Hope for ash conservation and propagation—single individuals can be highly resistant to an invasive pathogen

Abstract Common ash ( Fraxinus excelsior L.) has been considered an important candidate species for climate-resilient forest management in Germany. The occurrence of ash dieback, caused by the invasive fungus Hymenoscyphus fraxineus , severely limits its use. However, less damaged ash trees in heavily infested stands are observed. This suggests a tree-specific and genetically determined susceptibility to ash dieback. The potential of individual trees for selection and conservation programs has been investigated on 35 clones in a clonal trial and 30 families in a progeny trial since 2014. Ash dieback-related mortality, crown damage, and height growth were examined. Resistance to ash dieback was evaluated based on multi-year observations. Mortality of ramets occurred rapidly and crown damage of clones stabilized, whereas steadily increasing in progeny during the study period. About 34 percent of all offspring (0–56 percent per family) showed high resistance to ash dieback and 10 percent showed also good height growth. Broad-sense heritability and narrow-sense heritability were moderate and comparable to previous studies. We assume that at least some individuals from specific progenies will resist the disease in forest stands. In situ and ex situ conservation strategies should be promoted. Seed orchards with more resistant genotypes could provide less susceptible reproductive material, and breeding programs could help to accelerate the fixation of favorable alleles.