Clonal differences in susceptibility to the dieback of Fraxinus excelsior in southern Sweden

Ash dieback damage was assessed and analysed on 1622 year-old grafts in two ash seed orchards (Fraxinus excelsior L.). The grafts originated from 106 plus-tree clones selected from 27 stands in southern Sweden based on their phenotypes. The results obtained indicate that ash dieback disease is strongly genotypically controlled. There was considerable genotypic variation among individuals. None of the clones seemed to be totally resistant, but some exhibited reduced susceptibility and retained this resistance after 6 years under heavy infection pressure. Autumn phenology based on leaf coloration was subject to moderate genetic control (H 2 = 0.19). The genetic correlation between autumn phenology and damage was weak to moderate (r G from 0.38 to 0.60) and positive, indicating that susceptible clones have a prolonged growing season. There was no evidence suggesting that stands differed in susceptibility. Together with the high heritability of resistance, strong agexage correlations and weak genotypexenvironment interactions, this suggests there is good scope for breeding less susceptible trees for the future.