Thallus: crustose, episubstratal, indeterminate, irregularly spreading, up to a few cm in diam., thin and continuous to thick and warty, uneven; prothallus: generally present but mostly indistinct, forming a delimiting black line up to 250 µm wide, visible when abutting adjacent lichen thalli granules: up to c. 150 µm wide, arising over surface of thallus, even, often indistinctly areolate, rarely with esorediate areoles up to 0.l1 mm in diam., soon becoming completely dissolved in soredia surface: straw colored to dull sulfur yellow or yellowish fawn (in shade greenish yellow), sometimes with reddish brown tint, often entirely dissolved into a mealy sorediose crust soredia: pale yellow, usually with a yellowish brown tone, farinose to granular, diffuse, soon becoming confluent, forming a +thick leprose sorediate crust which may crack and form +angular portions, 1-2 mm across, in rounded consoredia up to 50 µm in diam. medulla: indistinct or absent; photobiont: green, coccoid, up to 12 µm in diam. Apothecia: often absent but when present not rare, 0.3-1(-1.5) mm in diam. disc: dark red-brown or rusty-red, sometimes blackening, becoming strongly convex, often irregular in shape margin: entire or undulate, soon excluded epihymenium: inspersed with reddish brown granules, K+ dissolving, purple hymenium: hyaline, c. 50 µm tall, I+ blue asci: clavate, 8-spored ascospores: hyaline, simple, short ellipsoid to ellipsoid, (7-)8-12(-14) x (5-)6-7(-8) µm Pycnidia: not found Spot tests: thallus K+ yellow (difficult to see against a yellow thallus), P- or weakly yellow, C+ and KC+ orange-red, UV+ orange or red Secondary metabolites: thallus of two chemotypes (J.A. Elix, personal communication): I. with 6-O-methylarthothelin (major), arthothelin (major), thiophanic acid (minor), thiophaninic acid (minor), 2,4-dichloronorlichexanthone (minor), 7chloroemodin (minor = orange pigment), atranorin (trace) and 4,5-dichloronorlichexanthone (trace); II. thuringione (major), arthothelin (major), thiophanic acid (minor), 2,4-dichloro-3-O-methylnorlichexanthone acid (minor), 2,4-dichloronorlichexanthone (minor), 7-chloroemodin (minor = orange pigment), atranorin (trace), 4,5-dichloro3-O-methylnorlichexanthone (trace) and 4,5-dichloronorlichexanthone (trace). Substrate and ecology: usually on acidic to intermediate, moderately nutrient-rich, rough bark (especially Quercus; also on Pinus, Prunus, Sambucus, Olea, Heteromeles, Artemisia), occasionally on wood, very rarely on sandstone rocks; mostly in open, well lit situations, in humid, warm, low elevation areas, including pine woodlands, pine-oak chaparral, oak woodlands, and coastal sage communities World distribution: oceanic regions of Europe, Macaronesia, China; Japan, and western North America Sonoran distribution: coastal, southern California, Baja California and Baja California Sur at 20-440 m. Notes: Pyrrhospora quernea is characterized by a leprose, pale yellow to yellowish fawn thallus with diffuse soredia, the presence of arthothelin, plus either 6-O-methylarthothelin or thuringione, as major metabolites, and the absence of phenolic carboxylic acid derivatives. When fertile it is easily recognized by its small dark red-brown apothecia. When sterile, it can be confused with various other sterile lichens. Lecanora expallens is thinner and less uniformly sorediate and has a much more distinctly yellowish green thallus (lacking a fawn tint) and lacks a distinct black prothallus; its thallus contains usnic acid and zeorin in addition to thiophanic acid as major constituents, plus unknown substances.