In agreement with the expectation that premating isolation should be selected for in sympatry, intersterility appeared lower among allopatric strains of different groups (from North America and Europe) than between sympatric strains. The differentiation of these Heterobasidion species was suggested to have occurred following the migration of the main spruce and fir hosts, into Europe from eastern Asia (Johannesson and Stenlid, 2003).