Arthonia caesia (aka “Frosted Comma Lichen”)

Identification Characteristics

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