Growth of Heterobasidion annosum (Fr.) Bref. through bark of one-year-old Pinus sylvestris seedlings grown in vitro

Pinus sylvestris L. seedlings grown for 12-17 months in pure culture after inoculation with S and P strains of Heterobasidion annosum (Fr.) Bref. were studied with the help of light and transmission electron microscopy. Standardized inoculation procedure and uniform in vitro conditions allowed evaluation of the host/pathogen interactions. Necrotic reaction of the bark cells associated with the accumulation of dark-stained material in vacuoles, and lignification of their walls in root regions in proximity to mycelium were the common host reaction. Hypertrophy of the bark cells in areas invaded by hyphae caused a tension that was responsible for strong deformation of the bark cells. Injury to the vascular cambium resulted in formation of barrier zone parenchyma and abnormal xylem elements. High amount of polyphenolic material accumulated in new formed parenchyma cells was an effective chemical “barrier” to the centripetal spread of hyphae. Resin-plugged tracheids and row parenchyma cells rich in polyphenolic material were common in wood areas colonized by the fungus.