Life habit: saprobic or parasitic Thallus: endosubstratic, the substrate not noticeably stained by its presence Apothecia: 0.5-0.7 mm tall, olivaceous to grayish brown to dark brown or almost black, epruinose, shiny, stalk: occasionally forked, 0.04-0.06 mm in diam., black or olivaceous brown, the surface often with a slight reddish tinge in section, K+ first darker and then strongly swelling and slightly reddish brown; hyphae: narrow, slightly intertwined, sclerotized, dark, with a reddish brown or aeruginose tinge, 4-5 µm in diam., mostly periclinally arranged, brown centrally; surrounded by a 2-5 µm thick hyaline gelatinous coat capitulum: cup-shaped to obconical; epithecium: brown to reddish brown, 5-12 µm thick, consisting of layers of anticlinallly arranged, sclerotized hyphae exciple: brown to reddish brown, with an aeruginose tinge in the inner part, 6-19 µm thick, formed by 3-6 layers of periclinallly arranged, sclerotized hyphaeexciple and epithecium, K- or slightly intentisified reddish; hypothecium: hyaline, 30-60 µm high, consisting of largely periclinally arranged, thin-walled hyphae with occasional branches at right angles asci: narrowly cylindrical, 75-83 x 4-5 µm, with uniseriate spores, 8-spored ascospores: , 1-septate (septum forming rather late, poorly pigmented and difficult to see without oil immersion), medium brown when mature, ellipsoid to narrowly ellipsoid, 12-17 x 5-6.5 µm; surface: indistinctly marked to distinctly papillate (appearing smooth under light microscope) Pycnidia: not seen Spot tests: stalk K+ slightly reddish brown, N+ slightly reddish brown in section Secondary metabolites: none detected. Substrate and ecology: on bark of thin, dead branches of living trees, trunks and dead trees, and main stems of Populus spp. World distribution: boreal and temperate zones of Eurasia and North America Sonoran distribution: Arizona. Notes: Phaeocalicium populneum is characterized by the one-septate spores with poorly pigmented septum, the olivaceous or grayish brown color of the stalks, the smooth spores, the usually faint K+ reaction of the stalk, and the occurrence on twigs of Populus. See Tibell (1996b) for more information.