Dynamics of genetic resistance to Hymenoscyphus pseudoalbidus in juvenile Fraxinus excelsior clones

Abstract

The study was aimed to evaluate a temporal change in genetic resistance to Hymenoscyphus pseudoalbidus and in level of its genetic variation and heritability in juvenile Fraxinus excelsior clones initially tested and selected for resistance in progeny trials. Data on tree growth and ash dieback symptoms of 47 (26 Lithuanian and 21 foreign) clones representing trees of improved resistance against H. pseudoalbidus and 7 control clones from random Lithuanian populations were collected during the three growing seasons in seven clonal trials in 2012 and compared with previously reported data of these trials. Evaluations revealed a considerable increase in disease occurrence and damage severity thus showing the continuous spread of infection. Incidence of disease increased from 10.7% in spring 2012 to 72.2% by spring 2013 and remained almost constant till the autumn 2013. In spring 2012, dieback symptoms occurred on 45.9% of the tested clones on 3-50% of their ramets while in 2013 disease occurred on 100% of the tested clones on 2-100% of their ramets. On 23.4% of tested clones disease incidence was equal or below 50%, while mortality increased very little and reached only 1.2%. There were evident differences in ash dieback severity between the investigated populations. The lowest disease mean incidence remained in clones originating from three Lithuanian populations - Ignalina, Žeimelis and Kėdainiai (52.2, 54.3 and 67.4% respectively), while the highest was found in clones of German - Farchau (86.0%) and Lithuanian - Sakiai (88.7%) populations. Most clones shoved significant growth recovery by the end of 2013 vegetation season regaining and slightly increasing the height that was lost after diseased leader shoots had dried-up