Leccinum aurantiacum

Despite the popularity of the name aurantiacum for North American Leccinum collections, current morphological and molecular data do not support the idea that Leccinum aurantiacum occurs in North America:

Leccinum aurantiacum is probably a European species, and no records are known from North America. The descriptions of L. aurantiacum in North American literature represent a mixture between a L. vulpinum-like, conifer associated taxon and North American species that are associated with broad-leaved trees, such as L. insigne, and L. brunneum.

As it is now defined, Leccinum aurantiacum is a mycorrhizal "generalist," associating with hosts ranging from Fagus and Quercus to Betula and Populus; it is the "one major exception" to high host specificity in the genus (den Bakker et al., 2004b; the paper did not consider species like Leccinum chromapes and Leccinum subglabripes in section Luteoscabra, which may also be generalists).

However, Leccinum aurantiacum does not associate with conifers--and this idea, combined with morphological analysis, places many North American "Leccinum aurantiacum" collections in the vicinity of the conifer-loving Leccinum vulpinum.

Morphologically, Leccinum aurantiacum can be separated from similar species (in Europe, at least) on the basis of the "brownish-reddish stipital ornamentation that is already reddish in young fruit-bodies," the red to reddish brown cap, the presence of overhanging flaps, and the reddish brown (in KOH) caulocystidia (den Bakker & Noordeloos, 2005).

It is worth pointing out that the original description of Boletus aurantiacus (Bulliard, 1780) was accompanied by a beautiful color illustration clearly depicting reddish brown, rather than black, scabers. To get an idea of what contemporary European researchers have in mind for the reddish brown scaber color in Leccinum aurantiacum, see these photos in the online Leccinum treatment of den Bakker & Noordeloos (2007).

Then compare the photo to color illustrations in your North American field guide's treatment of Leccinum aurantiacum. In my library, all the illustrations but one show black, rather than reddish brown, scabers--and close inspection of most of the photos also reveals conifer debris in the vicinity of the mushrooms.

One exception is the photo in Phillips (1991), which features a North American aurantiacum-like collection with reddish brown, rather than black, scabers.