The aim of this work was 1) to investigate the genetic diversity of H. annosum s. str. populations, 2) to recognize genets ability of their aggressiveness to Scots pine. The genetic similarity among genets varied from 0% to 74%. Totally different genets were found in 17.9% of cases. The lowest similarity occurred among 20.7% of genets and it was calculated on 18%. Six months after inoculations of Scots pine stems by H. annosum s. str. the distance of wood colonization by mycelium was measured. Wood was colonized to a distance varying from 0.52 cm to 2.06 cm. The most aggressive genets were isolated from stumps, and the less aggressive from trees’ stems. The infections in stands caused by H. annosum s. str. basidiospores and appearance of a new pathogen’s organisms, which might distinguish by high aggressiveness, could increase a damages in these stands. There is still a need to use a biological control against root rot disease to reduce spreading of a new generations of pathogens.

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