Does temperature regime govern the establishment of Heterobasidion annosum in Scandinavia?

We explored the reasons underlying the biogeographic distribution patterns of the economically important, wood-rotting basidiomycete Heterobasidion annosum in Sweden. Despite the commonness of suitable host trees, Heterobasidion annosum has not been recorded in the north of Sweden, whereas its relative, H. parviporum, is present throughout the country. To test the hypothesis that H. annosum has not spread to the north because of the effect of climate, mainly differences in the general temperature regime, we inoculated Norway spruce stumps and standing trees with H. annosum and H. parviporum at six field sites, three in the south and three in the north of Sweden. Three strains of both species were used in random combinations, so that each selected stump and tree was inoculated with both species at the same time. At 2 and 10 months after the inoculations, we compared the frequencies of detection of H. annosum and H. parviporum colonies at different distances from inoculation points in the stumps and in trees. The H. annosum colonies were detected only infrequently on disks cut from the inoculated stumps (0-4% of re-isolations) in both areas, whereas H. parviporum was detected much more frequently (26-47% of re-isolations). In standing trees, colonies belonging to H. annosum could be detected up to 210 cm (south) and 80 cm (north) and those belonging to H. parviporum up to 210 cm (south) and 140 cm (north) above the inoculation points. Our results suggest that difference in temperature regime does not provide an explanation for the distribution limit of H. annosum.