Chalara dieback of ash is a disease of ash trees (Fraxinus species) caused by an asexual fungal organism called Chalara fraxinea (C. fraxinea) and its sexual stage, Hymenoscyphus pseudoalbidus (H. pseudoalbidus). For ease of reference, Chalara fraxinea is used as the common term. The disease causes leaf loss and crown dieback in affected trees, and it usually leads to tree death. The C. fraxinea fungus has caused widespread damage to ash tree populations in continental Europe since it was first reported as an unknown new disease in Poland in 1992. It is especially destructive of common ash (Fraxinus excelsior), including its ‘Pendula’ ornamental variety. Narrow-leaved ash (Fraxinus angustifolia) is also susceptible. Chalara dieback of ash is particularly destructive of young ash plants, killing them within one growing season of symptoms becoming visible. Older trees can survive initial attacks, but tend to succumb eventually after several seasons of infection.
2. What is the situation in Great Britain?
It was unknown in Great Britain until the first case was confirmed in ash plants in a nursery in Buckinghamshire early in 2012, in a consignment which had been imported from The Netherlands. Since then, more infected plants have been confirmed in nurseries in a wide range of locations in England and Scotland, and in recent plantings of young ash trees at a variety of sites supplied by nurseries, including a car park, newly planted woodland and a college campus. Our colleagues in Fera and the Scottish Government are continuing work to trace and inspect plants which had already been sold on to retail customers from the infected nursery consignments.
In October and November 2012 infection was confirmed for the first time in the wider natural environment in longer-established situations, such as woodlands and hedgerows, in East Anglia, Essex and Kent. These trees appear to have had no recent connection with nursery supplied plants or imports of ash plants from mainland Europe, so we are investigating how the fungus got to these sites. Given their proximity to mainland Europe, we cannot rule out the possibility of some sort of natural introduction, such as wind-borne spores from mainland Europe, and we are investigating the likely consequences
We are treating C. fraxinea as a ‘quarantine’ plant pathogen, which means that we may use emergency powers to contain or eradicate it when it is found. This is being done in the form of Statutory Plant Health Notices which are served on affected owners. In the case of nursery plants and recently planted young trees, we require the owners to contain the site, and we may require that infected plants be destroyed to prevent disease spread. Equivalent measures are being taken on land managed by the Forestry Commission, and this is the only available treatment to get rid of the disease.
In the case of trees in established woodland and similar situations, where many of the affected trees are much larger, less accessible and in a mixture with other tree species, we require biosecurity measures to be taken to contain the infection on the site while we work to gain an overall national picture of the extent of the disease, and the likelihood that it will spread. Once we have completed that assessment, we will develop a Chalara control strategy.
On 29 October 2012, following the publication of a Pest Risk Analysis and a consultation with the industry and affected parties, the UK Government passed emergency legislation restricting imports into and movements within Great Britain of imported ash plants, seeds and trees in a bid to prevent any more accidental introductions into and spread within Britain of the disease. Details of this legislation are available in this Questions and Answers document.
Northern Ireland and the Republic of Ireland have intoduced similar legislation.
3. What are the symptoms?
4. What should I do if I think my ash trees have the disease?
If you think you have spotted the disease, please check our symptoms video and pictorial guide before reporting it using our Tree Alert form.
5. How much of a threat is it to Britain’s ash trees?
It is potentially a very serious threat. It has caused widespread damage to ash populations in continental Europe, including estimated losses of between 60 and 90 per cent of Denmark’s ash trees. We have no reason to believe that the consequences of its entering the natural environment in Britain would be any less serious. Experience on the Continent indicates that it kills young ash trees very quickly, while older trees tend to resist it for some time until prolonged exposure causes them to succumb as well.
6. How is it spread?
Local spread, up to some tens of miles, may be via 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.
7. How did it get into Britain?
The first interception of diseased ash plants found in a Buckinghamshire nursery had entered Britain in a shipment of plants for planting from a supplier in the Netherlands, who had obtained them from a nursery in Belgium. Many of the other interceptions of infected plants had come from suppliers in mainland Europe. The discovery in October and November 2012 of infected trees in established woodlands near the south-east coast of England raises the possibility that a natural introduction of the fungus might have occurred, such as spores borne by the wind from mainland Europe across the North Sea and English Channel.
8. What other countries have Chalara fraxinea?
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.
9. How were diseased ash plants allowed to enter Britain? What regulatory protection measures were in place to stop it coming in?
