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Chalara dieback of ash (Hymenoscyphus fraxineus)


First confirmed in Britain in 2012, Chalara dieback of ash, also known as 'Chalara', ash dieback or Chalara ash dieback, is a disease of ash trees caused by a fungus called Hymenoscyphus fraxineus. (The fungus was previously called Chalara fraxinea, hence the name of the disease. See 'The Science' below for an explanation of the name change.)

Chalara causes leaf loss, crown dieback and bark lesions in affected trees. Once a tree is infected the disease is usually fatal, either directly, or indirectly by weakening the tree to the point where it succumbs more readily to attacks by other pests or pathogens, especially Armillaria fungi, or honey fungus.

However, some ash trees appear to be able to tolerate or resist infection, and scientists are studying the genetic factors which make this possible so that tolerant ash trees can be bred for the future.

The Threat

We don't yet know what the full impact of Chalara will be in Britain. Evidence from continental Europe suggests that older, mature ash trees can survive infection and continue to provide their landscape and wildlife benefits for some time.

We do know that the disease has potential to cause significant damage to the UK's ash population. It has caused widespread damage in continental Europe, where experience indicates that it can kill young ash trees quite quickly, while older trees can resist it for some time until prolonged exposure, or another pest or pathogen attacking them in their weakened state, eventually causes them to succumb.

Ash can grow in a variety of soils and climatic conditions. It is one of our most useful and versatile native tree species, providing valuable habitat for a wide range of dependent species as well as strong, durable, flexible and attractive timber with a wide range of practical and decorative uses.

The Joint Nature Conservation Council (JNCC) in January 2014 published reports of studies into the potential ecological impact of Chalara ash dieback in the UK, and on the options for long-term monitoring of its impacts on biodiversity.

The best hope for the long-term future of Britain's ash trees lies in identifying the genetic factors which enable some ash trees to tolerate or resist infection, and using these to breed new generations of tolerant ash trees for the future. Government scientists, including our Forest Research agency, are working hard on this in partnership with a range of other respected scientific research institutions.

Susceptible species

Chalara dieback of ash is especially destructive of common or European ash (Fraxinus excelsior), including its ‘Pendula’ ornamental variety. Narrow-leaved ash (Fraxinus angustifolia) is also susceptible, and both species have been affected in the UK. It has not been reported infecting any non-ash species.

Method of spread

Local spread, up to some tens of miles, may be by wind. Over longer distances the risk of disease spread is most likely to be through the movement of diseased ash plants. Movement of logs or unsawn wood from infected trees might also be a pathway for the disease, although this is considered to be a low risk.

Outbreak stage

Ash trees suffering with the infection have been found widely across Europe since trees were first reported dying in large numbers in Poland in 1992. These have included forest trees, trees in urban areas such as parks and gardens, and also young trees in nurseries.

According to the European Plant Protection Organization (EPPO), Austria, Belgium, the Czech Republic, Finland, France, Germany, Hungary, Italy, Lithuania, the Netherlands, Norway, Poland, Slovenia and Sweden have confirmed its presence. On the basis of symptoms, the disease has also been observed in Denmark, Estonia, Latvia and Switzerland.

It was first confirmed in the UK in February 2012 when it was found in a consignment of infected trees sent from a nursery in the Netherlands to a nursery in Buckinghamshire, England.

In October 2012, a small number of cases were confirmed in Norfolk and Suffolk on ash trees at sites in the wider natural environment, including established woodland, which did not appear to have any association with recently supplied nursery plants. Further finds on trees in the wider environment have since been confirmed in most regions of the UK and Ireland, and it was first confirmed on the Isle of Man in 2017.

Hymenoscyphus fraxineus, the fungus which causes Chalara dieback of ash, is treated as a quarantine organism under national emergency measures, and any suspected sighting must be reported.

Upon discovery of infected ash trees in the wider environment in October 2012, we and others undertook a great Britain-wide survey of about a thousand sites which had received saplings from nurseries where Chalara dieback had been found to establish an understanding of the distribution of the disease.


Confirmed findings as at midday on 1st June 2018:

10-kilometre grid squares with one or more Hymenoscyphus fraxineus (Chalara) infections confirmed in the wider environment:

  2012 2013 2014 2015 2016  2017  2018  Total % of all 10k squares in country
Scotland 7 5 33 125 10 27 12 219 19.9%
England 82 60 161 224 310 86 29 953 64.8%
Wales 0 2 5 31 96 64 13 211 79.6%
N Ireland


0 0 0 17 0 0 17 9.1
Isle of Man 

0   0 1 1 0 1 7.1
UK (total) 89 67 199 380 433 178 54  1401 46.2%


The progression of numbers and appearance of new grid squares over time should not be interpreted as an indication of the rate of spread of the disease. It only indicates when infection sites were found, not when the fungus first arrived at the site, which in many cases cannot be known.



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Because ash trees have many genetic variants and occur right across the UK, they come into leaf at different times. Ash is also one of the last tree species to flush, sometimes taking as long as six weeks to do so, and can often occur as late as the end of May. Trees in the colder north flush later than trees in the warmer south. Some ash trees will break-bud, or flush, earlier than others, and some buds will produce flowers rather than new shoots. Some variation will be more apparent in older trees.

