Description
Outbreak stage
Distribution
Symptoms
Managing infected trees
Reporting suspect cases
The Disease
Origins
Import & movement restrictions
Further information
Latest:
New video on spotting symptoms in the spring.
First case in the wider environment confirmed in Wales.
Description
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.
Outbreak stage
Ash trees suffering with the infection have been found widely across Europe since trees 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.
In February 2012 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 do 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 south-eastern region 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. (Map below)
C. fraxinea is now being treated as a quarantine pest under national emergency measures and any suspected sighting should be reported.
Hundreds of staff from government agencies checked ash trees across the UK for signs of the disease during early November 2012. It was one of several actions to emerge from a meeting of the Government’s emergency committee, COBR, chaired by Environment Secretary Owen Paterson.
Plant health experts are also undertaking a survey of about a thousand sites which have received saplings from nurseries where Chalara dieback has been found.
Distribution
Confirmed findings at 20 May 2013:
Nursery sites - 23
Recently planted sites - 295
Wider environment, e.g. established woodland - 183
Total: 501
Symptoms
Video above: identifying symptoms in the spring.
Video: Year round symptoms
Symptoms picture guide
Exotic pest alert which gives more information about the disease
Video - main symptoms (FERA)
Because ash trees have many genetic variants and occur all across the UK, they come to leaf at different times. In general, they come into leaf later in spring than many other trees, often as late as the end of May. 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 of ash are best observed in August and September because in autumn infected leaves can be confused with leaves that are naturally changing colour.
Ash trees are starting to break-bud and will produce new growth some time in May, and we expect the vast majority to develop normally.
Some ash trees will break-bud, or flush, earlier than others, and some buds will produce flowers rather than new shoots.
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.
However, there will be some cases where ash will show symptoms of Chalara dieback. Along with our partners in Fera and Natural Resources Wales, we will continue to report on the condition of ash trees across Britain and plot the results on the map on this page. 
• ash is traditionally one of the last tree species to flush, sometimes taking as long as six weeks to do so
• variation will be more apparent in older trees
• trees in the colder north flush later than trees in the warmer south.
Managing infected trees
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. 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, collecting up and burning, burying or composting the fallen leaves.
Reporting suspected cases
If you think you have spotted the disease, please check:
Symptoms video
Symptoms guide
Guide to recognising ash trees
before using our Tree Alert form
Or our free App.
We are very grateful for the many reports we have received. We are working through these, and are sorry that we might are not able to respond to each one 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.
The disease does not spread via spores from the fungus during the winter, so we have the time to carefully examine all the reports.
The disease
Government scientists have set out the most up-to-date understanding of the disease. Their assessment agreed with the earlier Pest Risk Analysis carried out in August, and concluded that:
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the spores are unlikely to survive for more than a few days
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spore dispersal on the wind is possible from mainland Europe
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trees need a high dose of spores to become infected
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spores are produced from infected dead leaves during June to September
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there is a low probability of dispersal on clothing or animals and birds
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the disease will attack any species of ash
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the disease becomes obvious within months rather than years
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wood products would not spread the disease if treated properly
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once infected, trees can’t be cured
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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.
Consortium Q&A - science of chalara
Origins
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, and it spread rapidly to other European countries. However, it was 2006 before the fungus’s asexual stage, C. fraxinea, was first “described” by scientists, and 2010 before its sexual stage, Hymenoscyphus pseudo-albidus, was described. 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.
Video: history of the pathogen.
Pest risk assessment and consultation
A Pest Risk Assessment (PRA) on C. fraxinea was published, and a consultationon its management held in autumn 2012.
Import and movement restrictions
To prevent further spread of the disease in Britain a Plant Health Order prohibits all imports of ash seeds, plants and trees, and all internal movement of ash seeds, plants and trees.
How the legislation applies to timber and firewood
New requirements for statutory notification of imports of Ash, sweet chestnut, plane and oak.
Further information
Contact:
England and Wales
Chalara helpline: 08459 33 55 77 (8am - 6pm daily) or plant.health@forestry.gsi.gov.uk
Scotland
Forestry Commission Scotland: 0131 314 6156 (9am - 5pm weekdays + out-of-hours messaging system) or
fcscotlandenquiries@forestry.gsi.gov.uk
