A.Stephen Quarles, cooperative extension advisor on wood durability at the University of California Forest Products Laboratory in Richmond, Calif., responds: Based on your description, the degradation affecting your porch was caused by a decay fungus. The velvety appearance you observed on the surface of the wood was mycelium, a term used to describe a mass of hyphae (thread-like cells of the fungus that break down the chemical constituents that make up wood into food that the fungus can absorb).
The color of the mycelium can vary, depending on the particular fungal species and how dry it is. On the back of building paper installed on a wall, I have seen white mycelium so thick that other people present mistook it for housewrap.
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