Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: shrubby, compact, 2-8 cm long branching: isotomic-dichotomous, divergent basal part: distinctly paler than branches or of same color, with numerous and conspicuous annular cracks branches: usually ±irregular; lateral branches: not narrowed or slightly to distinctly narrowed at point of attachment; transverse furrows: rare on Mexican material but present on the type material from Taiwan segments: terete to distinctly ridged, cylindrical to weakly sausage-like papillae: absent tubercles: present, low and verrucous, whitish or eroded on top (probably very young stages of fibrils), not sorediate fibrils: short (1-3 mm), often spinulous, mostly present in oldest part of thallus isidiomorphs: absent cortex: thin to thick (7-10%), shiny like glass, distinctly transversally and longitudinally cracked at the base of main branches medulla: compact to dense and often orange pigmented close to the axis axis: thick Apothecia: present but not numerous, up to 10 mm in diam., mainly terminal to subterminal on terminal branches ascospores: ellipsoid, 8-10 x 5-6 µm Spot tests: medulla K+ yellow turning red, C-, KC-, P+ orang-ish yellow Secondary metabolites: norstictic (major) and salazinic (major) and usually galbinic (major) acids, or diffractaic acid (major). Substrate and ecology: on bark or rock in mixed pine-oak forests between 1600 and 2500 m or in thorn forest at lower elevations World distribution: Taiwan and Mexico Sonoran distribution: Sierra Madre Occidental in Chihuahua and Sierra de la Laguna in Baja California Sur. Notes: Among the apotheciate species without soralia, U. shimadai is distinguished by the irregular branches, the tubercles that are whitish on the top, the shiny cortex that is conspicuously cracked at the base of main branches and the production of norstictic, salazinic and ±galbinic acids / diffractaic acids in the medulla. Usnea strigosa lacks the eroded tubercles and has more densely and regularly disposed fibrils. The relationships with U. cristatula Motyka are not well understood yet and further detailed taxonomical work is needed. The specimens with diffractaic acid are only tentatively placed here.