Occurrence of heterobasidion annosum in pure and mixed spruce stands in Southern Finland

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Abstract
The distribution of different clones (genotypes) ofHeterobasidion annosum was investigated in 34 clear‐cut pure and mixed Norway spruce stands. The proportion of admixed tree species, mostly birch and Scots pine, varied between 0 and 60%. The total area investigated was 35 ha. H. annosum was isolated from 13.1% of the spruces: the S intersterility group from 11.8% and the P intersterility group from 1.3%. Only 0.7% of the pines and apparently none of the broad‐leaved trees had been attacked by H. annosum. The clones of this fungus were generally small: the mean number of spruces infected by one and the same clone was 1.8. The damage caused by H. annosum to spruce was slightly smaller in mixed stands than in pure spruce stands. However, other factors than the present stand composition proved to be much more decisive for the occurrence of H. annosum.