The wood of one Picea abies stump, including its roots (1·3 m in length), was sliced into 2 cm thick discs. The stump originated from a thinning conducted 7 yr prior to the investigation in a 30 yr old spruce stand planted on previous farmland that became heavily infected by Heterobasidion annosum. After incubation of the wood discs, interaction zones were observed on the surfaces and H. annosum was isolated from the areas between the zones and from the zone lines, resulting in 296 isolates. The isolates were tested with somatic incompatibility to detect 35 different genets. Of the 27 genets colonising the upper part of the stump (excluding the roots), 12 had proceeded into the roots. Seven genets found in the roots were not found in the upper part of the stump and one genet isolated from an interaction zone on the top of the stump was not found elsewhere in the stump or in the roots. Two of the genets had grown into root contact with other trees.