Variation in growth of Heterobasidion annosum among clones of Picea abies incubated for different periods of time

Thirty-seven 4-year-old clones of Picea abies were inoculated with one isolate of the S intersterility group of Heterobasidion annosum in a greenhouse. The dehardened cuttings were organized in three different groups with four to six ramets in each group. All groups were inoculated on the same day shortly after shoot elongation. The groups were sampled tur H. annosum growth after 34 (group 1), 83 (group 2) and 182 days (group 3), respectively. Measured parameters were cutting height and diameter, vigour index of the cuttings, infection incidence, mortality rare and fungal growth in the stem. The height of the cuttings was almost constant during the 6 months of incubation, whereas the diameter increased by about 10% during the same period. The proportion of living cuttings containing H. annosum decreased with time (99.5, 93 and 67% infection in groups 1, 2 and 3, respectively) and differed significantly among clones in group 3. Mortality rate increased with time (0.5, 22 and 37% mortality in groups 1, 2 and 3, respectively) and differed significantly among clones in groups 2 and 3; the same clones being must susceptible for both times. Mean fungal growth into the wood was significantly different among groups and among clones within each group. The ranking position for fungal growth was similar in the three groups. The results indicate that frequency of infection success, mortality rate and fungal growth are clone-dependent factors. Broad sense heritability varied between 0.08 and 0.25 for fungal growth and lesion length at the three incubation periods. Infection success frequency was initially not different among clones but in the longer incubation periods were significant differences among clones, indicating differences in resistance. Fungal growth in wood differed among separate host clones irrespectively of the length of inoculation period. The vigour of the cutting seems to influence the length of fungal extension only in the stage of the infection.