Heterobasidion occidentale sp. nov. and Heterobasidion irregulare nom. nov.: a disposition of North American Heterobasidion biological species

The genus Heterobasidion includes some of the most important pathogens of conifers in the world, and as such it is one of the most intensely studied genera of fungi. Because of the remarkable paucity of distinguishing morphological traits, the taxonomy of species within this genus has always been problematic. A partial resolution of the taxonomic issues regarding this genus was achieved by defining the most important and first described species within it, Heterobasidion annosum, as a species complex containing at least two partially intersterile biological species defined as intersterility groups (ISGs). With time, the number of ISGs has increased to include at least two distinct North American and three distinct Eurasian ISGs. Two additional, yet unnamed, taxonomic groups within Heterobasidion have been recently described in Japan. ISGs are distinguishable either by minor morphological differences, by partial intersterility, by ecological traits including host preference, and/or by their geographic range. Several studies employing a variety of molecular tools and analyses have confirmed the distinct genetic divergence among ISGs, identifying each of them as a monophyletic group. Using genetic markers, genotypes can always be unambiguously assigned to one ISG, and very few inter-ISG hybrids have been identified. In this paper, we summarize the available information, both genetic and ecological, that differentiates the two North American ISGs from each other and from other taxonomic units within the genus. We demonstrate that morphometric characteristics such as pore density and pore shape differentiate the two ISGs. Based on the cumulative genetic, ecological, and morphological evidence, we propose a disposition of ISGs of the North American H. annosum by replacing the P ISG with Heterobasidion irregulare, and the S ISG with Heterobasidion occidentale.