Arthonia caesia (aka “Frosted Comma Lichen”)
Description: Thallus thin, yellowish white to yellowish green, granular or powdery. Ascomata small (0.2 – 0.4 mm in diameter), heavily blue-gray pruinose, convex, marginless; internal tissues dark brown.
Spores: 15 – 24 x 4 – 6 um, 4-celled, with the upper 2 cells slightly larger or equal to the lower 2, & slightly constricted in the middle.
Photobiont: Green (unicellular, not Trentepohlia).
Chemistry: Spot tests unreliable (contains usnic acid & probably triterpenes in the thallus).
Substrate: On bark, mainly deciduous trees & shrubs such as willow, alder, & maple.
Lookalikes: The combination of the small, blue-gray, marginless ascomata on a powdery yellowish crust makes this lichen easy to identify. Sterile specimens, however, closely resemble species of Lepraria or Lecanora thysanophora. Old specimens of Arthonia caesia develop a fuzz of colorless needles, indicating the probable presence of triterpenes.
Bibliography: Lichens of North America, by Brodo, Sharnoff, & Sharnoff
Database Entry: Distance Everheart 12-26-13