Factsheet : Chalara dieback of ash (Chalara fraxinea)

Ash Tree
Ash Tree (Photo credit: fracker photographic)

Source: http://www.forestry.gov.uk/chalara

Outbreak stage
Reporting suspect cases
The Disease
Import & movement restrictions
Further information


Action on Chalara dieback

Highlights from the ash tree summit

Survey reveals disease in 6 further counties


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 C. fraxinea 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.

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 a number and variety of locations in Great Britain, including a car park, newly planted woodland and a college campus. All these sites had received stocks of young ash plants from nurseries within the past five years. Further cases have also been confirmed in the nursery trade.

In October 2012, Fera scientists confirmed a small number of cases in East Anglia 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 similar finds were confirmed in Kent, Essex and other counties in early November 2012.

C. fraxinea is being treated as a quarantine pest under national emergency measures, and it is important that suspected cases of the disease are reported.

Hundreds of staff from government agencies have been out checking ash trees across the UK for signs of the disease during early November. It’s one of several actions to emerge from a meeting of the Government’s emergency committee, COBR, which Environment Secretary Owen Paterson chaired on Friay 2 November.

Plant health experts are also undertaking an urgent survey of about a thousand sites which have received saplings (young trees) from nurseries where Chalara dieback has been found. They are also prioritising the examination of about 2500 blocks of land, each 10 kilometres square, where mature ash trees are present, to check for traces of the disease in established trees.


Confirmed findings at 8 November 2012:

Nursery sites – 15
Recently planted sites – 55
Wider environment, e.g. established woodland – 65
Total: 135

Outbreak map.



Large size map


Video: Spotting symptoms in the field (above)

Pictorial guide to recognising the main symptoms

Exotic pest alert which gives more information about the disease.

The Food & Environment Research Agency (Fera) has also produced this video presenting and explaining the main symptoms.

Reporting suspected cases

If you think you have spotted the disease, please check our symptoms video and pictorial guide , and our guide to recognising ash trees, before reporting it to one of the following:

In England and Wales

Chalara helpline: 08459 33 55 77 (open 8am – 6pm every day)  orplant.health@forestry.gsi.gov.uk

In Scotland

Forestry Commission Scotland: 0131 314 6156 (9am – 5pm weekdays +  out-of-hours messaging system) or

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:

  • 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;
  • the spores are produced from infected dead leaves during the months of 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 in trees 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, and some are likely to have genetic resistance.

Government scientists are working with their counterparts in other countries to learn from existing and emerging research and practical experience in combating the disease in countries which have had it for longer than the UK. They are also approaching companies with proposed treatment solutions for Chalara to rapidly evaluate their research to see whether they have potential for further testing and development.

key scientific facts paper has been prepared by the expert group led by the Chief Scientific Adviser, Sir John Beddington.


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 Great Britain on plants for planting 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 Britain by natural means. These include being carried on the wind or on birds coming across the North Sea, or on items such as footwear, clothing or vehicles of people who had been in infected sites in Continental Europe.

VIDEO: Our colleagues in Fera have produced this video outlining the history of the pathogen.

Pest risk assessment and consultation

A Pest Risk Assessment (PRA) on C. fraxinea  was published, and a formal consultationon its management held by Fera in September/October 2012.

Import and movement restrictions

Plant Health Order 2012 (pdf) prohibiting all imports of ash seeds, plants and trees into Great Britain, and all movement of ash seeds, plants and trees within Great Britain, was introduced by the UK Parliament on 30 October 2012 to prevent further spread of the disease. Detailed explanation of the effects of the legislation is available in this Questions and Answers document.

See Effects of new legislation on the timber and firewood trades for detailed advice about how the new legislation applies to these trades.

Further information

For further general information see our Questions and Answers about Chalara dieback of ash.



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