Rising out of the ashes: additive genetic variation for susceptibility to Hymenoscyphus fraxineus in Fraxinus excelsior

Ash dieback due to Hymenoscyphus fraxineus is threatening the Fraxinus excelsior species in most of its natural range in Europe and it is becoming obvious that other species in the Fraxinus genus are susceptible to the disease. The fungal pathogen apparently originates from Asia where it may act as an endophyte of local species like F. mandshurica. Previous studies reported significant levels of genetic variability for susceptibility in F. excelsior either in field or inoculation experiments. The present study was based on a field experiment planted in 1995, fifteen years before onset of the disease. Crown and collar status were monitored on 788 trees from 23 open-pollinated progenies originating from 3 French provenances. Susceptibility was modelled using a Bayesian approach where spatio-temporal effects were explicitly taken into account, thus providing accurate narrow-sense heritability estimates (h2). While moderate narrow-sense heritability estimates for Crown Dieback (CD, h2 = 0.42 in this study) have already been reported in the literature, this study is first to show that Collar Lesions are also heritable (h2 = 0.49 for prevalence and h2 = 0.42 for severity) and that there is significant genetic correlation between the severities of both traits (rspearman=0.40). Unexpectedly, their spatio-temporal dynamics followed almost opposite patterns. While no significant Provenance effect were detected for any trait, significant Family effects were found for both CD and CL. Moreover, the analysis strategy implemented here allowed to compute Individual Breeding Values (IBV) and to show that there is more genetic variability within families than between families. In agreement with previous reports, early flushing correlates with better crown status. Consequences of these results in terms of management and breeding