Twigs and leaves with chalara ash die back, showing the symtoms
Apr 04 2018

Chalara dieback of ash

Due to the current threat from Chalara dieback of ash the UK government has banned the movement of ash plants, trees and seeds within Great Britain until further notice. More information on the ban can be found on the Forestry Commission’s website (link). Given the virulence of Chalara (potentially ≥ 90% mortality) planting ash would not be prudent even if it were available.

Unfortunately no ash trees have yet been found that are totally resistant to Chalara, so widespread sowing would not be effective and could even help to spread the disease in the wider environment (hence the ban). There are a number of trials underway looking to identify resistant trees and researchers have identified a few specimens that appear to exhibit tolerance to some extent (i.e. they don’t display any symptoms of dieback despite being surrounded by other diseased trees); however, this could be misleading (they may simply have avoided infection by chance) and long-term resistance to the disease cannot yet be confirmed. While disease resistant ash trees may have a role to play in the future, widespread replanting is likely to be some way off.

The Trust’s current policy is to replace ash (both pre-emptively and where felling has taken place) with a diverse mix of other native tree species. The UK’s trees and woods will be more resilient to tree disease if they contain a wider range of species, have good genetic diversity and a more varied structure. For landowners who are keen to retain the ‘look’ of ash, green ash (Fraxinus pennsylvanica) or white ash (Fraxinus americana), which are American species that appear to be resistant to die-back, could be considered – although the Trust’s policy is to use UK natives exclusively.

Neil Ingram – Tree Procurement Coordinator

  • AlanSlack

    Are we still collecting samples in new squares to send to forestry research for recording?

    April 14, 2018 at 8:17 pm
    • CharlotteArmitage

      Hi Alan,
      I have just sent out the Observatree monthly update which hopefully should answer your query, if not please send me an email to and I will try to be of further help.

      April 18, 2018 at 9:11 am
  • ChrisStephens

    Good the have Neil’s update . many thanks

    April 16, 2018 at 5:46 pm
    • Sarah Shaw

      Thanks Chris, we’ll get regular updates from Neil throughout the tree procurement season.

      April 17, 2018 at 3:53 pm
  • DavidTaylor-Gooby

    If we find diseased. trees. Can isolated?

    April 23, 2018 at 3:04 pm
    • Sarah Shaw

      Hi David, good question. The mechanism of transmission of Chalara is not fully understood but it is believed to have blown over from the continent so unfortunately it would be almost impossible to isolate affected trees, there is also the chance that some trees will demonstrate a genetic resistance. This is why projects such as Observatree are so important in helping us tackle and learn more about tree disease.

      April 23, 2018 at 4:49 pm

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