False Tinder Fungus (Phellinus igniarius) is the most common cause of White Trunk Rot in North America. It infects 21 genera of hardwood trees but is found most often on birch. It turns the wood of infected trees into a soft, white, spongy mass. The fruiting body is a woody, bracket-shaped conk with pores on the underside. It is usually found on the trunk of a living tree.
This species is identified by the host (hardwoods); the black, often cracked upper surface of older specimens; the dark brown or rusty brown flesh; the brown underside; the layered pores; and the white lines running through the flesh, visible in cross section.