Wood-rotting fungus. Close-up of a wood-rotting fungus (Pulcherricium caeruleum) growing on bark. This is a corticiod or crust fungus. Crust fungi are found growing on the moist undersides of fallen logs and branches. These fungi are largely responsible for the decay of dead wood. Their fruiting bodies form a brightly coloured crust (here purple) which drops reproductive spores.