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 - Šakiai (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.

Variance analysis of tree damage traits such as health condition and disease incidence in 5th and 6^th growing seasons revealed constantly increasing clonal variance component, coefficients of genetic variation and heritability which indicated permanent substantial genetic control of the disease resistance and therefore good prospects for further tree breeding for resistance at a clonal level. Clonal variance component (vcc) of disease incidence over all trials increased from 8.15% (19.0-70.5% in individual trials) in spring 2012 to 14.76% (23.9-63.2% in individual trials) in autumn 2013 and broad sense heritability (H2) increased from 0.33 to 0.40 (means from all 7 trials) thus indicating increasing genetic variation in susceptibility to disease. The vcc for tree health conditions increased from 13.60% to 16,35% and H2 decreased from 0.40 to 0.38. Meanwhile vcc for length of dry/necrotic part of leader shoot decreased from 29.86% to 8.12% and H2 decreased from 0.74 to 0.51 thus indicating decreasing genetic variation in disease spread rate within trees. Genotype-environment interaction (G × E) vc_c in most of tree damage traits decreased although remained significant thus suggesting presence of genetic variation in plasticity and reaction norms of clones across a range of infection load/pressure environments.

Key words: Hymenoscyphus pseudoalbidus, Chalara fraxinea, common ash, dynamics of dieback, disease resistance, dynamics of genotypic variation, heritability