Genetic and environmental variation in rust frequency on mature mountain birch trees

This study investigated genetic and environmental variation in the frequenc y of birch rust, the most important leaf disease of birch species. The same half-sib families of mature mountain birch trees were studied in two areas corresponding to their natural growing habitats over 3 yrs. The frequency of birch rust was examined both in the field and from detached leaves inocu lated in the laboratory. The frequency of birch rust varied among the mount ain birch families. However, the heritability of birch rust resistance was found to be fairly low, with the heritability of naturally occurring birch rust varying between 0.27 and 0.41. The frequency of birch rust varied high ly between the two study areas and among study years. Nevertheless, the rel ative frequency of birch rust among tree individuals and tree families rema ined similar and as a result no notable genotype x environment interaction was observed. The field and in vitro results differed with respect to the r anking of birch families by birch rust resistance.