Summary:
Features include 1) a nearly spherical fruitbody with a dark brown surface that is slightly villose, 2) a light brown spore mass with small, maze-like chambers, 3) a large rhizomorph from which it arises, 4) absent columella, 5) growth in duff under conifers, 6) an olive reaction on the fresh peridium with KOH or FeSO4, 7) oblong spores on basidia that are 6-spored, 8-spored, or more rarely 4-spored, 8) septate paraphyses with mucilaginous thickenings, and 9) a peridial epicutis of loosely interwoven hyphae with cinnamon-brown walls. Molecular evidence has been presented to support the contention that the Smith and Zeller species R. colossus var. colossus (2 paratypes studied), R. hawkerae (paratype studied), and R. parksii (holotype studied) are synonyms of this species, (Martin, M.P.(2)); on the other hand, other molecular evidence suggests that R. villosulus (different collection), R. colossus (holotype), R. hawkerae (same paratype), and R. villescens (non-type) are close and could be a single species that shows variation, whereas two non-type collections of R. parksii were more distant, (Grubisha(2)). Rhizopogon villosulus using the Smith & Zeller(1966) concept is not rare in the Pacific Northwest according to Smith(4), "Not rare in Idaho but it is one of the most difficult to collect since the basidiocarps are often about the size of rabbit pellets and the same color as the soil", (Smith(30)). It is abundant among false truffles in the Pacific Northwest (Trappe(13)).

Chemical Reactions:
"in KOH olive on fresh peridium, FeSO4 olive", (Smith(30))

Interior:
"light brown drying buffy brown (dingy avellaneous in the type); cavities small and labyrinthiform", (Smith(30)), "light brown, drying buffy brown; cavities small, labyrinthiform", (Zeller), white when young, dark olive-brown when mature, (Trappe, M.(3))

Odor:
when mature "of spicy, garlicky sausage" (Trappe, M.(3))

Taste:
mild (Trappe, M.(3))

Microscopic:
spores 6-8 x 2-2.5 microns, brachybasidioles soon thick-walled, "epicutis of loosely interwoven hyphae with cinnamon-brown walls, very few flagellate hyphal ends present", (Smith(4)), spores 6-8 x 2-2.5 microns, oblong or nearly so, in Melzer''s reagent colorless individually and pale reddish brown in aggregations, in KOH colorless or nearly so singly, dingy ochraceous to dingy pinkish buff in masses, basal scar inconspicuous; basidia 6-spored, 8-spored, more rarely 4-spored, 15-20 x 4.5-6 microns, "subcylindric, readily collapsing"; paraphyses "septate, terminal cell ovate, obovate to clavate", 15-25 x 7-12 microns, "mostly with mucilaginous thickenings both in the terminal cell and those (2-3) beneath it, the paraphyses readily disarticulating at the septa and then found floating separately in the mount", the amorphous central body not coloring in Melzer''s reagent; subhymenium "cellular and the cells thickened as indicated above as well as separating at the septa readily under pressure"; tramal plates of "highly refractive gelatinous hyphae more or less interwoven"; inner peridial layer (subcutis) of interwoven hyphae colorless in KOH "but with dull brown pigment pockets present and in places in KOH becoming dark green, red pigmentation observed in the layer on some collections, no pockets of distinctly inflated cells present", amyloid particles present on sections of peridium revived in Melzer''s reagent; peridial epicutis of hyphae that are loosely interwoven, brown-walled (near cinnamon-brown), smooth, "often with thickened walls, the hyphae branched and often enlarged near the septa, some with short branches but these not thorn-like, very few flagellate hyphae seen, greatly inflated dark brown cells present and often abundant"; all tissues inamyloid; clamp connections absent, (Smith(30)), spores 6-7.5 x 2.2-2.7 microns, narrowly elliptic, slightly colored; basidia 2-spored and 4-spored, 14-25 x 3-5 microns, exceeding the paraphyses, narrowly clavate, sterigmata about 1-1.5 microns long; paraphyses 12-20 x 8-10 microns, "obovate to oblong, highly gelatinized"; septa "narrow, of highly gelatinized hyphae"; peridium "very thin, 32-60 microns thick, when dry, duplex", the inner layer composed of compactly woven, colorless hyphae, the outer layer of hyphae 4-7 microns wide, loose, brown, some somewhat erect, "this layer sometimes rupturing and exposing the inner layer", (Zeller)

Notes:
Rhizopogon villosulus is found in the Rocky Mountains, southern BC to northern CA, Mexico, (Trappe(13)). It was described from OR by Zeller. Collections were examined from OR, ID, and CA, (Smith(30)). A specimen from WA used in study (Grubisha(2)). It was introduced into Europe, Australia, and New Zealand on Douglas-fir seedlings, (Trappe, M.(3)).

Synonyms and Alternate Names:
Hymenogaster caespitosus (Rolland) Soehner
Leccinum caespitosum "(Rolland) M. Kuo & B. Ortiz, Mycologia"