Gymnopilus penetrans, also known as the Common Rustgill, is a small to medium-sized saprotrophic agaric with a widespread distribution across temperate regions. It typically grows on decaying wood, especially coniferous stumps and fallen logs, where it plays an essential role in breaking down organic matter. The mushroom features a rusty-orange to cinnamon-brown cap, which is convex to flattened, often with a dry and slightly scaly surface. Its gills start out pale, gradually turning rusty-brown as the spores mature, giving the Common Rustgill its characteristic appearance. Gymnopilus penetrans is found throughout the autumn months, sometimes persisting into early winter, adding its warm hues to forest floors.

Cap ochraceous with an apricot hue, lighter at the margin and gradually developing a more cinnamon tone as it ages. It has a silky-smooth texture and starts out convex, eventually expanding and becoming wavy. The flesh is the same colour as the cap, firm, and relatively thin. Gills initially straw-coloured or ochraceous, gradually turning more cinnamon at maturity, with orange-brown spots appearing. They are adnate with a slight tooth or sometimes slightly emarginate, and are densely crowded. Stem similar in colour to the cap, slender, and more or less uniform in thickness. It features a silky striate or fibrillose texture. A ring is either absent or appears as a faint zone when young. The flesh is yellowish, turning orange-brown when damaged, especially near the base, and is firm with a stuffed interior. Spore print rusty orange-brown.

Microscopic Features: The spores are ellipsoidal, measuring 7-9 x 4-5.5 µm, and are covered in very fine warts.

Gymnopilus penetrans on the First Nature Web site.
Gymnopilus penetrans on the California Fungi Web site.