Chalara Dieback of Ash (Hymenoscyphus fraxineus)

Background to Chalara Dieback of Ash Disease in Britain

Chalara dieback of ash (Hymenoscyphus fraxineus) (formerly known as Chalara fraxinea) is one of the most significant fungal pathogens to be introduced to Britain in recent years. It is a vascular wilt fungus of ash trees (Fraxinus spp.) and is thought to have been introduced to Europe in 1992, when the first cases were found in Poland. Since that time, the fungus has spread over a wide area, and has had a devastating impact on ash populations in several countries; Denmark, for example, has seen a very high proportion of its ash infected within just three years of the arrival of the disease.

In October 2012, The UK Government announced that cases of Chalara dieback of ash had been found in the UK. Plant health monitoring and surveys have been initiated and throughout the winter of 2012-2013 the number of confirmed reports of infection has increased at a steady rate. Three categories of infection are recognised:

  1. Tree Nurseries – nurseries where seedlings have been imported from growers in Europe
  2. Recently Planted Sites – seedlings planted out from infected nurseries
  3. Wider Environment – established woodlands

New and recent plantations of ash are currently being checked where seedlings were procured from infected nurseries. This is known as “trace forwarding”. Most of the wider environment locations where the disease is present are in the south-east of England (especially Kent, Suffolk and Norfolk).

Ongoing surveys have now confirmed the presence of ash dieback disease in nearly 1000 sites.

Distribution (2 March 2015):

  • Nursery sites – 26
  • Recently planted sites – 404
  • Wider environment (established woodland) – 617
  • Total – 1047

AshStat – Chalara dieback of ash disease statistics tracking since the start of the current outbreak in Britain. Link to AshStat page.

Identification of Chalara Dieback of Ash

Ash trees occur all across the UK and have many genetic variants. They leaf out at different times in the spring, but as a general rule they tend to flush later than most other tree species. Most ash trees will be in leaf by late May; all healthy ash trees should be in full leaf by mid-June. The leaf symptoms of Chalara dieback of ash are best observed in August and September. Later in the growing season it becomes more difficult to assess tree health because infected leaves can be confused with leaves that are naturally changing colour in the autumn.

The video below, produced by the Forestry Commission, provides a guide to symptoms of Chalara in spring. Other Forestry Commission videos can be found on the Forestry Commission YouTube Channel.

This second video, produced by the Tree Council, provides another look at the symptoms of ash dieback disease.

Science and Silviculture

The UK Government has commissioned several projects and reviews pertaining to ash dieback disease. Initial papers and information can be accessed here. The latest report (Mitchell et al 2014) on the potential impact of ash dieback disease in the UK can be found accessed here. Work has also been undertaken on approaches to monitoring the long-term effects of ash dieback disease on biodiversity (Hinsley and Pocock 2014).

There is currently limited advice on how best to manage stands of ash that are infected with ash dieback disease. A perspective from northern Europe is provided by Thomsen and Skovsgaard (2012), who suggest a range of alternative silvicultural strategies for managing infected stands. The relevant strategy depends on stand age and the degree of dieback. Generally, the strategy should be conservative, if the dieback is less severe. An operational approach would be to identify and mark healthy trees. In case of severe dieback, the suggested approach is to harvest remaining timber as soon as possible and replant the area.

A report by Rick Worrell (2013) outlines the potential impacts of ash dieback in Scotland. This includes useful facts and information on the ecology and management of ash.

Several organisations are now actively engaged in research on ash dieback and ash tree genetics. Fast track funding has been awarded to a new consortium of researchers for studying ash dieback fungus. The consortium is supported by funding of £2.4M through the Biotechnology and Biological Sciences Research Council (BBSRC). OpenAshDieBack is a newly created hub for crowdsourcing information and genomic resources for ash dieback.

Among the key organisations leading on ash dieback research, and the genetics and silviculture of ash trees, are Forest Research, the Earth Trust and the Future Trees Trust.


