The epidemiology of the ash dieback pathogen, Hymenoscyphus fraxineus, in Northern Ireland

  • Emma Baxter

Student thesis: Doctoral ThesisDoctor of Philosophy


Ash dieback, caused by the fungal pathogen Hymenoscyphus fraxineus, was first identified in Northern Ireland in November 2012. This study, started in autumn 2013, investigated the behaviour of H. fraxineus in Northern Ireland.

H. fraxineus isolates and DNA samples were tested alongside European samples to compare their phenotypic (colony growth and morphology) and genotypic characteristics. The Northern Irish H. fraxineus population could not be distinguished from the European populations. H. fraxineus isolates from both populations showed a wide range of morphological characteristics, no one characteristic linked to a single location.

H. fraxineus isolates from N. Ireland and Europe were inoculated into ash trees obtained from Co. Armagh, N. Ireland to determine if there were any differences in aggressiveness between the two populations. Lesion length was measured as an indicator of aggressiveness. There were significant differences in lesion length between individual H. fraxineus isolates, but the diversity in the N. Irish H. fraxineus population mirrored that of the European population.

Urea was investigated to determine if it aided in the decomposition of ash leaves and thus might have potential as a control measure for ash dieback. Urea applications did not enhance the degradation of ash leaves; the rachises (on which the apothecia of H. fraxineus are produced) remained largely intact. It was therefore concluded that urea was unlikely to be effective in reducing ash dieback.

In 2015 a site recently planted (2006) with ash in Co. Antrim, N. Ireland was found through routine plant health inspections to be infected with H. fraxineus. This site was used for an in-depth case study. Previously published work had suggested that apothecia could only form on fallen, infected ash rachises; raising the question of the initial source of infection of N. Ireland ash trees and inoculum build up if ash was imported as leafless whips. A unique finding from the site was that apothecia were observed on ash stems, branches and roots (in addition to the leaf rachises). This could explain the environmental spread of ash dieback into N. Ireland. A disease timeline was constructed for the site, showing that there was a disease lag time between two and three years from planting to symptom development.
Date of AwardDec 2021
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsDepartment of Agriculture, Environment and Rural Affairs
SupervisorIrene R. Grant (Supervisor) & Louise Cooke (Supervisor)


  • Ash
  • ash dieback
  • plant pathology
  • forest pathology
  • hymenoscyphus fraxineus
  • H. fraxineus
  • hymenoscyphus
  • fraxinus excelsior
  • Northern Ireland
  • forestry
  • epidemiology

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