AbstractAlthough the suberized rhytidome and phellem of Norway spruce bark appeared to be the strongest barriers against penetration byHeterobasidion annosum, artificial inoculations on standing trees showed that the living phelloderm and phloem bark were also able to contain the fungus. Thickened cell walls and excretions in the lumen of necrotic cells, as well as morphological changes in the infecting hyphae, indicated that this protection is of both mechanical and chemical character. Rate of wound pcriderm formation was correlated to bark thickness and tree age.