An ascomycete on Alnus (alder) bark, visible as groups of hard, hemispherical cushion-like stromata or “warts” of several mm diam. Indistinguishable from Hypoxylon fuscum (hazel woodwart) in the field, but producing a vinaceous purple colour reaction in 10% KOH, and possessing smaller ascospores with more acute ends.
Stromata erupting from bark, cushion-shaped, slightly constricted at the bases, in groups, separate to merging together, 1.4–3 mm diam. × 0.8–1.4 mm thick, containing perithecia. Surface covered in fine powder (pruina), brown vinaceous in colour, slightly uneven, with perithecial contours not exposed, with a thick layer of yellowish waxy granules beneath the surface. Pruina made up of red brown granules. Tissue beneath the perithecia 0.5–1.2 µm thick, greyish brown with blackish marks, soft-textured. Perithecia subglobose to obovoid, rarely slightly tubular, 0.32–0.38 mm high × 0.13–0.22 mm diam. Ostioles (perithecial openings) with a central depression, inconspicuous. Asci cylindrical, short-stipitate, 8-spored, readily deliquescent, 100–120 µm total length, the spore bearing-parts 70–84 µm long × 7–8 µm broad, the stipes 24–42 µm long, with a discoid apical ring 0.5–0.8 µm high × 3–3.4 µm broad, bluing in Melzer’s reagent. Paraphyses filiform (thread-like), septate. Ascospores 9.5–12.5 × 5–6 µm, ellipsoid slightly inequilateral with narrowly rounded to acute ends, brown, smooth, with a conspicuous spore-length sigmoid (s-shaped curve) germ slit, swelling rapidly in water. Perispore (spore covering) detaching in 10% KOH, thin-walled, with faint transverse striations; epispore (outer spore surface) smooth. Colour reactions in 10% KOH: The whole stroma (or fragments) yields vinaceous purple pigments; subsurface granules turn from yellowish to colorless; pruina granules turn from red brown to bluish green.
Description adapted from Fournier, J., Köpcke, B., & Stadler, M. (2010). New species of Hypoxylon from western Europe and Ethiopia. Mycotaxon, 113(1), 209-235. Available online here or here.