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DescriptionBasidiomata: at first absent, fungus inducing the formation of olivaceous to brownish, sometimes reddish brown, flat to slightly convex galls with a dull surface, 0.4-2 mm in diam., when old, developing into dark brown and strongly convex, sometimes tuberculate, gelatinous basidiomata; hyphae: context hyphae: 3-4 µm in diam.; clamps: not observed; haustorial branches: frequent, tremelloid, with clamps; mother cell: ellipsoid to subspherical, 3-4 x 2.5-3 µm; haustorial filament: 0.5 µm in diam., 1.5-2.5 µm long; hymenium: hyaline, 35-55 µm tall, containing numerous conidiogenous cells intermixed with probasidia; probasidial initials: clavate, proliferations occurring through the basal clamp; hyphidia and cystidia: absent; basidia: when mature, 2-celled, with one transverse septum, with the lower cell generally attenuated at the base, with the upper cell often shorter but larger, 15-27 x 4.5-8 µm; epibasidia: subcylindrical, 2-3 µm in diam., 25-30 µm long; basidiospores: subspherical to ellipsoid, with a distinct apiculus, 6-8 x 5.5-7.5 µm; Anamorph: asteroconidia: frequent in older basidiomata, with 4 arms, 8-11 µm in diam.; individual arms: 2.5-5 µm long; conidiogenous cells: irregularly cylindrical, with a few short, 1-4 µm long ramifications near the apex, 18-28 x 2.5-3.5 µm; conidia: sometimes present, catenate; individual cells: 3-4 x 2.5-3 µm.; Hosts: thalli of Phaeophyscia species, inducing the formation of slightly to distinctly convex, olivaceous to brownish galls; World distribution: Europe (very common) and North America; Sonoran distribution: known from southern California, probably overlooked.; Notes: Tremella phaeophysciae is easily recognized by its darker, olivaceous, +round or ellipsoid spots on the Phaeophyscia thalli that soon become swollen and convex. Microscopically it is the only species of Tremella studied here having 1-septate basidia with a transverse septum. The species should not be confused with Syzygospora physciacearum, a relatively common fungus occurring on Physcia, Physconia and Heterodermia, but unknown on Phaeophyscia.