Annillaria root rot, one of the most dangerous diseases in our forests, is caused in Poland mainly by Armillaria ostoyae, especially severe in young Scots pine stands, 'established after broadleaved stands or with participation of broadleaved species. in Forest District Zielonka young stands are severly affected by Armillaria root 'rot. Only one species, A. ostoyae, was found in the young (8-14 yrs) Scots pine stands, despite the presence of other Armillaria species in the district. The pathogen's frequent occurrence may be due, interalia, to favouring environmental, roots.