Thallus: shrubby, compact, 2-5 cm long branching: isotomic- to anisotomic-dichotomous, divergent basal part: usually distinctly jet black, sometimes concolorous or brownish black, often but not always with few to numerous longitudinal cracks branches: tapered; lateral branches: not narrowed at attachment points segments: terete, cylindrical papillae: few to numerous and then dense on main branches, verrucous tubercles: few to numerous, especially on main branches fibercles: generally absent fibrils: usually slender and ±long (2-3 mm long when mature), few to numerous especially on main branches, irregularly distributed soralia: conspicuous, even to ±excavate, as large as the branches or larger when mature, irregularly rounded to often oblong cylindrical when mature, often confluent, arising initially on the cortex and on top of tubercles isidiomorphs: only present on young soralia (sometimes very few and difficult to find), always absent on mature soralia cortex: dull to shiny, thin to thick (8-13%) medulla: in average thin, dense axis: thick Apothecia: not seen Spot tests: K-, C-, KC+ yellow, P-; or K+ yellow turning red, C-, KC+ yellow, P+ orangish yellow; or K+ yellow intense turning slowly orange, C-, KC-, P+ orange Secondary metabolites: barbatic acid (major), ±4-0-demethylbarbatic acid, ±salazinic acid (minor); or salazinic acid (major) alone; or thamnolic acid (major) and ±squamatic acid (minor). Substrate and ecology: mainly on bark of Quercus spp. or on scrubs in the chaparral between 200 and 500 m World distribution: Eurasia and western North America Sonoran distribution: rare on the Channel Islands (Santa Catalina, San Clemente, Santa Cruz and Santa Rosa) and more rarely on the mainland of southern California. Notes: Usnea wasmuthii is a difficult species to identify in western North America. The thallus is generally small and its typical characters are badly developed. Therefore, TLC is often needed to confirm identifications. Although it is close to U. subfloridana, it differs from that species by its larger soralia that are usually concave and oblong longitudinally when optimally developed. Rare morphs of U. subfloridana with large and concave soralia have numerous isidiomorphs. In U. wasmuthii, isidiomorphs are always few in number and only on young soralia. Usnea fulvoreagens has deeply excavate soralia adjacent places where the cortex is torn off, no isidiomorphs and a different chemistry. Usnea wasmuthii might be sometimes difficult to separate from U. substerilis. The latter species has, however, irregular branches (often with foveoles or transversal furrows), a distinctive anisotomic-dichotomous branching pattern, a basal part without longitudinal cracks and has a continental type of distribution (mountains of Arizona).