Outbreak stage and national plans
Reporting suspect cases
Managing infected trees
Pest risk analysis
Import & movement restrictions
Dark-coloured clumps of keys (seeds) hanging on ash trees in the autumn can be easily mistaken for the blackened leaves which can be a symptom of Chalara dieback disease. See 'Symptoms' below for further guidance.
Chalara dieback of ash is a serious disease of ash trees caused by a fungus called Chalara fraxinea (C. fraxinea), including its sexual stage, Hymenoscyphus pseudoalbidus (H. pseudoalbidus). The disease causes leaf loss and crown dieback in affected trees, and usually leads to tree death.
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 will be any less serious. Experience on the Continent indicates that it can kill young ash trees very quickly, while older trees tend to resist it for some time until prolonged exposure causes them to succumb as well.
Chalara fraxinea is especially destructive of common ash (Fraxinus excelsior), including its ‘Pendula’ ornamental variety. Narrow-leaved ash (Fraxinus angustifolia) is also susceptible. It is particularly destructive of young ash plants, and can kill 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.
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.
Ash trees suffering with the infection have been found widely across Europe since trees now believed to have been infected with this newly identified pathogen were 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.
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. Since then it has been found in young ash trees in a number and variety of locations in Great Britain, including urban landscaping schemes, newly planted woodland, and more nurseries.
In October 2012, Food & Environment Research Agency (FERA) scientists confirmed a small number of cases in Norfolk and Suffolk in ash trees at sites in the wider natural environment, including established woodland, which did not appear to have any association with recently supplied nursery stock. Further finds in trees in the wider environment have since been confirmed in a number of places, mostly on the eastern side of England and Scotland, and mostly concentrated in the east and south-east of England. In May 2013 the first wider-environment case was found in south-west Wales, which is the farthest west site in Britain that a wider-environment case has been confirmed. C. fraxinea is now being treated as a quarantine pest under national emergency measures and any suspected sighting must be reported.
Plant health experts are undertaking a survey of about a thousand sites which have received saplings from nurseries where Chalara dieback has been found.
Confirmed findings at 2 December 2013:
Nursery sites - 24
Recently planted sites - 347
Wider environment, e.g. established woodland - 240
How to identify Chalara dieback of ash (Chalara fraxinea) in the forest. multimedia module
Video 1: identifying Chalara symptoms (hosted by external party)
Pest alert - for more information
Because ash trees have many genetic variants and occur right across the UK, they come into leaf at different times. Ash is also traditionally one of the last tree species to flush, sometimes taking as long as six weeks to do so, often 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.
The leaf symptoms of Chalara dieback are best observed in August and September, because in autumn infected leaves can be confused with leaves that are naturally changing colour.
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.
We are sorry that we are not able to 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.
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. You should, however, 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. You can also help to slow the spread of the disease by, where practicable, removing and disposing of infected ash plants, and collecting up and burning (where permitted), burying or composting the fallen leaves.
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 can’t be cured; and
not all trees die of the infection - some are likely to have genetic resistance.
Scientists are working with their counterparts in other countries to learn from existing and emerging research and practical experience in combating the disease in other countries. They are also approaching companies with proposed treatment solutions for Chalara to rapidly evaluate their research.
Our Forest Research agency is part of a consortium awarded £2.4m research funding to gather an in-depth understanding of the ash dieback fungus and to provide genetic clues about the natural resistance of some ash trees to attack.
Forest Research is also leading a mass screening trial to identify inherent resistance in UK ash trees.
Ash trees were first recorded dying in large numbers from what is now believed to be this newly identified form of ash dieback in Poland in 1992. It is believed to have entered Britain on plants imported from nurseries in continental Europe. However, now that we have found infected older trees in East Anglia, Kent and Essex with no apparent connection with plants supplied by nurseries, we are also investigating the possibility that it might have 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.
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.
Video: history of the pathogen.
A Pest Risk Analysis (PRA) was published in May 2013. (The document title uses the name of the sexual stage of the fungus, Hymenoscyphus pseudoalbidus.)
This followed publication of a Rapid Assessment of the Need for a Detailed Pest Risk Analysis in 2012
A Plant Health Order prohibits all imports of ash seeds, plants and trees, and all internal movement of ash seeds, plants and trees.
New requirements for statutory notification of imports of Ash, sweet chestnut, plane and oak.
England and Wales
Chalara helpline: 08459 33 55 77 (8am - 6pm daily) or email@example.com
Forestry Commission Scotland: 0131 314 6156 (9am - 5pm weekdays and out-of-hours messaging system) or