Phellinus robiniae, also known as the cracked cap polypore, is a member of the Phellinus genus. They are perennial polypore mushrooms, both parasitic and saprotrophic. Commonly found on locust trees, but may also grow on acacia, chestnut, hickory, mesquite, oak, and walnut trees.
You'll find Phellinus robiniae growing on nearly every black locust tree due to the locust borer beetle (Megcallene robiniae). As their name states, the beetles bore tunnels into locust trees. These tunnels are ideal conditions for Phellinus robiniae spores to germinate. Unfortunately for the locust trees, the tunnels often lead directly to the heartwood, which the fungus attacks, causing heart rot.
Phellinus robiniae is sessile (no stipe), and resupinate, meaning that the spore-bearing tissue (hymenium) connects directly to the substrate. The shape of the mushroom typically appears as semicircles growing from the margins (edge). Each year a new layer of pore surface accumulates at the cap margins.
Giant Phellinus robiniae
The largest species I found was 56 cm in diameter, 16 cm high, and 24 cm protruding from the tree. Surprisingly the specimen was ground level allowing for close examination.