Fruiting body perennial,woody, typically sessile, 6-60 cm broad, 5-10 cm thick, fan-shaped to slightly convex, rarely hoof-like, usually solitary; margin rounded early, becoming narrowed at maturity; surface a hard crust, dull grey, grey-brown to brown, irregular, often furrowed, nodulose and zonate, frequently dusted with brown spores. Flesh up to 6.0 cm thick, brown, tough, corky, blackening in KOH.
Pores 4-6 per mm, white, quickly bruising brown when injured, fading to pale yellowish-buff when dried; tubes multi-seried, 4-13 mm long, brown, each layer separated by a thin layer of tissue; tubes and pores blackening in KOH.
Spores 6-9.5 x 5.7 µm, broadly elliptical, blunt at the distal end, thick-walled, ornamented with minute spines; spores brown in deposit.
Solitary or in small groups on downed logs of both hardwood and conifers, also on living trees; especially common on Umbellularia californica (California bay).
Our best known shelf fungus, Ganoderma applanatum is distinguished from other woody polypores by a dull grey-brown, bumpy, usually zonate cap, often powdered brown from released spores, and a white pore surface which instantly darkens when injured. The latter feature makes it a favorite of artists which use the smooth white surface as a canvas. Like a number of perennial woody conks, the age of a fruiting body can be roughly determined by sectioning and counting the tube layers. Related fungi include Ganoderma tsugae, uncommon in our area, with a shiny red/brown to mahogany brown cap and pores that do not bruise and Fomitopsis pinicola with a reddish-brown cap margin and pores that bruise slowly pale yellowish-buff. One other Ganoderma that occurs in California is Ganoderma brownii. According to Gilbertson & Ryvarden, it is similar to Ganoderma applanatum but differs in having larger spores and a yellow pore surface.