Initial laboratory- and field experiments were made in order to find a suitable wood-decomposing fungus for biological control of Heterobasidion annosum (Fr.) Bref. Strains of Resinicium bicolor (Alb. P Schw. ex Fr.) Parm., Phanerochaete sanguinea (Fr.) Hjortstam, Coniophora sp. DC. ex Me'rat, Armillaria borealis Marxmueller and Korhonen and Hypholoma capnoides (Fr.) Kummer were paired against Heterobasidion annosum in two different competitiontests using wood blocks, equal and unequal in size. The most successful species in the laboratory experiments. Resinicium bicolor, was used in field experiments in first rotation field plantations of Norway spruce (Picea abies). Wood blocks, preinoculated with Resinicium bicolor, were buried in the soil beside stumps at different time interval after thinning and with different number of wood blocks. Half of the stumps were treated with a suspension of Heterobasidion annosum conidia sprayed on the surface. Investigations of roots from one of the field experiments, showed that the percentage of Heterobasidion annosum infected roots was less on the roots that were infected with Resinicium bicolor. This tendence was the same in both, with Heterobasidion annosum, naturally and artificially infected stumps. However, the average length of growth of Heterobasidion annosum was longer on the roots with Resinicium bicolor present, at least on the naturally infected stumps.
