Distribution of Heterobasidion butt rot in northern Finland

Heterobasidion parviporum and H. annosum are the most damaging root and butt rot pathogens of conifers in southern Fennoscandia where their distribution and frequency are reasonably well described in literature. However, in the most northern boreal forests of Europe the occurrence and significance of these pathogens is poorly known. In the future global warming may increase root and butt rot damages in the North. In this research the distribution of Heterobasidion spp. in northern Finland was compiled from literature and various available databases and from a field study in two counties of northern Finland, Northern Ostrobothnia and Kainuu. Conifer stumps from 101 regeneration cutting sites were inspected and sample discs from stumps with signs of decay were investigated for the presence of Heterobasidion spp. In all, 11,846 stumps were investigated. Decay was found in 429 Norway spruce stumps and 50 Scots pine stumps. H. parviporum was isolated from five spruce discs, each from a different site. H. annosum was not detected. According to previous records H. annosum has been identified only twice in northern Finland and these locations are over 600 km south of the northernmost Scots pine stands in Fennoscandia. Instead, H. parviporum occurs sporadically in the whole distribution area of Norway spruce in northern Finland. Most probably H. parviporum can be found in overmature dense stands at fertile sites but even there only at low frequencies. In northern Finland butt rot of conifers is caused mainly by other decay fungi than Heterobasidion. Low pH of the organic soil layer is probably a significant factor hampering H. parviporum from spreading in the North.