National Control Strategies and Management Plans

Chalara control strategies have now been implemented for all jurisdictions in Britain and Ireland. These can be accessed here:

Work on monitoring and and impacts of ash dieback on biodiversity has been undertaken by the JNCC. Their report can be accessed here.

Ash pollard in Cumbria

Ash is the most common tree outside woodlands in Cumbria. It can be found in fields, along road-sides, in towns and on the fells. The county is especially rich in ancient ash pollards, which are important for their ecological and landscape values. Photo: Edward Wilson 2012

Silviculture Research International and Ash Dieback Disease

We are actively involved in public engagement and policy development in relation to the recent outbreak of ash dieback disease (Chalara fraxinea) in Britain. This fungal pathogen has already had a devastating impact on populations of ash (Fraxinus excelsior) in many parts of Europe. Our primary focus is to promote research and strategies aimed at reducing/minimising the potential impact of the infection in Cumbria, which is the location of many of the most important and ancient ash populations in Britain. However, we are also active at the wider regional and national levels.

Included on this page is background information about the disease, infection statistics (updated regularly) and links to other organisations.

Call for Action

We were the first to publicly call for action to identify key populations of ash and initiate strategies for disease control. Our calls appeared in the Guardian, Daily Telegraph, Sunday Times and in several interviews on BBC and ITV news programmes. We have also made contributions at a number of policy fora, including the Forestry Think Tank, chaired by Rory Stewart MP.

We have given several presentations on Ash Dieback Disease, to both professional and non-professional audiences. We are pleased to provide a range of technical and scientific services, in addition to presentations and public talks on ash dieback disease. Details of how to get in touch are on the Contact page.

The following resources are available for download:

1. “Ancient Ash Trees and their Future in Cumbria”. Presentation notes. [download pdf] pdficon_small

2. “Ash trees and their future in Britain”. Public Talk.

Link to SlideShare: Ash trees and their future in Britain by Edward Wilson

Citizen Science – The Ancient Ash Trees in Eden Project

We have launched a pioneering new citizen science and public engagement project on ash trees and their conservation in Eden District, Cumbria. Project partners include the Heart of Eden Development Trust, OPAL Tree Health Survey and Silviculture Research International. Financial support is being provided by the Heritage Lottery Fund.

There will be a series of workshops on ash trees and tree health issues, followed by a publication aimed at increasing awareness of the importance of ash trees in our landscape. Resources will be made available and archived online to help sustain interest in ash trees and tree health issues within the local community. Further information can be found on the Ancient Ash Trees in Eden Project webpage.

Useful Links

For more information on the ash dieback disease, please see the following:

  • Forestry Commission – ash dieback disease – link here
  • Defra – ash dieback disease – link here
  • JNCC Resources on ash dieback disease – link here
  • Food and Environment Research Agency (fera) – ash dieback disease – link here
  • Forest Service, Department of Agriculture, Food and the Marine, Ireland – ash dieback disease – link here
  • BSBI Ash dieback disease – link here
  • FRAXBACK – Understanding ash dieback disease (Europe) – link here
  • EUFORGEN – European Forest Genetic Resources Programme – link here
  • AshTag – phone app – link here
  • Future Trees Trust – link here
  • Scottish Government Ash Dieback video – link here

News Links

  • Ash dieback disease – Guardianlink here
  • Ash dieback disease – BBC – link here
  • Ash dieback disease – Countryfile Magazine – link here

Links to Video Presentations

  • A brief history of ash dieback disease in Britain (fera) – link here (YouTube)
  • Ash dieback (Chalara fraxinea) – Life Cycle and Symptoms (fera) – link here (YouTube)
  • Identifying Chalara fraxinea in the winter – link here (YouTube)
  • Spotting Chalara fraxinea in the field – link here (YouTube)

Field Guides and Identification

 Page Updated: 8 March 2015

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