The basidiomycete Heterobasidion annosum is responsible for root- and butt-rot on conifers and has long been considered as a single species with a wide host range. However, experimental crosses have revealed that not all strains would mate one with each other (Stenlid and Karlsson, 1991). Three groups have been defined based on intersterility, and they showed different ecological preferences: The S group (renamed H. parviporum) is a pathogen almost exclusively of Norway spruce (Picea abies) in most parts of Europe and of Siberian fir (Abies sibirica) in northeastern Europe, while the P group (H. annosum) is found mostly on pine, and the F group (renamed H. abietum) is more saprophytic, although sometimes also pathogenic on spruce species. Experimental crosses revealed that five genes were involved in intersterility (Chase and Ullrich, 1990).