Etymology: The species epithet refers to the pruinose [Latin: pruinosus] discs [Latin: discus] Thallus: superficial, well-delimited, rimose to rimuloseareolate; prothallus: absent areoles: 0.1-0.5 mm across, plane to slightly convex, irregular in outline, occasionally becoming crenate surface: yellowish white goniocysts: absent soralia: absent cortex: dirty yellow-brown in upper part, 50-75 µm thick, paraplectenchyamtous, with 3-4 µm in diam. lumina, covered by thin layer of fine, brownish granules (insoluble in K) medulla: with coarse irregular hyaline granules (insoluble in K) and very coarse (15-20 µm across), hyaline crystals; photobiont: arranged in distinct groups c. 100 µm deep Apothecia: mostly irregularly arranged, in small groups, not parallel, +round to ellipsoid or fusiform, or occasionally elongate, sometimes branched, 0.3-0.5(-1) x 0.2-0.3 mm disc: brown to yellowish brown, open, plane to slightly convex, pale, +distinctly whitish pruinose (appearing pale yellow or orange, or less often bluish gray) margin: narrow, +level with disc, paler or blacker than disc exciple: yellow-brown epihymenium: brown, 10-15 µm thick, with coarse, irregular yellow-brown granules (insoluble in K) hymenium: hyaline, 75 µm high; paraphyses: gelatinized, indistinct, with brown tips; ascogenous layer: hyaline; hypothecium: yellow-brown asci: clavate, c. 60 x 15-20 µm, 8-spored ascospores: hyaline or a few old spores becoming gray-brown, simple, ellipsoid, 13-16 x 5-6 µm Pycnidia: not infrequent, 0.1-0.2 mm in diam., +round; wall: grayish brown to black conidia: filiform, curved, 12-15 µm long Spot tests: apothecia (and thallus?) K+ yellow then red (numerous acicular crystals seen in squash mounts under the compound microscope), P+ orange Secondary metabolites: norstictic acid. Substrate and ecology: on wood, in spruce-fir communities World and Sonoran distribution: Arizona (Apache and Cochise Counties), 2770-2800 m. Notes: Xylographa pruinodisca is similar to X. crassithallia in having a thick thallus with few or no goniocysts, but differs from that species (and X. parallela) in having somewhat larger, sessile to constricted apothecia, irregularly arranged in small groups (mostly not parallel), with paler, mostly more pale yellow or orange and distinctly pruinose discs, and apothecial margins often paler or darker than the discs, and in containing norstictic acid (K+ red). It differs from X. trunciseda (known from outside the Sonoran region) in having sessile and +constricted apothecia with pale discs. Furthermore, its thallus is well developed, its apothecia are larger and irregularly arranged in small groups, its discs are distinctly pruinose, and its spores are over 13 um long. It is also similar to X. hians Tuck. s. lato (which occurs from Madera Co., California north to British Columbia, and includes X. micrographa G.K. Merr.) in chemistry (K+ red, containing norstictic acid) and in having mostly discrete (grouped but not aggregated or confluent), more sessile apothecia with +pale, mostly short and irregularly oriented discs. It differs from that species in having pruinose, simply structured (not convolute or aggregated) discs, and a distinct and rather thick epixylic thallus. Xylographa pruinodisca differs from X. opegraphella Nyl. ex Rothr., which also contains norstictic (or stictic) acid, but occurs on driftwood in northeastern North America, in having larger apothecia and spores.