Brittle Cinder (Kretzschmaria deusta)
Kretzschmaria deusta, commonly known as Brittle Cinder, is a worrying plant pathogen which has the ability to seriously damage trees by causing deep seated decay within the root system and lower stem. The fungus produces wavy-edged cushions or crusts clinging to the substrate timber. Initially greyish with white edges, as shown in the image above, the fruitbodies turn black and become extremely brittle with age. Easily spotted when young, K. deusta is probably under-recorded because it can so easily go unnoticed once it darkens; old fruitbodies looking more like charred wood than anything produced by a fungus.
Maybe not one of the glamour species but certainly fascinating when viewed in close detail. To give an idea of size, the above specimen, recently photographed in local Surrey woodland, was approximately 35mm in diameter.
Sterry, P. and Hughes. B. (2009). Collins Complete Guide to British Mushrooms and Toadstools. London: HarperCollins, p. 328, fig. p. 329.