01/31/2020
Phellinus robiniae, commonly known as the “Cracked Cap Polypore,” is a woody bracket fungus that is most easily identified by its habitat. This fungus grows almost exclusively on locust trees. In fact, the fungus is such a common pathogen of locusts that nearly every Black Locust tree has at least one P. robiniae mushroom on it. The mushroom is also distinguished by its furrowed cap – which gives the fungus its common name – and its dull brown pore surface. Phellinus robiniae is just one of many woody brown polypores that can be found growing on living or recently deceased trees. It usually appears on the side of a tree and grows out and down in semicircular layers from its point of attachment. The Cracked Cap Polypore is a parasitic and saprobic species that infects living and dead wood of Robinia trees. P. robiniae selectively degrades the lignin in trees, causing a white rot. Infected trees are usually either Black Locust (Robinia pseudoacacia) or New Mexican Locust (Robinia neomexicana). Infection by P. robiniae is so pervasive among Black Locust trees that nearly every tree-sized Black Locust you find has at least one Cracked Cap Polypore mushroom on it. The success of P. robiniae is aided by the Locust Borer beetle, Megcallene robiniae, which tunnels into the wood of locust trees and creates openings for the fungus to attack. P. robiniae occasionally appears on other trees including: acacias, chestnuts, walnuts, mesquites, and oaks (although very similar species also occur on those trees, making identification much more difficult). The Cracked Cap Polypore grows wherever its host trees are present, which today includes most of North America and some parts of Europe and Asia.
Photo: Judy Gallagher
