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.
Not formally assessed. Hypoxylon fuscoides was recently described in 2010, and is currently known from only three sites in Great Britain and Ireland. The FRDBI contains around 80 records of Hypoxylon fuscum on Alnus, distributed throughout the UK, and it is unknown what proportion of these records (if any) may actually have been H. fuscoides.
Hypoxylon fuscum is indistinguishable from H. fuscoides in the field. The two species can be differentiated by coloured pigments released when fragments of stromata are treated with 10% KOH, and also by ascospore shape and size.
In 10% KOH, H. fuscum produces orange-brown/amber, pale cream-brown, or olivaceous gray/green diffusible pigments, rather than the vineous purple of H. fuscoides.
H. fuscum ascospores are typically more rounded at the ends, and slightly larger (11–16 × 5–8 µm), while H. fuscoides ascospores are slightly more acute and smaller (9.5–12.5 × 5–6 µm). Differences in ascospore shape may be more reliable than differences in size, due to the range of ascospore dimensions reported for the current concept of H. fuscum.
H. rosieri, described in 2008 from the USA but not yet known from Europe, also produces a purple colour reaction in 10% KOH, but possesses ascospores which are longer and thinner than those of H. fuscoides (13.5–15 × 5–6 µm).
Further details and a key to European Hypoxylon species can be found in Fournier et al. (2010).
Hypoxylon fuscoides has only been reported on Alnus incana Betula pendula.
Probably a weak parasite (and perhaps pathogen) of stressed or weakened trees, and a capable saprotroph of dead wood, as reported for other Hypoxylon species including H. fuscum.