Fruiting body annual, effused-reflexed, shelving, 4.0-13.0 cm long, up to 5.0 cm in width; cap shell-shaped, the margin persistently incurved, arching over, and partially covering the pore surface; upper surface when young covered with a whitish matted-tomentum, becoming buff to pale-tan in age, eventually nearly glabrous to sometimes wrinkled; context approximately 1.0-2.0 cm thick, two-layered, the upper half tan, cottony, soft, the lower portion whitish and rigid; odor not distinctive; taste acrid to somewhat bitter.
Pores 2-3/mm, angular, the edges minutely fringed in youth, dentate in age (use hand lens), at first cream, pale-tan in age; tube layer up to 5.0 mm thick, becoming hard/rigid at maturity.
Spores 4.5-6.5 x 1.0-1.5 µm, narrowly-oblong to sausage-shaped, smooth, thin-walled, hilar appendage not obvious, inamyloid; spore deposit not seen.
Solitary or imbricate in small groups on wood of montane conifers, particularly red fir (Abies magnifica) and Lodgepole pine (Pinus contorta); fruiting in late spring, dried fruiting bodies persisting through the summer; common.
Inedible; tough, unpleasant taste
Oligoporus leucospongia is a snowbank species found throughout the montane regions of the western United States. It occurs commonly in the spring at higher elevations of the Sierra Nevada and presumably the Coast Ranges as well. An important fieldmark is the soft, white, upper surface which partially encloses the pore layer. Oligoporus leucospongia is sometimes found fruiting with another annual polypore, Pycnoporellus alboluteus. The distinctive orange hues of the latter, however, make it unlikely to be confused. Both species have fruiting bodies with a foam-like quality and are surprisingly light for their size.