Fungal species associated with butt rot of mature Scots pine and Norway spruce in northern boreal forests of Northern Ostrobothnia and Kainuu in Finland

Diversity of fungi was investigated in Scots pine and Norway spruce stumps showing symptoms of butt rot in managed coniferous forests located in the northernmost distribution area of Heterobasidion butt rot in Finland. During a survey of 101 recently clear-cut stands, a total of 479 stumps showing signs of decay were sampled. Fungi were isolated from wood chips taken at the transition zone between healthy and decayed wood. A subsample of 209 isolates from Norway spruce and nine from Scots pine was selected for sequencing ITS-DNA. In Norway spruce 59 species and in Scots pine six fungal species were detected based on ≥98% similarity and 62 genera based on ≥95% similarity with sequences in GenBank. Three sequences did not match to any identified sequence in GenBank. The majority of the isolates (74%) were Ascomycota while 18% were Basidiomycota and 6% Zygomycota. The dominance of Ascomycota over Basidiomycota was pronounced in spruce stumps of stands with a butt rot incidence above 20% and the decayed area was larger in sample discs from which only Ascomycota were isolated than in discs inhabited by Basidiomycota. The result suggests that there is a high site- and/or tree -specific variation of the mycoflora in the Norway spruce decaying butt. Based on these results no single fungal species can be suspected to act as causal agent of butt rot occurring in Norway spruce nor in Scots pine in Northern Ostrobothnia and Kainuu counties.