Once the short peg like isidia are spotted, the combination of these and the sulphur yellow thallus and the very bright yellow-orange UV fluorescence makes this a distinctive Pertusaria. Found on older well lit trees on mesic bark and is characteristic of species rich Mature Mesic Bark Communities (Pertusarietum amarae). Appears to be declining, probably from a combination of pollution and habitat loss.
Thallus pale or bright sulphur yellow to yellow-green-grey or yellow-brown, sometimes with a pale to dark grey margin, rather thick and uneven, ± coarsely rimose; upper surface coarsely warted, flat, smooth or somewhat roughened; isidia numerous, globose or shortly cylindrical-clavate, occasionally dissolving into soredia and obscuring the thallus, leaving faint cortical scars when shed. Apothecia very rare, 2-5 immersed in hemispherical sorediate-isidiate warts; disc black-brown, punctiform. Asci (4-) 8-spored. Ascospores 60–100 × 25–40 µm, wall 7–10 µm thick, less than 20 µm thick at the ends. Thallus C+ orange, K–, KC+ orange, Pd–, UV+ bright orange or yellow-orange (thiophaninic and an unidentified substance).
Distinguished by the thick, abundantly isidiate yellow-tinged thallus and C+ and UV+ orange reactions. Three yellow-green C+ orange crustose species can cause confusion: Lecanora expallens has a less robust, thinner, distinctly sorediate thallus containing usnic, zeorin and thiophaninic acids, and is UV+ dull orange; Pyrrhospora quernea has a brownish tinge, is ± continuously finely sorediate, and has arthothelin and thiophaninic acid; Lecanora alboflavida has convex bright ± discrete yellowish-grey efflorescent soralia and contains atranorin as well as arthothelin.
A single occurrence of Sphinctrina turbinata is the only lichenicolous fungus reported.
On smooth and rough mesic bark of well-illuminated older broad-leaved trees, especially old Oak, in open woodlands, parklands and on wayside trees. A characteristic species of well developed Mature Mesic Bark Communities (Pertusarietum amarae) on field and parkland trees and a useful lead in species when searching for the rarer lichens found in this community.
Throughout S. and E. Britain, central and N.Wales, scattered in Scotland, rare in Ireland.
Not assessed as threatened in 2012, but the large number of older records, suggests a both failure to recover from past acidifying pollution and that there are going threats. The latter are probably from a combination of pollution from raised ammonia levels and habitat loss of older trees directly to felling and disease and loss of habitat to the spread of Ivy on trunks.
Cannon, P., Chambers, S., Coppins, B., Sanderson, N. & Simkin, J. (2021). Pertusariales: Pertusariaceae, including the genus Pertusaria. Revisions of British and Irish Lichens 6: 1-12.
Text by Neil A Sanderson based on Cannon et al (2021)