Cap 2.0-8.0 cm broad, convex, expanding to plano-convex, sometimes with a low umbo; margin decurved, occasionally wavy in outline, normally not appendiculate; surface more or less glabrous, at times shallowly pitted or with small depressions, viscid when moist, buff to pale olive-grey, often spotted or streaked rusty-brown; context soft, thick, pale, dingy-buff, slightly darker when cut and developing dull vinaceous tones; odor harsh, similar to that of Suillus pungens; taste mild.
Pores radiating from the stipe, dull ochraceous, angular, elongated, up to 2 mm in the longest dimension, not discoloring when bruised; tubes 0.4-1.0 cm long, colored like the pores or darkening somewhat when cut, adnate to subdecurrent at the stipe.
Stipe 2-5 cm long, 0.5-1.0 cm thick, round, solid, equal to tapering to a narrowed base, sometimes twisted or with a basal bend; surface at the apex ochraceous, longitudinally striate or obscurely reticulate from partially descending tubes, pallid below when young, but in age developing brownish glands or discolorations where bruised; partial veil membranous-viscid, translucent buff-brown, forming a ragged, superior veil, typically appressed to the stipe and colored brown from spores.
Spores 8-10 x 3.5-4 µm, smooth, thin-walled, narrowly elliptical to subfusiform; spore print brownish-olive.
Solitary to gregarious in conifer woods; believed to be mycorrhizally associated with beach pine (Pinus contorta); relatively common north of San Francisco along the Mendocino coast; fruiting from late fall to mid-winter.
Probably edible but untried due to a viscid cap and minimal texture.
Suillus umbonatus is among the drabbest, least conspicuous of our local slippery jacks. It is recognized by a viscid, buff to pale olive-grey cap often with reddish-brown streaks, ochraceous tubes that do not discolor, and a viscid brownish annulus. Despite the species name, the presence of an umbonate cap is not a consistent character and may be lacking in some collections.