Pliūra, A., Lygis, V., Suchockas, V. and Bartkevičius, E. 2011. Performance of twenty four European Fraxinus excelsior populations in three Lithuanian progeny trials with a special emphasis on resistance to Chlara fraxinea.  Baltic Forestry 17 (1): 17 – 34

The main aim of the present study was to estimate genetic variability and the level of genetic control of disease resistance in common ash populations based on progeny testing, and to select resistant populations, families and genotypes for production of reproductive material resistant to dieback caused by Chalara fraxinea. A total of 320 half-sib families from ten Lithuanian and fourteen foreign European Fraxinus excelsior populations have been tested in three progeny trials established in Telšiai, Pakruojis, and Kėdainiai State Forest Enterprises. Because of severe attacks by C. fraxinea, only 10% of trees survived in all progeny trials at the age of eight years. Variance analysis of health condition scores revealed significant effect of genetic factors i.e., populations and families. Differences in resistance among populations were significant and ranged up to 52.8-55.1% in a degree of damage and survival rate of populations varying from 0.13 to 0.59. At the age of eight years, genetic family variance components ranged 5.3-7.5%, while the additive coefficients of genetic variation ranged 29.9-38.7% indicating the existence of strong genetic differences in progenies’ resistance to C. fraxinea both at population and family levels. There was a significant site effect on the level of damage which could be due to site conditions and/or different infection pressure from the surrounding stands. However, genotype by environment interaction (G x E) for resistance traits was weak and non-significant which indicated that populations and families do not differ much in their performance across environments and thus specific adaptations and genetic variation in ecological response are not pronounced. The obtained high heritability estimates (h²=0.40-0.49±0.03) for health condition indicated that resistance is genetically predetermined and inheritable, and allows forward selection and breeding of the resistant genotypes. Health condition of foreign populations was worse than that of the Lithuanian ones by 17.5-23.5% in degree of damage and by 48.5-73.5% in survival rate which could be a consequence of more advanced process of natural selection in resistance against C. fraxinea in damaged Lithuanian F. excelsior populations. The obtained results demonstrated that strategy of the extended introduction of foreign populations to restore Lithuanian ash stands is of doubtful potential. Only few most resistant foreign genotypes could be selected as potential candidates for breeding of ‘synthetic’ F. excelsior populations in order to increase genetic diversity. Among the Lithuanian populations, the best in terms of growth characteristics, resistance, survival, and proportion of most healthy individuals were those originating from Ignalina, Pakruojis, Šakiai and Kėdainiai. Based on estimated BLUP breeding values, complex breeding indexes and on so-called ‘Pyramid selection principle’, fifty superior individuals were selected within best families from the best populations for establishing resistant breeding populations of common ash in different provenance regions of Lithuania, as well as for establishing clonal trials and resistant seed orchards of second generation.

Key words: Chalara fraxinea, common ash (Fraxinus excelsior), dieback, genetic variation, half-sib families, heritability, populations, resistance, selection