Population dynamics of the root rot fungus Heterobasidion annosum following thinning of Picea abies

Populations of Heterobasidion annosum were studied in two equally aged stands of Norway spruce Picea abies at the same site. The stands were 2 and 7 ha large and had been thinned one and seven years, respectively, before the investigation. The 7-yearold thinning was more thoroughly investigated since spread of the disease could be followed. One third of thinning stumps were free of H. annosum, and 36% hosted one genet; the rest were colonised by between 2 and 13 genets each. In contrast, only a single genet was found in each of the decayed trees. Groups of decayed trees infected by the same genet were found. Sixty-three percent of the trees had started to decay within 7 years of thinning. Of 262 genets examined, 158 were restricted to thinning stumps, 18 infected both stumps and trees and 86 were restricted to trees. Based on mating tests, 95% of the isolated strains were assigned to the S intersterility group and 2% to the P group; the other 3% could not be classified. Some stumps contained genets of both intersterility groups, but trees were not infected by the P group. The number of genets per stump was correlated better with the stump diameter than with stump surface area. The percentage of decayed trees increased with stem diameter. Similarities in the population densities between the two stands despite the 6-year difference in the time since thinning indicate that disturbance is essential for fungal establishment. It was concluded that trees were infected by mycelial spread from stumps rather than by spore infection of wounds.