Pileus 4-15 cm broad, convex, expanding to plano-convex; margin at first incurved, then decurved, at maturity nearly plane to slightly upturned, often wavy; surface subviscid to viscid when moist, disc grey-brown with matted hairs or fibrils, elsewhere often squamulose, the squamules appressed, typically dark-brown, dispersed over a cream-buff ground color; context, 1- 2.5 cm thick, soft, cream-yellow, unchanging when cut; odor like that of Suillus pungens, i.e. fruity, harsh; taste, mild to harsh.
Pores fine when young, angular at maturity, 1-1.5 mm in longest dimension, subboletinoid; at first cream-yellow-buff, becoming dull-yellow, finally dingy-ochraceous, to dark olive-brown, unchanging or slowly darkening when bruised; tubes 0.7-1.5 cm long, usually colored like the pores, adnate, depressed to subdecurrent in attachment.
Stipe 4-7 cm long, 2-3.5 cm thick, solid, equal to enlarged at the base; surface more or less glabrous, subviscid, yellowish at the apex, sometimes slightly reticulate from tube remnants, lower portion tinted variously cream, pale-yellow, pinkish to apricot-buff; surface dotted with pinkish-buff to cinnamon-brown glands which in age become dark-brown; context, soft, colored like that of the cap, unchanging; stipe base of young specimens pinkish-buff to pale-lavender; veil absent.
Spores 9.5-11.5 x 3.5-4.5 µm, ellipsoid to subfusoid, smooth, thin-walled; spore print olive-brown.
Scattered to gregarious under 3-needle pines, e.g. Monterey pine (Pinus radiata), knobcone pine (Pinus attenuata), and Ponderosa pine (Pinus ponderosa); fruiting from late fall to mid-winter.
Edible, but generally considered mediocre.
This dark, usually squamulose Suillus is one of several slippery jacks found in the San Francisco Bay Area including the ubiquitous Suillus pungens, and much less common S. brevipes. Suillus fuscotomentosus can be told from S. pungens by a less viscid, typically squamulose cap and from S. brevipes by a gland-dotted stipe. Another squamulose slippery jack that resembles Suillus fuscotomentosus but is associated with 2-needle pines along the coast (Pinus muricata and P. contorta) is S. tomentosus. Besides habitat differences, it is distinguished by a cap with lighter colored scales which redden in cold weather, and pores which bruise blue. Finally, Suillus fuscotomentosus is also very similar to Suillus acerbus. As noted by Arora, these two taxa appear to intergrade, specimens within a collection varying enough to fit either species. For the present, we have chosen to use the name Suillus fuscotomentosus for material in this group, acknowledging that the choice is arbitrary and with further study could change.