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 isol ate of the S intersterility group of Heterobasidion annosum in a greenhouse . The dehardened cuttings were organized in three different groups with fou r to six ramets in each group. All groups were inoculated on the same day s hortly after shoot elongation. The groups were sampled tur H. annosum growt h after 34 (group 1), 83 (group 2) and 182 days (group 3), respectively. Me asured parameters were cutting height and diameter, vigour index of the cut tings, infection incidence, mortality rare and fungal growth in the stem. T he height of the cuttings was almost constant during the 6 months of incuba tion, whereas the diameter increased by about 10% during the same period. T he proportion of living cuttings containing H. annosum decreased with time (99.5, 93 and 67% infection in groups 1, 2 and 3, respectively) and differe d 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 differe d significantly among clones in groups 2 and 3; the same clones being must susceptible for both times. Mean fungal growth into the wood was significan tly different among groups and among clones within each group. The ranking position for fungal growth was similar in the three groups. The results ind icate 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 period s. 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 amon g 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.