Arthonia invadens

Thallus absent, lichenicolous; weakly parasitic in the thallus of the host, Schizotrema quercicola, suppressing or sometimes eliminating production of soralia. Apothecia evenly and mostly densely scattered, developing below the surface of the bark but soon erumpent, dark brown, not pruinose, linear to more usually stellate or stellate-dendritic, 0.2–0.8 (–1) mm diam., linear apothecia or stellate-rays very narrow, 30–80 (–100) μm broad; in section 54–60 μm tall, I+ blue; epithecium 7–12 μm tall, red-brown, K+ olive; hymenium 30–40 μm tall, colourless or pale red-brown in places; hypothecium indistinct, less than 10 μm tall, colourless; paraphysoids rather scanty, branched, 1(–1.5) μm diam.; apices often brown-walled and to 2 μm diam., without dark caps. Ascospores 11–15 (–17) × (4–) 4.5–5.5 (–6) μm, 2- or 3-septate, the apical cells enlarged, cylindric-obovoid, at first colourless and often with a thin perispore, later covered in brown, granular warts. Pycnidia few, immersed, 40–50 μm diam., the wall red-brown, K+ pale green; conidia bacilliform, 3.8–5 × ca 0.8 μm. Lichen products not known. 

Recently sequenced and shown to be in the Coniocarpon-Reichlingia clade (Ertz et al. 2019), but its systematic position needs to be fully established. In circumstances where the lichenicolous nature is not recognised, the stellate non-pruinose apothecia distinguish this species from Reichlingia anombrophila. There are nearly always some relic soralia present from the host on the fringes of the invasion, but rarely these are absent altogether.