Wet rot - Coniophora puteana
Commonly know as the cellar fungus

Habitat:

Found in softwoods and hardwoods. This is the commonest cause of decay in woodwork which has become soaked by water leakage.

Damage characteristics:

The wood darkens, with cracks along and across the grain, but usually less deep than those caused by Serpula lacrymans. Where conditions cause the wood surface to dry, there may be an apparently sound skin of timber which may crack longitudinally as the decay progresses beneath. Freshly colonised wood usually shows a yellow colouration.

Fungal characteristics:

Mycelium Only present in conditions of high humidity. Yellow to brownish in colour, off-white under impervious floor coverings. May spread superficially over damp plaster or brickwork.

Strands are thin, usually brown to black, though yellowish when young.

Fruit-body is rare in buildings. Thin, lying flat on substrate and with small irregular lumps. Olive green to olive brown with a cream margin, paler when young.