Individual resistance of Fraxinus angustifolia clones to ash dieback

The presumed resistance of individual ash trees to ash dieback caused by invasive pathogen Hymenoscyphus fraxineus is an important issue for the maintenance of ash in European forests. All known studies regarding the resistance of ash trees to ash dieback were conducted in plantations and stands of F.excelsior; however, no such data exist for F.angustifolia. Crown damage assessments were performed over four consecutive years between 2009 and 2012 at a F.angustifolia clonal plantation in Hraica, Slovenia. Inoculation of H.fraxineus into the branches of the most and least damaged clones of F.angustifolia and leaf phenology assessments was performed to verify the presence of defence mechanisms that limit fungal growth or promote disease escape. Additionally, root collars of selected clones were inspected for fungal infections. The crown damage assessments showed considerable differences among F.angustifolia clones, indicating genetic variability in susceptibility to ash dieback. Crown dieback progressed significantly over the 4-year time period; the mean crown damage of individual clones in 2012 varied between 16.7% and 83.8%. Significant differences among F.angustifolia clones were found in the inoculation trials and leaf phenology assessments. However, defence mechanisms such as early leaf flushing, early leaf shedding and the ability to inhibit pathogen growth in host tissues were not confirmed. High frequency of Armillaria spp. and H.fraxineus root collar infection demonstrated the need for whole tree inspection to determine causal agent of damages on individual ash trees. Armillaria spp. may be highly associated with ash decline epidemiology.