E-Flora BC: Electronic Atlas of the Flora of British Columbia

Summary:
Postia leucospongia forms a whitish shelf or bracket with whitish pores growing on the underside of a tough white layer that is covered on the upper side with soft spongy cottony material that usually extends over the margin below the pore surface and often partly covers it. Fruitbodies develop under snowbanks in the spring on dead wood in the mountains. The online Species Fungorum, accessed April 20, 2020, gave the current name as Tyromyces leucospongia (Cooke & Harkn.) Bondartsev & Singer, and Mycobank, accessed the same day gave the current name as Spongiporus leucospongia. The description is derived from Gilbertson(1).

Postia leucospongia has been found in BC, WA, OR, ID, AB, YT, AK, AZ, CA, CO, MT, NM, NV, UT, and WY, (Gilbertson). It is also known from the Himalayas.

Cap:
2-10cm long, 1-5cm wide, shelf-like or bracket-like to somewhat irregular, usually elongated along wood, convex; "white to buff, grayish-buff, pinkish, or even brown or cinnamon; usually velvety and very soft or spongy to the touch, but sometimes papery and fairly firm if the velvety layer wears away; margin usually extending below the pore surface and sometimes partially covering it", (Arora), up to 3cm x 7cm x 3.5cm, flat on wood with pore surface exposed but bent back to form shelf-like cap, to bracket-like, dimidiate [roughly semicircular] to elongated; upper surface white, not zoned; smooth or rugose [wrinkled], tomentose to papery, soft; margin growing down and partially enclosing pore surface, rounded, sterile underneath, (Gilbertson), 3-10cm long, 1-4cm wide, 0.5-2.5cm thick, "somewhat rounded, soft and watery when fresh"; "white sometimes tinted dull cinnamon-pink"; smooth to slightly rugose [wrinkled]; "margin extends down partially over the pore surface and is sometimes reddish brown", (Bessette), single shelves, or elongated, or sometimes flat pore layers on the underside of logs, (Trudell)

Flesh:
a rather tough white layer covered by a thick (up to 1.5cm), "soft, spongy layer of cottony tissue that is pallid to pinkish-buff or cinnamon", (Arora), up to 2cm thick, duplex: soft and cottony in upper part, firmer near the tubes; white, not zoned, (Gilbertson), up to 1.5cm thick, 2-layered, "with a soft, cottony upper layer and a firm zone just above the pores"; white, azonate, (Bessette), soft, cottony flesh gives light weight to the fruitbodies (Trudell)

Pores:
2-3 per mm, angular, often becoming torn and tooth-like when old, "white when fresh, often discoloring (buff to tan, brownish, or cinnamon)" when old or when dried; tube layer 0.2-0.6cm thick, (Arora), 2-4 per mm, circular to angular, white to pale buff; rough, walls at first thick, becoming thin and deeply torn; tube layer up to 0.5cm thick, pale buff, hard and brittle, (Gilbertson), "3-4 per mm, circular to angular, minutely roughened, surface soft, white to cream color", (Bessette)

Stem:
absent or rudimentary (Arora)

Odor:
not distinctive (Bessette)

Taste:
mild (Gilbertson)

Microscopic:
spores 4.5-6 x 1-1.5 microns, allantoid [curved sausage-shaped], smooth, inamyloid, colorless; basidia 4-spored, 16-23 x 4-5 microns, clavate, with basal clamp connection; cystidia absent, hyphal pegs present; hyphae monomitic, hyphae of context 3-7 microns wide, "with abundant clamps, rarely with double clamps, thin- to thick-walled, with frequent branching", "some contorted, with short, nodular branches or swellings and appearing twisted", hyphae of trama similar to non-contorted type, (Gilbertson), spores 4-6 x 1-1.5 microns, more or less sausage-shaped, smooth, (Arora)

Spore Deposit:
white (Arora)