## 1 Introduction  
Based on several independent studies that utilized the morphological, biological and phylogenetic species concepts, it is clear that _Phellinus sensu stricto_ (s.s.) is composed of a limited number of species that cause a delignifying trunk rot of deciduous hardwoods, with minor exceptions [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. Numerous other species previously classified as _Phellinus_[13] are now members of various segregate genera (e.g., _Fomitiporia_, _Fuscoporia_, _Inonotus_, _Onnia_, _Phellinitium_ and _Porodaedalea_, among others) [7; 8; 9].  
White trunk rot of hardwoods, caused by _Phellinus_ s.s., has been described as the single most important rot of hardwoods in North America [14]. [PERSON] ([15] p. 413) noted: \"The false tinder fungus causes more loss than any other wood destroyer of hardwoods\". In North America, _Phellinus_ s.s. is known to be most destructive in northern hardwood (_Acer-Betula-Fagus_) and aspen (_Populus_) forest types [14; 15; 16]. The presence of a single basidiocarp on the trunk of an infected tree can indicate a column of decay approximately 2 m in length for American beech (_Fagus grandifolia_) and sugar map (_Acer saccharum_), and nearly 3 m in length for yellow birch (_Betula alleghaniensis_) [17]. _Phellinus tremulae_, one of the first species to be distinguished from the _P. ignarius sensu lato_ (s.l.) complex [18], acts as an aggressive pathogen of aspen (_Populus_; especially _P. tremuloides_) and has been shown to cause significant losses of merchantable timber in the U.S. Lake States and Intermountain West [19; 20].  
For decades, white trunk rot of hardwoods was attributed to the actions of a single species, _P. ignarius_, even though several varieties or types were described [21; 22]. While still referred to as the \"_P. ignarius_ group\", there are up to nine species of _Phellinus_ s.s. known or suspected to occur in North America, and include: _P. alni_, _P. actrostaphyli_, _P. ignarius_ s.s., _P. laevisatus_, _P. lundelli_, _P. nigricans_, _P. populicola_, _P. tremulae_, _P. tuberculosus_[8; 9; 16; 23; 24]. There are additional species still described as \"_Phellinus_\" in North America (_P. bicuspidatus_ and _P. spiculosus_, for example) that prior studies have shown are not members of _Phellinus_ s.s. [8; 10]. Ironically, it has been hypothesized that _P. ignarius_ s.s. is a European species restricted to _Salix_[6; 9]. However, a broader survey of species incidence by [PERSON] _et al._[12] demonstrated that while _P. ignarius_ s.s. is most common on _Salix_, it also occurs on _Populus_ and _Malus_ in Europe and Asia, signifying this species could potentially occur on all members of the Salicaceae and Rosaceae growing in northern temperate regions. An additional _Phellinus_ species found only on _Populus_, _P. populicola_, is genetically similar to _P. ignarius_ s.s. and is present throughout northern Europe and Asia [6; 12]. To date, it remains unclear if either _P. ignarius_ s.s. or _P. populicola_ occur in North America.  
Phylogenetic analyses have vastly improved the ability to discriminate genera and species in _Phellinus_ s.l., yet most studies that include _Phellinus_ s.s. have primarily utilized isolates collected from Europe [6; 7; 8; 9; 12], leaving North American researchers to speculate about species distribution and host specificity. The goals of this study were to determine the incidence and host distribution of _Phellinus_ s.s. species using isolates that originated from northern temperate forests in North America. It was hypothesized that eight species of _Phellinus_ s.s. occur in North America: _P. alni_, _P. arctostaphyli_, _P. nigricans_, _P. laevisatus_, _P. lundelli_, _P. populicola_, _P. tremulae_ and _P. tuberculosus_.