Thallus dark red-orange to medium orange; lobes rounded or finely divided, 0.2-0.6 mm wide, flat to somewhat convex, ascending to erect (especially in the northwest) or mostly horizontal (especially in the southeast), with mealy soredia on the edge of the lower surface, not forming hoods; lower surface white, with few rhizines or holdfasts.  Apothecia rarely seen; pycnidia common, resembling dark orange pimples; conidia baciliform, 3-4.5 um long.  HABITAT: Mostly on bark, sometimes on wood, and rarely on rock.  COMMENTS: Among the sorediate species of Xanthoria, X. fulva, X. ulophyllodes, and X. oregana are most similar to X. fallax.  Xanthoria fulva has narrower, more ascending lobes and lacks rhizines, and the grandular soredia are mainly on the lower surface.  Xanthoria fulva and X. oregana have soredia on the lower surface of the lobes, and the pycnidia are more prominent.

Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.

Thallus: foliose, forming small cushions up to 9 mm wide or extensive colonies, adnate to loosely adnate, lobate hymenium: hyaline below, 50-100 µm tall; paraphyses: simple to branched, cylindrical, septate, mostly with oil droplets in the uppermost cells; hypothecium: hyaline to pale brown, 25-55 µm thick asci: clavate, 8-spored ascospores: ellipsoid, polarilocular, hyaline, 11-20 x 5-9 µm; septum: 2.5-9 µm wide Spot tests: upper surface K+ purple, C-, KC-, P- World distribution: widely distributed mainly in boreal and temperate regions Sonoran distribution: rare in southern California and Baja California. Notes: Compared to other small sorediate species in the genus, Xanthomendoza fulva is generally darker orange pigmented. The thallus is smooth and somewhat shiny, and the lobes appear thin and delicate. Pycnidia, when present, are reddish pigmented and large compared to the lobe width, which make them very conspicuous. The thalli often coalesce into extensive colonies. Small thalli of X. oregana sometimes resemble X. fulva, but generally X. oregana is larger with longer and broader lobes, that are lighter orange pigmented, and the conidia vary in shape. See notes for X. 'galericulata' and X. fallax for comparisons with these taxa. upper surface: light to dark orange, smooth and shiny soredia: granular, blastidious, below lobe tips, in submarginal soralia Pycnidia: rare to abundant, protruding, dark orange to red-orange Secondary metabolites: parietin (major), fallacinal (major), emodin, teloschistin (major) and parietinic acid.