Note from Anders: Very difficult to motivate a decline based on one known site only. Suggest D only. I also rather suggest NT than VU, as it explicitly states that it is difficult to identify.
Leucoagaricus dyscritus is a fungus known from just one locality in central coastal California, a very special Monterey cypress grove with an undisturbed litter layer, an open structure without invasive weeds and grasses in the understory, on an east-facing slope.The particular grove this species in known from is rich in rare fungi of Lepiota and Leucoagaricus species and under threat of disturbance and of being converted into an oak woodland. The species is not known to occur elsewhere. This species, though it has been looked for, is not that easy to recognize, so the actual distribution may be wider than just this one Monterey cypress grove, and it might also be in other habitats. For those reasons, it is assessed as Vulnerable based on having a very small population.
Endemic to California, U.S.A. Known with from only one locality in San Mateo County.
Known from one locality only. This is a Monterey cypress grove near Crystal Spring reservoir, where La. dyscritus has been collected repeatedly from 1999 to 2008. This is in a protected area. Despite surveys and inventories in other Monterey cypress habitats in coastal central California, not encountered. This species, is not that easy to recognize, so the actual distribution may be wider than just this one Monterey cypress grove, and it might also be in other habitats. However, it has been much searched for and the total population is estimated not to exceed 1000 mature individuals , i.e. less than 50 sites with 20 mature individuals each (cf. Dahlberg & Mueller, 2011).
Leucoagaricus dyscritus is a slender fungus with a reddish-brown cap. Microscopically the short elements of the cap covering and the simple cheilocystidia are characteristic. It belongs to the group of species that turn green in ammonia. It is saprotrophic, growing singly or in small groups in deep undisturbed litter of a >100-year old Monterey cypress grove on an east facing slope. The grove is open in character, without a dense shrub or herbaceous understory. This particular habitat is rare, because of the relatively undisturbed litter layer, the openness without invasive weeds, and the east facing aspect. It is rich in various very rare Lepiota and Leucoagaricus species, such as Lepiota luteophylla, L. rhodophylla, L. scaberula, and Leucoagaricus hesperius. Lepiota luteophylla and L. rhodophylla are only found in this habitat and this location. Leucoagaricus dyscritus’ spores are wind-dispersed.
The Monterey cypress location is under various threats. Access to the area is restricted, but its mycological richness attracts trespassers. The undisturbed duff layer and the lack of undergrowth are the main characteristics that allow for the mycological richness. A similar grove in the same restricted area is overgrown by Vinca major, basically eliminating all fungal growth. Trampling by humans and deer would change the character and the suitability for Lepiota species. The other fact that sets this particular grove apart is its location on an east facing slope. The Monterey cypress grove was planted in the early 1900’s and is considered non-native in the present location. Monterey cypress is a native tree species of California, but considered native in only a very few locations on the coast. Plantings have also mostly taken place on west-facing slopes right on the coast. Management proposals to convert the present Monterey cypress grove into native oak forests have been thwarted for the time being, but could be rescinded.
Awareness of the rarity of the taxon has to be raised, and the known population has to be monitored. Monterey cypress groves in general have to be put off-limits to disturbance, so that the duff layer can develop. The Monterey cypress grove is home to a number of very rare and endangered Lepiota and Leucoagaricus species, and should be maintained and not converted into Oak and Douglas fir woodlands.
Vellinga, E.C., 2010. Lepiotaceous fungi in California, U.S.A. Leucoagaricus sect. Piloselli. Mycotaxon 112: 393–444.