Department of Forest Mycology and Pathology, Swedish University of Agricultural Sciences, Box 7026, S-750 07 Uppsala, Sweden

Background and objectives
Heterobasidion annosum is a basidiomycete that causes root and butt rot to conifers all over the Northern hemisphere. Economic losses are substantial; recent estimates in EU countries alone amount to 700-1500 million ECU annually. The fungus spreads by basidiospores to newly exposed wood, e.g. stump surfaces created in thinning operations. Subsequent spread is via mycelial growth in root wood and transfer between root systems in root contacts. This presentation describes studies on pathogenicity, population dynamics and intersterility carried out in Scandinavia.

Results and conclusions
The fungus consists of three intersterile groups, of which two are present in Scandinavia; one is specialized on spruce (S) and the other (P) has a broader host range with Pinus as its main host genus. Pathogenicity tests show that the P type is a slightly more aggressive pathogen and a more efficient decomposer. However, differences in host preferences are not pronounced on exotic tree species other than pines. Studies of local populations of the fungus show that individual mycelia can prolong for several decades and infect up to 15 trees. Local spore sources are likely to dominate spore infections but long-distance spread accounts for a large enough gene flow to counteract genetic differentiation among Scandinavian populations. Complex interactions among homo- and hetero-karyotic mycelia in stumps contribute to population diversity and this relatively nonselective substrate may also be a site for hybridization between the S and the P type. Interfertility can be shown in the laboratory between the intersterility groups - increasingly so between allopatric compared with sympatric populations. For example, Scandinavian S populations mate more readily than south European S types do with south European F types (associated with Abies alba). On an evolutionary scale, both host specialization and geographic isolation are likely to have contributed to differentiation in this species. Five to six distinct clades within the species can be distinguished from Euroasia and North America based on phylogeny of ITS, IGS and lignin peroxidase: North American and European P, European S and F, Japanese S and North American S. The rDNA genes do not clearly distinguish between the European S and F, while differences in lignin peroxidase support the differentiation detected by intersterility tests.