commission using the Tree Alert reporting tool. Three good-quality digital photographs are required to aid identification.

For more information visit the Forestry Commission.

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