Roots of 18 mature Picea abies were inoculated in situ with Heterobasidion annosum. Within one month a dry transition zone formed in advance of the growing hyphae and around non-infected wounds. After 1, 3, and 12 months, the fungal growth and the phenolic and resinous content of the root wood were determined. After 12 months, accumulation of phenolic compounds in the transition zone resulted in concentrations similar to those found in sound heartwood and 12 times higher than those found in sound sapwood. The amount of phenolic glucosides in Heterobasidion annosum-infected wood was higher than in the other samples. The relative toxicity of the phenolics was highest in the transition zone and lowest in the heartwood. At concentrations comparable to those in wood, transition zone phenolics were the strongest inhibitors of conidial germination in Heterobasidion annosum and sapwood phenolics were the weakest.