This thesis describes the results of an extensive survey of Rhizoctonia solani, a fungal pathogen of potato, in Finland. Over 500 isolates were collected from potato fields. Nearly all characterized isolates belonged to anastomosis group 3 (AG-3), the most typical strain of R. solani infecting
potato. Additionally, three other AGs (AG-2-1, AG-4 and AG-5) were found associated with potato plants, as also reported previously in many other countries. Comparison and phylogenetic analysis of the internal transcribed spacer sequences (ITS-1 and ITS2) indicated that all Finnish AG-3 strains are closely related, despite the wide variability of their other biological features, e.g., virulence, growth rate and fungicide tolerance. Detailed analysis of AG-3 isolates revealed polymorphism within the ITS sequences.