R. subcaerulescens appears to be a synonym of R. ellenae. This is based on identical ITS sequences of the holotypes. R. ellenae (or R. subcaerulescens) is the host to snow plant (Sarcodes), but not Pterospora. So, even though R. subcaerulescens was initially reported with Pterespora by Cullings (and me); that’s wrong, and I can share some of the blame.
The nomenclature gets messy because R. ellenae and R. subcaerulescens were described by Smith in monograph with Zeller. Martin Bidartondo, who is at Kew Gardens now, found the connection between Snow plant and R. ellenae before he found out that R. subcaerulescens and R. ellenae were the same thing. So we have used the name R. ellenae over R. subcaerulescens, but this was an arbitrary decision and either name could be chosen as the correct one (which no one has done yet).
In the lake states there is an undescribed species of Rhizopogon that we know only from sequence that associates with disjunct populations of Pterospora. When found it will be the only member of the amylopogon group in the East.
Morphologically no one seems to be able to tell R. ellenae, R. salebrosus, or R. arctostaphylos apart. I say this with confidence because Martin sequenced a bunch of collections identified by Smith and by Trappe and nobody got the distinctions between these things right. I think this is because the color changes that are usually focused on are very developmentally plastic.
Smiths holotypes for the amylogon group have all been sequenced and its an amazing mess. Many appear to identical, and “paratypes” are often different. Bottom line – that part of genus is confusing.