Additional key words: Armillaria ostoyae, diversity, genets, RAPD-PCR, rhizomorphs

Abstract: Incidence of Armillaria root disease and the population structure of associated Armillaria spp. were studied in 5-17-year-old Scots pine plantations in west-central Poland. Two infection centres (1.14– 9.30 ha) in each of three forest districts (Siemianice, Zielonka and Złotów) were intensively sampled. Root collars were examined for mycelial fans, decayed wood, and rhizomorphs. Twenty two isolates of Armillaria ostoyae collected from epiphytic rhizomorphs from 20 living and two dead trees in the six infection centres were identified with somatic incompatibility group. Only one somatic incompatibility group for A. ostoyae was found. Twenty one isolates produced rhizomorphs on oak-wood discs submerged in a sand-forest soil substrate. Isolates from Siemianice formed the smallest rhizomorph networks and those from Złotów the most abundant. There were 16 different genets among 22 isolates of A. ostoyae distinguished by RAPD analysis. Genetic similarity among genets was 25.6–97.5%. The large diversity in A. ostoyae suggests that sexual reproduction may occur in nature more often than expected