The mould, so rare it is known only by its scientific name Badhamia foliicola, was spotted in Castle Eden Dene Nature Reserve, near Peterlee, by an eagle-eyed member of the public during a Fungus Foray guided walk last October.
Initially thought to be a more common type of fungus, experts have since been analysing the discovery and it has now been confirmed by Tom Kirby, of the North-East Fungus Study Group, as the rare species.
The Association of British Fungus Groups only knows of 24 sightings of the species in the UK since 1965, the last being in Kent five years ago.
Joe Davies, of Natural England, said: “The identification of Badhamia foliicola at Castle Eden Dene is an unexpected new record for Castle Eden Dene National Nature Reserve and a significant addition to the bio-diversity of the site.
“The discovery underlines the Dene’s role as a natural treasure house for a wealth of wildlife, which ranges from the tiniest fungi to huge and ancient yew trees.”