Byssoloma brunneodiscum is a species of foliicolous (leaf-dwelling) lichen in the family Ectolechiaceae. Described in 2020 from specimens collected on Hainan Island, China, this lichen forms smooth, dull-green films on living leaves in montane rainforest understorys. It is distinguished by its brown to dark-brown fruiting discs surrounded by thick white margins and its production of a chlorinated xanthone compound in its tissues.
Byssoloma brunneodiscum is a foliicolous member of the family Ectolechiaceae that was described in 2020 by Wei-Cheng Wang and Jiang-Chun Wei. Molecular analysis of mitochondrial small-subunit rDNA places it, together with B. annuum, B. melanodiscocarpum and B. rubrofuscum, in the well-supported B. subundulatum clade within Byssoloma. The holotype was collected by Wang on 6 September 2017 from Jianfeng Ridge, (Ledong County, Hainan), at 960 m elevation. The specific epithet brunneodiscum refers to the brown that are characteristic of the apothecia of this species.
The thallus forms a smooth, continuous, dull-green film on living leaves and lacks a distinct marginal . Algal cells are , 5âÂÂ7.5 üm in diameter. Apothecia are sessile, 0.3âÂÂ0.5 mm wide, with a plane brown to dark-brown that is encircled by a thick, persistent white margin. Beneath the disc, the excipulum and basal tissue consist of loosely interwoven colourless hyphae studded with crystals that dissolve potassium hydroxide solution (KOH) to produce a positive pink reaction; the is pale brown and measures 25âÂÂ30 üm high. The hymenium reaches 40âÂÂ50 üm tall; asci are 8-spored and amyloid, while the ellipsoid, colourless ascospores have three transverse septa and measure 11âÂÂ14 à3âÂÂ4 üm.
Asexual propagation occurs through , wart-like pycnidia (10âÂÂ13 üm across) that release non-septate, pear-shaped conidia 2âÂÂ2.5 à1.5 üm. Thin-layer chromatography detects the chlorinated xanthone compound 2,5,7-trichloro-3-O-methylnorlichexanthone in the thallus.
The species is known only from montane rainforest on Hainan Island, China, where it grows sparsely on smooth leaf surfaces in the damp, shaded understory at around 960 m elevation.