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