Genetic evaluation of damage caused by ash dieback with emphasis on selection stability over time

Abstract Damage caused by Hymenoscyphus fraxineus , the causal agent of ash dieback, was genetically evaluated based on scorings made on five occasions during the years 2006–2016 at two seed orchards of ash ( Fraxinus excelsior L.) established in southern Sweden in 1992 and 1995, respectively. The studied population included grafts originating from 106 plus-tree clones selected from 27 stands in southern Sweden. The study verified previous results of (1) high genetic control of damage traits, (2) high genotypic variation in disease susceptibility and (3) that only a small proportion of the natural population are at least partly resistant. The clonal response for dieback damage was stable over the 10-year period, i.e. the least susceptible clones selected in 2006 still belong to the healthiest ones after several years of heavy infection pressure. There were no clear trends in the development of damage over time for trees being alive at each assessment. However, the proportion of trees that died was substantial (7–8% per year), but this estimation was influenced by other factors than the disease, such as wild game and sanitary cuttings. Altogether, the results are promising for initiating breeding programs to improve the dieback resistance. However, this is a heavy and costly task, since only a very small proportion of the ash trees are partially resistant to the dieback disease. Thus, a joint breeding program involving all the northern European countries is justified.