Managing Ash Dieback Case Studies
Ash dieback (formally known as Chalara) is caused by the fungal pathogen Hymenoscyphus fraxineus and was first identified on ash trees in the UK in 2012. It is now thought to have been present at lower levels since 2005/6 or earlier, having arrived both on imported planting stock and via spore masses blown across the Channel from mainland Europe.
This document draws together current information to provide examples of interventions some landowners/managers are using to tackle ash dieback and the issues they are encountering along the way. All those featured in the case studies are happy to be contacted for further information.
Ash dieback is present in all counties of England and in large areas of Wales. Experience in mainland Europe suggests that the majority of ash trees in woodland infected with the disease could decline and die over the next 10–15 years. The current population of the pathogen in Europe consists of two genetically divergent strains and the introduction of further strains could have significant consequences for the level of virulence. Therefore, a Plant Health Order continues to prohibit all imports and internal movement of ash seeds and planting stock.
The total cost of ash dieback to the UK is estimated to be £15 billion, half of which will be incurred over the next ten years (report published in Current Biology by University of Oxford). Almost one third of this cost arises from safety felling of dead and dying trees and almost two thirds from loss of benefits provided by trees, e.g. water and air purification and carbon sequestration. The remaining few percent is composed of research and replanting costs as well as a loss of profits to the forestry and nursery sectors predicted to be £78 million and £2 million respectively.
Symptoms of ash dieback include leaf loss, crown dieback and bark lesions in affected trees. Spores from the fungus colonise ash foliage which eventually falls to the ground where the fungus overwinters in the central leaf stalk (rachis) before fruiting and releasing more spores in the spring. The fungus often goes on to colonise branches and can infect the lower stem through lenticels – raised pores in the bark that allows gas exchange between the atmosphere and the internal tissues. These symptoms can cause mortality directly or stress the tree to the point where it succumbs to secondary pathogens. Honey fungus (Armillaria spp.) in particular can colonise weakened trees resulting in root or lower stem rot. Heavily affected trees can become brittle and unstable making them dangerous to climb or to fell by chainsaw or manually, and present a potential hazard particularly along road sides and in areas of public access.
Away from woodland and especially in urban settings, low spore levels and a lack of secondary pathogens may mean trees are barely affected. In addition, research suggests that some ash trees (1-5 %) will have genetic tolerance to the fungus. Clones taken from these trees are being planted as an archive to provide material for a breeding programme to generate stocks of resilient native ash to plant in the future.