It can only grow in the faeces of horses and ponies fed on acid grasslands with no use of artificial fertiliser. It also has to pass through their bowels before it can grow at all. Once fairly widespread in Britain, it is now thought to be almost completely confined to the forest, in Hampshire, because of the decline of horse grazing in that kind of habitat.
In autumn these discs eject spores on to the surrounding grass. They - or something they grow into (no one has yet found out what happens to the spores) - have to be eaten by a pony during the next autumn before the fungus can begin to grow in newly deposited dung.
This species, Poronia punctata, is one of four fungi for which rescue plans have been proposed by a committee of civil servants, wildlife scientists and conservation groups.
Costed plans for a total of 116 plant and animal species and 14 types of habitat were published by the committee last December. This week the Secretary of State for the Environment, John Gummer, will give his response, and indicate the kind of backing the Government is willing to give to the initiative, part of Britain's response to the 1992 Rio Earth Summit and the biodiversity treaty signed there.
The action plan for the nail fungus calls for it to be restored to 10 former sites by 2004, to study its requirements and maintain its New Forest population. The key requirement is to maintain horse and pony grazing on the right type of pasture.