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DescriptionMycelium: I+ blue, 2-4 µm diam.; Apothecia: superficial, 200-400 µm in diam.; disc: black, rarely pruinose when young, from the beginning strongly convex; epihymenium: dark greenish brown to black, K+ green, sometimes with a granulose layer giving a yellow solution in K; hymenium: hyaline to pale brown, in the upper part pale green, K+ green; hypothecium: dark reddish brown, K-; asci: elongate ellipsoid to clavate, 8-spored; ascospores: medium to dark brown, verrucose, 1-septate, cells unequal in width, (13-)13.5-16 x 5-7 µm; Pycnidia: black, immersed, or the upper third erumpent, ostiolate; wall: 20-35 µm thick, pale brown in the lower part, dark blackish brown in the upper part, dark olivaceous around the ostiole, K-; conidiogenous cells: holoblastic, elongate ampulliform to subcylindrical, hyaline, 10-17 x 3-4.5 µm; conidia: hyaline to pale yellow, ellipsoid to elongate ellipsoid, simple, basally distinctly truncate, surface ±smooth, 7-14 x 4.5-6.5 µm.; Hosts: thallus and apothecia of Xanthoparmelia species; World distribution: Europe, North America (U.S.A.) and South America (Bolivia); Sonoran distribution: Arizona and California.; Notes: Abrothallus caerulescens is easily recognized by its host, Xanthoparmelia species. A specimen published by Triebel et al. (1991) as Abrothallus bertianus De Not. and growing on Xanthoparmelia somloënsis is here included in the species concept of A. caerulescens. The mycelium is usually blue in I (epithet "caerulescens"!), although some specimens on Xanthoparmelia were reported in which the mycelium would not react. One such specimen was recently published as the new Abrothallus tulasnei (Cole and Hawksworth 2001). As long as no revision of the genus exists, I prefer to treat the entire material on Xanthoparmelia as a single species, for which A. caerulescens is the oldest name.