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Consultation launched on risk assessment for Chalara dieback of ash trees

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Chalara dieback of ash - stem canker on young ash caused by Chalara fraxinea fungus, showing bark splitting

Views are invited on a Rapid Pest Risk Assessment for Chalara dieback of ash, a highly destructive disease of ash trees which has been found this year in nurseries and recently planted trees in Great Britain.

The risk assessment has been drafted by plant disease scientists at the Forestry Commission’s Forest Research agency. It assesses the risks to the United Kingdom’s ash trees, the feasibility and practicality of eradicating the outbreaks which have been found, and measures to prevent more outbreaks occurring.

The Commission and the Food & Environment Research Agency (Fera) are now inviting comments and suggestions on the assessment, and on options for a control strategy. If an eradication strategy is adopted, the risk assessment will be used as evidence to support a case for national legislation to strengthen the UK’s protection from accidental introductions of Chalara fraxinea (C. fraxinea), the pathogen which causes the disease.

Dr John Morgan, Head of the Commission’s Plant Health Service, said,

“It is important that everyone involved in the forestry, nursery, arboricultural and related sectors understands what we are doing to assess the threat which C. fraxinea poses, and to understand the possibilities for action to prevent it getting established here.

“I strongly encourage people to give us their responses to the risk assessment, and I can assure them that all responses received will be considered in any policy decisions which are taken.

“Ash is our third most common native broadleaf tree. It contributes a great deal to our biodiversity and landscape beauty, it is widely planted, and we must do all we can to protect it from this very destructive pathogen.”

The rapid assessment is available to download from the plant pests and diseases consultation pages of the Fera website ( The deadline for submission of comments is 26 October, 2012.

Further information about C. fraxinea is available on the Forestry Commission website at


  1. Chalara dieback of ash trees has been found in four tree nurseries in England and at three sites which were recently planted with ash trees – at a Forestry Commission Scotland woodland west of Glasgow, at a Leicester car park, and in the grounds of a South Yorkshire college. In all cases it is believed the infection arrived with the young ash plants. Diseased ash plants are being destroyed, and other plants sold from the same consignments are being traced to their final planting sites and destroyed. Surveys are being conducted in the surrounding areas to check whether the disease has spread into ash trees in the wider environment. 
  2. Ash trees with disease symptoms now believed to have been caused by C. fraxinea were first reported in Poland in 1992, followed by further reports from many other European countries in subsequent years. The causal agent of this disease is an ascomycete fungus which is characterised by two stages. The asexual stage, C. fraxinea, was first formally 'described' by scientists in 2006 and is not thought to be involved in its reproduction. The sexual stage, Hymenoscyphus pseudoalbidus (H. pseudoalbidus), was only described and confirmed as a new species in 2010. It is the spores from this stage which are thought to be responsible for its reproduction and dispersal.
  3. For a time, scientific evidence indicated that the sexual stage of the fungus was the native European species Hymenoscyphus albidus (H. albidus), and one suggested reason for the outbreak of disease in Europe was that environmental changes might have caused this previously benign organism to become damaging to its host plant, ash.  H. albidus is endemic (widespread) throughout continental Europe and the British Isles and, during the period when it was implicated as the pathogen associated with ash dieback, it did not appear to be feasible to tackle the disease by means of EU or national regulation. The origin of the new species H. pseudoalbidus, which is now recognised as the sexual stage of the pathogen C. fraxinea, is not yet known.
  4. Because C. fraxinea is not currently a “regulated” plant pathogen in European Union plant health law, the trade in ash plants between Member States is not subject to plant health controls. Member States may, however, consider a case for legislation to control ash imports from areas where the disease is known to be present. In order to do this, Member States must accept that it is not established in certain parts of the EU or, if it is present, that it can be either eradicated or contained. The rapid risk assessment which Forest Research has drafted is the first step towards making a case for legislation to protect the UK from further imports of infected ash trees.
  5. Common ash (Fraxinus excelsior) is a deciduous species native to much of Europe, including the British Isles. It is the third most common native broadleaf tree in Great Britain after oak and birch. Ash timber is a dense, strong but flexible, easily worked hardwood which was traditionally and commonly used for making tool handles and furniture. Usage has declined in these markets because of 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.
  6. Government departments and agencies from across the UK, working with partners in the private and third sectors, have come together to draw up a Tree Health and Planty Biosecurity Action Plan to ensure that the UK's defences against tree and plant pests and diseases are as robust as possible. In addition, the European Union is reviewing its plant health regime to ensure that it is fit for purpose in the 21st-century global trading environment.

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