C. fraxinea is not a “regulated” plant disease in European Union plant health law, which means that ash plants moved between Member States are not subject to inspection. EU legislation allows Member States to take national measures to prevent the entry and spread of pests and diseases not found on their territory, and the UK introduced such legislation for Great Britain on 29 October 2012. Northern Ireland and the Republic of Ireland have similar legislation.
10. What are you doing to deal with the current known introductions?
Fera and Scottish Government inspectors have been following up plants involved with the different interceptions, requiring destruction of associated plants. A multi-agency, cross-border Outbreak Management Team has been formed, including representatives from all five countries in the British Isles. Forestry Commission staff have been redeployed from usual duties to survey the British countryside for signs of the disease, and a strategy to deal with it is being developed as research information and information about its extent is obtained and analysed.
11. Will you be able to eradicate it?
Where the disease is established it will be impossible to eradicate, but we are giving ourselves the best prospects by responding promptly to findings. We need to determine the extent to which the organism is present and whether it is established, which is why we encourage all those with an interest in trees and woodland to work with us to report any suspected findings.
12. Why did FC/Fera not act before now?
This has been an evolving situation. The organism which was at one time thought to be causing this disease has been present in Great Britain since the 1800s and is already widespread, so legislative action against it would not have been appropriate. But with better scientific techniques we now know that a different organism is responsible. The origins of this organism are not known.
13. Why is this organism not regulated at EU level?
The disease is already established in much of eastern and northern Europe, so action across the EU is not realistic. However, parts of the UK which remain free of the disease can be considerd for “protected zone” status, which would introduce requirements for ash plants being moved into the UK to come from a designated "pest-free area" for C. fraxinea. This could be the next step after having introduced national legislation on this issue. No such pest-free areas have yet been designated in any country.
14. Why can't we grow our own ash trees here instead of importing them?
We can and do grow our own trees, and people have the option to specify British-grown trees and plants if they wish. We strongly advise tree and plant buyers to be very careful to specify healthy stock from reputable suppliers, to practise good plant hygiene and biosecurity in their own gardens and woodlands etc to prevent accidental spread of plant diseases, and to report any plant diseases. Buyers should also be aware that seed gathered from British trees is sometimes sent to nurseries in continental Europe to be cultivated before being reimported as seedlings.
15. I own or manage ash trees - how can I help?
There are several things you can do to help us get this disease under control.
a. Be vigilant – Chalara dieback could appear in ash trees anywhere in Britain. Early action is essential if we are to eradicate this disease from Britain before it becomes established. We therefore urge you to inspect frequently any ash trees in your care, and especially any which have been planted during the past five or so years. Make yourself familiar with the symptoms of Chalara dieback from the materials here. There are other causes of ash dieback, so it is important to distinguish them from Chalara dieback. However, if in doubt, report it.
b. Report it - Report suspicious symptoms to us or Fera - see Question 3 for details of where to report them.
c. Buy with care – Be careful when buying plants to buy only from reputable suppliers, and specify disease-free stock. A list of countries where C. fraxinea is known to be present is at Question 7.
d. Be diligent - Practise good plant hygiene and biosecurity in your own gardens and woodlands etc to prevent accidental spread of plant diseases. See our biosecurity advice for guidance on basic hygiene and biosecurity measures which you can take.
e. Keep up to date – Check our website regularly for updates on developments. ‘Follow’ our Tree Pest News account on Twitter at www.twitter.com/treepestnews to receive rapid intelligence of new developments, delivered by text or email.
Information about a wide range of other tree pests and diseases can be accessed via our Tree pests and diseases page.
16. I have a woodland planting grant or felling-licence agreement with the Forestry Commission to plant ash trees this season. If I do not wish to take the risk of losing the ash trees to Chalara dieback, may I plant another species instead?
Now that movements of ash plants are prohibited, it is not possible to plant ash seedlings which are not already on the site. We are operating a flexible approach for those customers with existing grant or licence agreements which specify ash as a planting species, but it is essential that owners discuss the situation with their local Forestry Commission woodland officer before planting alternatives.
Further information about felling licences and how to obtain them is available on the following pages:
17. What species can I plant instead?
Species choice should be guided by management objectives and site conditions, and the decision tool Ecological Site Classification ESC3 is the key tool to help review options which are likely to be sustainable in the future climate.
Detailed guidance on species choice in native broadleaved woodland can be found in Harmer, R., Kerr, G. and Thompson, R. 2010 Managing Native Broadleaved Woodland, from The Stationery Office, Edinburgh.
There is a wide range of alternatives species for sites with brown-earth soils, including aspen, beech, birch, field maple, hornbeam, oak, lime, rowan, sweet chestnut and sycamore.
