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News Article

The origins of Ash dieback in Europe

New genetic study has revealed the low diversity in original UK ash dieback introduction

England’s first recorded observance of Ash Dieback disease was 2012, 20 years after the first European report of the disease in Poland, likely thought to be an import from Asia. The current toll for the impacts of the disease in Europe has been heavy across the continent with over 80% of young ash trees in Norway killed by the disease and a quarter of ash trees in southern Sweden damaged or dead as a result. In total, the disease threatens 95% of European ash trees and a new study on the original genetic diversity of the first agents to come to the UK has suggested it could get even worse for Britain’s trees.

The causative agent of the disease is a fungus, Hymenoscyphus fraxineus, which produces tiny fruiting bodies that release thousands of windborne spores. The disease progression is as follows: dark brown/orange lesions appear on the leaves followed by wilting, necrotic lesions on the shoots and then diamond shape lesions on the stem. The crown of the tree then dies back with the plant either succumbing to the disease itself or another able to take advantage of its weakened state.

Research published in Nature Ecology & Evolution has traced the origin of the first spores of ash dieback to reach the UK isles and discovered that they were likely 2 haploid individuals from Asia. The divergence between the two original founders has implied a larger more diverse source population, creating worry that even just one more introduction from this source could wipe out even the currently resistant trees. Authors were surprised at the level of genetic diversity being so small across UK, Norway, France, Poland and Austria in proportion to the damage it has created and warn of the consequences should any more diversity be introduced. With only <5% of Fraxinus excelsior resistant to the pathogen in its current form, any increase in its diversity could mean even this small fraction become susceptible. Given that there is 8 times more genetic diversity in the fungus when found in a single Japanese woodland than across Europe this would seem to be a very real possibility.

Because trees have such a long generation time, populations are relying on what genetic resistance is already present in order to survive. Whilst the disease is relatively harmless to Asian ash species, the overwhelming majority of UK trees are highly susceptible. Breeding trials have already taken place in the UK and Denmark and DEFRA is now funding larger screening trials to develop a stockpile of resistant trees destined to replace those lost. However this is a slow process and currently, preventing the spread of the disease, through restriction of trade and movement, is all that can be done to help the remaining populations until resistance is successfully bred. Unfortunetly there as yet currently no confirmed pest free areas in the UK.

The results of global traffic and climate change mean the incidence of foreign pests and diseases in the UK will only increase. Ash dieback is one of many new diseases and pests reaching European shores, with the Emerald ash border beetle putting 5 ash species in the US on the IUCN red list. The use of phytosanitary certificates and border checks for the import of material that could carry fungi and insects is already relied upon heavily to protect our native flora. Now, the use of genetics to understand the origin and diversity of emerging diseases represents a new and important approach to combating these threats.

The reference for the original article can be found below:

M. McMullan, M. Rafiqi, G. Kaithakottil, B. J. Clavijo, L. Bilham, E. Orton, L. Percival-Alwyn, B. J. Ward, A. Edwards, D. G. O. Saunders, G. G. Accinelli, J. Wright, W. Verweij, G. Koutsovoulos, K. Yoshida, T. Hosoya, L. Williamson, P. Jennings, R. Ioos, C. Husson, A. M. Hietala, A. Vivian-Smith, H. Solheim, D. MaClean, C. Fosker, N. Hall, J. K. M. Brown, D. Swarbreck, M. Blaxter, J. A. Downie and M. D. Clark (2018) The ash dieback invasion of Europe was founded by two individuals from a native population with huge adaptive potential. Nature ecology & evolution, (DOI:

To find over 2000 similar articles use the search below in the Forest Science Database: (Ash OR fraxinus OR “fraxnius excelsior”) AND (“Hymenoscyphus fraxineus” OR disease OR resistance OR “ash dieback” OR “chalara disease” OR “chalara dieback”)

Article details

  • Author(s)
  • Ellen Baker
  • Date
  • 30 April 2018
  • Source
  • The Earlham Institute
  • Subject(s)
  • Environment