Ash dieback which is also known as Charlara dieback of ash is a serious disease which is killing ash across the whole of Europe and is spreading considerably across the UK.

What is Ash Dieback?

Ash dieback affects ash trees (Fraxinus excelsior) and is caused by the fungus Hymenoscyphus fraxineus. It affects the trees vascular system, the pathogen causes necrosis in the sapwood and affects the trees ability to draw nutrients up into its upper branches. The pathogen first inhabits leaves and twigs which it damages by producing a chemical called viridiol, causing leaf loss, lesions in the wood on the bark and ultimately the dieback of the crown of the tree.

The disease was first detected in Poland in 1992 but has since spread westwards and throughout Europe with devasting effects. Although it could have been in the country longer it was first identified in Britain in 2010 in nursery stock and then in the wider environment in 2013. The number of findings is increasing and reported by the Forestry Commission on a regular basis. Younger trees are particularly vulnerable and die quickly once infected.

ash dieback overhead picture of problem
ash dieback branch example photo
ash dieback leaves photo
example of ash die back photo

Older trees can be slowly killed by a yearly cycle of infection. The spread of the disease in the UK is most likely to be from the planting of infected nursery stock however, windblown distribution of fungal spores also occurs and the disease is also seed-borne too. There are several signs to look out for however these symptoms can also be caused by other problems and therefore diagnosis should be sort by an expert. The summer season is the time to look for symptoms as leaves are in bloom as opposed to the autumn and winter when leaves are shedding making it tricky to identify.

Symptoms to Look Out For

  • Lesions – often long, thin and shaped like a diamond, they appear on the trunk at the base of dead side shoots
  • Tips of shoots are black and shrivelled
  • Leaves may look frost-damaged so blackened with small white fruiting bodies can be found on the blackened leaf stalks around July-October.
  • The veins and stalks of the leaves turn brown when they are normally pale in colour
  • Saplings have dead tops and side shoots
  • In older trees, dieback of twigs and branches in the crown with bushier growth further down the branches where new growth shoots have produced.

Causes of Ash Dieback

  • The spores from the fruiting bodies of the fungus which can disperse naturally via wind and can travel tens of kilometres.
  • It is more than likely via movement of infected plants prior to the ban in October 2012 on the movement of ash trees.


As we know the disease spreads via spores from the fruiting bodies of the fungus produced on the leaves and fallen leaves. These airborne spores can disperse naturally via wind to a wider area, therefore, the infection risk is extremely high. After 7 years from the official identification of ash dieback in the UK, it has already started having a significant impact on the country’s treescape which is likely to increase rapidly.

Scientists have developed techniques to identify individual trees which are less susceptible to the disease, this technique combined with resistance breeding trials can be used to grow trees that are more likely to survive the disease.

Although it is too early to establish if any trees will prove to be resistant to the fungus, the reality is the likelihood is 90% of the 2 billion ash trees across the UK will be infected in the years to come.

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