The species range is more restricted for calcareous soils, particularly shallow ones, and includes beech, birch, field maple, hawthorn, holly, lime, rowan, whitebeam and yew.
Alder, aspen, willows and oaks are possible alternatives on moist to wet soils.
On sites where there are few restraints, non-native species can also be considered, and guidance can be found in the tree species information on the Forest Research website and links therein.
Some of the alternative species to ash, such as beech, sycamore and Norway maple, are particularly susceptible to bark stripping by grey squirrels.
There is a wider range of species to choose from for the urban environment, and the Right Tree for a Changing Climate website provides information on more than 300 species.
18. What advice do you have for the trade?
Be careful about the sourcing of, and the specification for, your plants. (See question 7 for countries where C. fraxinea is present.) Keep good records of any imported stock, remain vigilant, inspect any recent plantings of ash, and report any suspicious signs to Fera or the Forestry Commission – see Question 3.
19. What advice do you have for the public?
We welcome reports of ash with Chalara dieback symptoms. We do ask that you take care first to ensure that the infected tree really is an ash, because they can look very similar to rowan trees (Sorbus aucuparia), which do not get the disease. (To add to the confusion, rowan trees are sometimes called mountain ash.)
Please also take care to ensure that the symptoms you report are Chalara dieback symptoms, and not the symptoms of some other, less-serious form of dieback or disease of ash tree. You can familiarise yourself with the symptoms with our guide, symptoms pdf and this video.
You should also follow the ‘biosecurity’ advice on any signs at affected sites, to avoid accidentally spreading the disease on your boots, clothes, bicycle wheels etc.
20. What does a Plant Health Notice involve?
Owners of any recently planted ash plants which are found to be infected, or infected ash plants in nurseries or garden centres, can be served with statutory Plant Health Notices requiring them to destroy the plants, either by burning or deep burial on site, or to take steps to contain the disease on site.
Owners of infected ash plants plants in a new-planting site are advised to voluntarily remove and dispose of the plants to prevent the disease spreading regardless of whether they express symptoms of the disease. This is because experience with other plant diseases shows that we must presume that asymptomatic plants in close proximity to symptomatic plants are almost certainly infected, but are not yet showing symptoms.
In an established woodland or similar site, the Plant Health Notice will require movement restrictions and biosecurity measures to prevent the disease being spread from the site while we consider our disease control strategy.
21. Is there any compensation available for people who have to destroy ash plants under a Plant Health Notice?
Unfortunately we are unable to offer compensation for plants destroyed to comply with a Plant Health Notice. It is felt that the available resources are best used for surveillance, research and eradication work. Plants are therefore purchased at buyers’ risk, and any questions about recompense would be between the customer and supplier of the plants involved.
22. Can the timber from infected ash trees still be used?
The implications for growers of ash for the timber trade would be significant if the disease were to become established in Britain. The timber in infected trees might still be usable for some purposes, although staining by the fungus might limit the range of end uses.
23. How many ash trees are there in Britain?
Common ash (Fraxinus excelsior) is the third most common native broadleaved tree species in Great Britain after oak and birch. The National Forest Inventory interim report 'Preliminary estimates of quantities of broadleaved species in British woodlands', published in December 2012, estimates that ash trees in woodlands greater than 0.5 hectares (1.25 acres) cover about 142 thousand hectares in Great Britain. It also estimates there are approximately 126 million live ash trees in woods greater than half a hectare. The report is available in the National Forest Inventory pages of this website.
In addition, the complementary Countryside Survey Report estimates there are 38,500 hectares of ash trees in woodland smaller than 0.5 hectares, and that there are approximately 2.2 million individual ash trees outside woodland.
24. What is the distribution of ash trees?
Common ash is a deciduous, broadleaf species native to much of continental Europe and the British Isles, and a map of its European distribution is available on the pest alert.
This map of ash distribution shows its distribution in Great Britain, and indicates those managed by the Forestry Commission and those belonging to other owners. (Note that this map does NOT show where Chalara dieback has been found.)
25. How important are ash trees in Britain? What are their benefits?
Ash is a common component of many native woods and makes an important contribution to biodiversity and wildlife habitat. It is popular for landscaping urban facilities such as car parks. It is grown commercially for its dense, strong but elastic, easily worked hardwood, which was traditionally and commonly used for making tool handles and furniture. Usage has declined in these markets due to the advent of other materials, but the good-quality timber is still sought after for flooring and high-end, bespoke uses. It also makes excellent firewood, smoking wood and barbecue charcoal.
26. Where can I find more information?
There is further information about Chalara fraxinea on the EPPO website