Arthonia destruens (All Fungi)
Anamorph: conidia formed by fragmentation from outer wall cells of the ascomata, 3.5-5 µm diam, ± spherical, hyaline, aseptate.
Teleomorph: Ascomata 100-250 (-400) µm diam, scattered to crowded in groups of about ten, then often confluent, circular to irregular in outline, erumpent from the host thallus, sessile and cushion-like, blackish (also when moist), with a rough surface, 50-100 µm tall. Ectal excipulum formed by a down-turning epithecial layer, occasionally forming thalloconidia at the periphery. Epithecium brownish (due to presence of intercellular and intralaminar granular pigment) with a yellowish tinge, 5-15 µm tall, composed of dense but not interwoven cells ca 5 µm diam, covered by a hyaline gelatinous layer ca 5 µm thick. Hymenium yellowish (containing amorphous pigments), 25-50 µm tall. Hypothecium 20-45 µm tall, yellowish brown, composed of ± roundish cells 3-4 µm diam, pigmentation as in the epithecium, often with integrated groups of algae which are densely entwined by hyphal cells. Interascal tissue of paraphysoids, scanty, occasionally branched and anastomosed, 0.7-1 µm diam. Asci 29-43 x 11-15 um, clavate, with an indistinct foot, 4- to 8-spored, lacking any 1+ or KI+ structures. Ascospores 10.5-17 x 5-7 µm, ovoid-cylindrical, becoming brown and verrucose, 1-septate, slightly constricted at the septum and with a slightly larger upper cell.
Chemistry: Epithecium and hymenium I+ dark blue, hypothecium I+ reddish brown, all parts KI+ deep blue; yellowish pigments K+ turning purplish violet and dissolving.
Superficially similar to A. epiphyscia, which also grows on Physcia spp., but distinguished by having an internal yellowish, K+ purple-lilac pigment and longer ascospores (10.5–17 vs. 10–14 µm).
Parasitic on thalli of Physcia species, causing bleaching of the substratum and sometimes reddish discoloration, possibly also independently lichenized. In GB&I known from P. tenella but also reported from P. aipolia and P. stellaris. Records from other lichen host genera are probably erroneous.
In GB&I known from Wales (Cardiganshire), also found in Austria and Germany.