Performance of susceptible and tolerant juvenile Fraxinus excelsior infected by Hymenoscyphus fraxineus, the invasive pathogen causing ash dieback

Sammanfattning: European ash (Fraxinus excelsior), a tree species with both economic and ecological importance, has experienced a steady population decline in Europe since the early 1990’s due to ash dieback, caused by the pathogen Hymenoscyphus fraxineus. In 2001, the first observations of ash dieback were made in Sweden and by 2010, ash had become a red-listed species. In its origin in east Asia, H. fraxineus exists on native Fraxinus species, such as Fraxinus mandshurica, mainly as a benign associate causing little to no damage, suggesting a balanced equilibrium between host plant and pathogen. In Europe, H. fraxineus infects F. excelsior trees of all sizes, however there are several reports indicating large genotypic variation in the level of disease susceptibility among different individuals. The aim with this thesis is to evaluate heritability of ash tolerance to H. fraxineus by studying young progeny and clonal trials. The results aim to elucidate differences in susceptibility to ash dieback among individuals. This study has included a progeny trial, located in Alnarp, planted in 2014 with progeny from both known susceptible and tolerant mother trees. During 2014-2016, assessments were conducted to determine any variability among selected progeny in leaf phenology (bud burst), tree health status and stem form. Included in this thesis was also, a trial of selected ash genotypes planted in 2016, in Snogeholm, consisting of 65 clones, including susceptible and tolerant F. excelsior individuals and Asian Fraxinus species. In this trial, only health assessment was carried out. The results show that none of the ash families were unaffected by H. fraxineus but showing differences in disease severity. However, progeny from tolerant mother trees had higher survival, lower mean health class rating (HCR) and in general better stem form. This work is a first step to enable targeted genotypes to be selected for further commercial propagation, breeding and future establishment of new seed orchards.