The viability of a breeding programme for ash in the British Isles in the face of ash dieback

Abstract

Societal Impact StatementThe current ash dieback epidemic in Europe caused by Hymenoscyphus fraxineus poses a key question to policy makers: whether or not to commit time and resources to the initiation of a breeding programme for the development of more resistant ash, as a long‐term policy of adaptation to the epidemic. Here we review current evidence on the potential viability of such a programme, from a biological perspective. We conclude that a breeding programme for ash aimed at resistance to current strains of H. fraxineus in the British Isles is biologically feasible.SummaryTo evaluate the viability and feasibility of a future breeding programme to produce trees resistant to an emerging pest or pathogen, it is helpful to ask the following questions: How much variation in resistance exists in tree populations? To what extent is this resistance heritable? How many genetic loci are involved? What level of resistance is found in other species of the same genus? Here, we survey current knowledge of these issues in relation to the degree of resistance of European ash (Fraxinus excelsior) to H. fraxineus, the fungus causing ash dieback (ADB). Several studies have found a low frequency of heritable resistance in F. excelsior populations, which seems to be determined by many genetic loci. This suggests that a breeding programme is viable and that natural selection may also increase the mean resistance of populations over time. More research is needed on the genetic basis of resistance to ADB to understand how quickly natural selection can operate in woodlands and what acceleration may be possible in breeding programmes, including via use of genetic markers. Hybrid breeding programmes may also be a possibility, as some ash species appear to be more resistant to ADB than is F. excelsior, but more research is needed on this issue. We do not yet know if it will be possible to breed F. excelsior to have resistance to both ADB and the emerging threat of emerald ash borer. We recommend short‐term mitigation measures for the ADB epidemic and future research directions.