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Pure cultures of Heterobasidion annosum were isolated from diseased trees in a 120-year-old Norway spruce stand. Clones of the fungus were identified by three different methods: studies of somatic incompatibility, identification of mating factors, and comparative studies of isoenzyme patterns. All of these methods yielded similar results. The study of somatic incompatibility was found to be the easiest and most reliable method for future investigations of population structures of Heterobasidion annosum. Nine clones were identified within a 60 × 60 m area. The number of trees infected by one clone varied from 1 to 13. The largest area occupied by one single clone was 30 m in diameter. Altogether, 10 different mating alleles were found, of which 2 occurred in four different clones and 2 occurred in two different clones. The vegetative spread of H. annosum between trees within the area occupied by a particular clone was confirmed by excavating root systems.
Jan Stenlid (Sun,) studied this question.