Ash dieback, caused by the invasive pathogen Chalara fraxinea T. Kowalski (teleomorph, Hymenoscyphus pseudoalbidus V. Queloz), has emerged and spread across Europe in the last two decades and currently affects most of the natural distribution of Fraxinus excelsior L. (common ash, European ash). This thesis comprises five papers that aim to contribute to the knowledge on the interaction between the pathogen and its host and the inherent natural resistance found within the host species.
Paper I reviews the current literature on the aetiology of the fungus, the epidemic and its consequences and the evidence of genetic variation in susceptibility and inheritance of resistance and discusses the potential for preserving the species through management of genetic resources.
Paper II confirms the invasiveness of H. pseudoalbidus in Denmark by comparing collections of the native H. albidus and H. pseudoalbidus in a fungal herbarium where only samples collected after 2005 were molecularly identified as the invasive species. Furthermore systematic collections of apothecia in forests where H. albidus had been previously confirmed suggest that the species is being outcompeted by the invasive H. pseudoalbidus.
Paper III estimates the inherent resistance in Danish populations of F. excelsior. A strong genetic variation in susceptibility to C. fraxinea was observed among 39 tested clones. The susceptibility was highly genetically correlated with leaf senescence suggesting that the observed resistance could be an effect of disease escape. The results suggest that a small fraction of the natural population may be able to resist the epidemic and proposes prospects for maintenance of the species through selection of highly resistant trees.