Fruit-body a bright-orange mold growing on the surface of members of the Russulaceae (in California usually Russula brevipes); gills of the parasitized host reduced to folds; in age, minute, reddish-orange perithecial mounds develop (use hand lens); context of host firm, brittle, if Lactarius, then oozing a latex; odor at first fungal, then of shellfish; taste mild in R. brevipes, potentially peppery if Lactarius.
Ascospores 35.0 x 43.0 x 6.0-7.5 µm, spindle-shaped, apiculate, equally two-celled, thin-walled, warted, contents globular; asci uniserate; spores white in deposit.
Parasitic on Russula brevipes, and possibly other members of the Russulaceae; north coastal in distribution; fruiting shortly after the fall rains; occasional to common.
Edible and very good, but some allergic reactions have been reported; caution is advised since the identity of the host mushroom is not always apparent.
This Ascomycete parasite transforms its host, often Russula brevipes, from white to brilliant orange, and causes the gills to abort their development. The "gills," still produce spores, but only those of Hypomyces lactifluorum. These emanate from countless reddish-orange perithecial mounds which dot the surface at maturity.
While Hypomyces lactifluorum is most conspicuous member of the genus, other species are occasionally encountered. These include: Hypomyces chrysospermum which forms the familiar white to yellow cottony growths on boletes; Hypomyces lateritius, which despite its name meaning "reddish," produces a cream to pale-tan growth on the gills of Lactarius deliciousus and L. sanguifluus; Hypomyces aurantius is a yellowish-orange mold found on the spore-bearing surface of senescent polypores; Hypomyces luteovirens is an olive-colored species, occasional in California, that attacks the gills of Russula spp; Hypomyces cervigenus is the common whitish to pinkish mold seen on the surface of Helvella lacunosa; finally Hypomyces hyalinus, is a pallid species which attacks Amanita species, especially Amanita novinupta.