Some shoots on ash trees will fail to flush altogether, while others will flush normally before showing signs of ill-health or dieback later. These events might mean that the trees are damaged in some way, but shoot death and dieback in ash trees can have a number of causes.

So if an ash tree does not have any leaves on it in April and May, it does not necessarily mean that it is diseased or dying, but by mid-June all healthy ash should be in full leaf.

Mid- to late summer (August and September) is a good time of year to undertake surveys, because once autumn has begun, visual symptoms can be confused with leaves that are naturally changing colour.

ASH KEYS (Fraxinus excelsior)

In the autumn you might see clumps of sometimes dark-coloured ash keys (seeds), retained on the trees after the leaves have fallen. This is quite normal, but from a distance they can be mistaken for the blackened leaves which can be a symptom of the disease.

Reporting suspected cases

Tree Alert icon If you think you have spotted the disease, please check our symptoms section before reporting it using Tree Alert or one of the Further Information contact points below.


Sorry, but we cannot respond to each report individually. However, every one of them will be assessed, and for each report we will:

  • prioritise action according to our existing knowledge of the disease's distribution and
  • decide it isn’t Chalara dieback of ash; or
  • ask for more information, which might include asking for photographs; or
  • arrange for someone to do a further investigation on site.

Management, grants, treatment

  • You are not required to take any particular action if you own infected ash trees, unless we or another plant health authority serves you with a statutory Plant Health Notice requiring action.
  • Keep an eye on the trees' safety as the disease progresses, and prune or fell them if they or their branches threaten to cause injury or damage.
  • In low-density situations such as parks and gardens, you can help to slow the spread of the disease by removing and disposing of infected ash plants, and collecting up and burning (where permitted), burying or composting the fallen leaves. This breaks the fungus's life cycle.

  • Detailed information and guidance for woodland owners and managers.
  • Grants for replacing ash trees in England

The science

Chalara is caused by a fungus called Hymenoscyphus fraxineus. This fungus has two phases to its life-cycle: sexual and asexual. The asexual stage, which grows in affected trees, attacking the bark and girdling twigs and branches, was the first to be described by science, and was originally called Chalara fraxinea. This gave rise to the common name of the disease which it causes.

The sexual, reproductive stage, which was only discovered some years later, occurs as tiny, mushroom-like fruiting bodies on infected rachises, or stalks, of the previous year's fallen leaves. Infective spores from these fruiting bodies are spread by the wind on to the leaves of healthy trees in summer. This sexual stage was initially called Hymenoscyphus pseudoalbidus (H. pseudoalbidus) before a taxonomic revision suggested the name should be Hymenoscyphus fraxineus. The International Botanical Congress has also agreed that a single fungus should have only one name, even if different stages of the organism have previously been given separate names. Therefore Hymenoscyphus fraxineus is now widely accepted as the name to use.

Government scientists have set out the most up-to-date understanding of the disease. Their assessment concluded that:

  • the spores are unlikely to survive for more than a few days;
  • spore dispersal on the wind is possible from mainland Europe;
  • trees need a high dose of spores to become infected;
  • spores are produced from infected dead leaves during June to September;
  • there is a low probability of dispersal on clothing or animals and birds;
  • the disease will attack any species of ash;
  • the disease becomes obvious within months rather than years;
  • wood products would not spread the disease if treated properly;
  • once infected, trees cannot be cured; and
  • not all trees die of the infection - some are likely to have genetic factors which give them tolerance of, or resistance to, the disease.

Scientists are working to learn from existing and emerging research and practical experience in combating the disease in other countries.

Our Forest Research agency is part of the Nornex Consortium, which was awarded £2.4m research funding to gather an in-depth understanding of the Chalara dieback fungus, and to provide genetic clues about the natural tolerance or resistance of some ash trees to attack. Forest Research is also leading a mass screening trial to identify inherent tolerance or resistance in UK ash trees.

Preliminary results from testing of selected fungicides for treating ash trees with Chalara dieback of ash for their efficacy in laboratory tests and field trials are now available on the Defra website.


The fungus is believed to have originated in Asia, where Asian species of ash are able to tolerate it after of thousands of years of co-evolution.

It entered Britain on ash plants imported from nurseries in continental Europe. However, now that infected, older trees have been found in South-East England with no apparent association with plants supplied by nurseries, it is thought possible that it also entered by natural means. These include being carried on the wind or on birds coming across the North Sea and English Channel, or on items such as footwear, clothing or vehicles of people who had been in infected sites in Continental Europe.

Video: history of the pathogen. 

Pest Risk Analysis

Pest Risk Analysis (PRA) was published in May 2013. (The document title uses the former name of the sexual stage of the fungus, Hymenoscyphus pseudoalbidus.)

Import and movement restrictions

A Plant Health Order prohibits all imports of ash seeds, plants and trees, and all internal movement of ash seeds, plants and trees.

Further information



: 0300 067 4000


: 0300 067 6156 (9am-5pm weekdays and out-of-hours messaging system)



Last updated: 5th June 2018