European ash (Fraxinus excelsior) dieback - A conservation biology challenge

Common ash (Fraxinus excelsior) is a keystone tree species throughout temperate Europe whose future existence is threatened by an emerging invasive fungal disease. Ash dieback, which first appeared in Poland in the 1990s, has rapidly spread to most eastern, central and northern European countries. The causal agent of the disease, the ascomycete Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea), was recently described as a new species. Given that the disease lethally affects ash trees of all age classes, and that ash tree mortality levels are high, F. excelsior and the many organisms dependent on ash trees are under threat. Based on a literature survey, we provide an overview of the present knowledge on ash dieback, identify practical recommendations and point out research needs. The observation of relatively resistant individual ash trees (although at very low frequency) calls for a rapid germplasm collection effort to establish a breeding program for resistance or tolerance to the disease. Ash trees that appear to be tolerant to the pathogen should not be felled, unless they pose an unacceptable risk to people's security. Given that the pathogen does not form propagules on wood, and given the importance of deadwood for biodiversity conservation, dead and dying ash trees should be left in the forest. Landscape pathology and genetic tools can be used to reconstruct the dispersal pathways of H. pseudoalbidus and to identify environmental features associated with variation in disease severity, so as to better predict the further development of the epidemic. Observations on differences in susceptibility of various ash species are needed to locate the geographic origin of the pathogen and to identify Fraxinus species which might be used for resistance breeding or even replacement of F. excelsior. Conservation biologists, landscape managers, restoration ecologists, social scientists and tree geneticists need to engage with forest pathologists and the various stakeholders throughout the distributional range of F. excelsior so as to tackle this pressing conservation challenge. (c) 2012 Elsevier Ltd. All rights reserved.