Studies on the ecology and genetics of hybridization in Heterobasdion
The Heterobasidion taxonomic complex comprises two of the most important hngal tree pathogens in North America (Okosina 1989), and includes at least three other taxa in Eurasia (Niemela 1998). These taxa were initially identified as intersterility groups (ISGs) (Korhonen 1978) within the species H. annosum, but the current trend is to assign them the species rank based on molecular, morphological, and host-association traits. European ISGs have already been accepted as different species (Niemela 1998). Although yet to he defined as species by taxonomists, the North American Heterobasidion ISGs have a much wider genetic divergence and isolation than their European counterparts (Otrosina 1993), and are also ecologically specialized on different hosts (Otrosina 1989; Worrall 1983). In this paper we will use the terms "species" and "ISGs" interchangeably when referring to the two North American taxa of Heterobasidion.