Resistance and tolerance mechanisms of common ash (Fraxinus excelsior) against the ash dieback pathogen Hymenoscyphus fraxineus: latest advances and knowledge gaps

Abstract

The common ash (Fraxinus excelsior) populations in Europe are suffering from ‘ash dieback’ caused by the fungal pathogen Hymenoscyphus fraxineus. The disease was introduced to north-eastern Europe more than three decades ago and has spread throughout the continent since then. Hope that common ash will not completely disappear is provided by findings that a small percentage of ash trees remain healthy despite being exposed to high pathogen pressure and that this reduced disease susceptibility has a heritable, genetic component. Here, we review recent studies related to the mechanisms underlying the resistance and tolerance of common ash to H. fraxineus. Our aim is to provide up-to-date information relevant for ash conservation and breeding programmes, and to identify knowledge gaps where research would provide further insights. To date, the identification of resistant and/or tolerant trees is mostly based on crown health assessments of common ash trees exposed to high infection pressure in forests or in experimental nurseries. Other assays are being developed, and lesion length in artificial stem inoculation experiments seems to correlate well with crown health in the field. The extent to which phenological, physiological, and morphological traits may contribute to resistance and tolerance is still unclear. The availability of genomic resources for common ash has led to the identification of molecular markers putatively associated with resistance and/or tolerance. Similarly, phytochemical markers in bark/stem and leaf tissue with a putative association with resistance and/or tolerance have been identified. However, a thorough validation of these markers to clarify their utility for breeding programmes is still lacking. Although recent research attributes high importance to the defence mechanisms acting in the bark or stem tissues of common ash, the ability of ash to prevent foliar infections or inhibit fungal growth from petioles to shoots requires further investigation. Overall, current research shows that the mechanisms underlying resistance and tolerance are highly complex and have not yet been fully explored.