What is Ash Dieback?

Ash dieback (also kown as Chalara Dieback) is a serious disease of ash trees caused by a fungus called Hymenoscyphus fraxineus. The fungus progressively damages the vascular tissue of trees resulting in dieback, often at the crown. The fungus causes dead or dying branches to fall and can eventually kill a tree, or allow other pathogens e.g. honey fungus to accelerate wood decay and lead to its eventual death. Ash dieback is the most significant tree disease to affect broadleaved trees in the UK since Dutch elm disease gained prominence in the 1960s. Based on data from European infections, up to 85% of UK ash trees outside woodlands are expected to die from it. It is inevitable that all ash trees will be exposed to it (including those near roads or public rights of way); experience from the continent shows that we can expect 70-95% of ash trees to die over the next two or three decades. Some locations will experience this loss more rapidly. There is no cure for ash dieback and it is not possible to prevent its spread.

What are the implications of the disease?

Dead and diseased ash trees can pose real risks to human safety, in addition to considerable economic and environmental impacts. Ash dieback will not be ‘business as usual’ for tree and landscape managers in Scotland. Scottish Forestry believes that we are now at a level of infection that means land managers must take action to identify and monitor infected trees and to manage them accordingly. Estimates are that the cost of managing the safety aspects alone of the disease across Scotland could cost land managers collectively anything from £4 million to £20 million each year as the disease progresses.

Costs of the problem

The national cost of managing trees with ash dieback (which could include monitoring, pruning and, where necessary, felling) is difficult to calculate but it has been estimated that across the UK, and leaving aside the notional cost of the loss of ecosystem services, the cost is likely to exceed £15 billion across the UK in the next 10 years. As part of that, Scotland’s costs could, potentially, run to £180 million.

Priority

Roadside Ash trees growing close to public roads are a priority for monitoring as tree condition (the hazard) and location (the risk) need to be considered together when assessing the danger to road users. In Scotland, Roads Authorities have responsibility for managing and maintaining public roads, including road verges, as outlined in the Roads (Scotland) Act 1984. The duty of care outlined in the Occupiers Liability (Scotland) Act 1960 is particularly relevant to those who own or manage land adjacent to public roads.