Lectotype. Sweden, without data, Fries, Scleromyc. Suec. no. 145, as Sphaeria stilbostoma (UPS:BOT:F-117590, lectotype here designated; MBT390467)). Epitype, here designated: Austria, Tirol, Prägraten, Umbalfälle, grid square 8939/4, on Betula pendula, 28 Aug 2000, W. Jaklitsch W.J. 1543 (BPI 748447; ex-epitype culture CBS 109778 = A.R. 3501 = ME11; AFTOL-ID 936; MBT390383; iso-epitype WU 31897).

Sexual morph: Pseudostromata 1.3–3.6(–4.5) mm diam., scattered to aggregated, slightly or distinctly projecting from bark surface, pulvinate with bluntly conical centre (projecting disc), circular to elliptical in outline; consisting of an ectostromatic disc and perithecia embedded in an entostroma around a central column and often chambers filled with conidia. Ectostromatic discs 0.4–2.4(–2.7) mm diam. or length, fusoid to circular, projecting from the bark surface to 0.5 mm, less commonly 1 mm including pseudostroma, white or yellow, brown when old, flat, concave or convex, often completely filled by tips of ostiolar necks; central column beneath disc brightly white to yellow, consisting of hyaline hyphae and colourless crystals. Ostiolar necks cylindrical, laterally or centrally attached on perithecia, convergent and densely and irregularly or evenly disposed in the disc or around the margin; visible part in the discs (106–)139–231(–283) µm (n = 68) diam., 1–25 per disc, shiny black, convex papillate, discoid with depressed centre or conical to cylindrical and projecting to 300 µm. Entostroma paler than surrounding inner bark, consisting of hyaline to white hyphae and bark cells, sometimes forming white patches. Perithecia (450–)540–700(–780) µm (n = 45) diam., arranged in valsoid configuration around and below central column, globose to subglobose, collapsing upon drying. Peridium pseudoparenchymatous, consisting of a dark brown small-celled outer and a hyaline to brownish, large-celled inner layer. Hamathecium absent at maturity. Asci floating free at maturity, (69–)80–123(–141) × (10–)13–18(–21) µm (n = 64), fusoid to oblong or narrowly clavate, with an apical ring distinct in water and staining in Congo Red but invisible or indistinct in 3% KOH, containing 4–8 biseriate or obliquely uniseriate ascospores. Ascospores (13.7–)16–19(–23) × (4.7–)6.5–8.5(–9.7) µm, l/w (1.9–)2.1–2.7(–3.6) (n = 186), first narrow, fusoid or oblong and with small roundish appendages (1.5–)2–5(–7.3) × (2.2–)3.3–5.5(–6.8) µm, l/w (0.3–)0.5–1.1(–1.7) (n = 60) within asci, later becoming broadly ellipsoid with rounded ends, symmetric or inequilateral, slightly constricted at the central to slightly eccentric septum, hyaline, thick-walled, smooth; appendages fugaceous and absent on released ascospores.

Asexual morph acervular, intermingled with pseudostromata of the sexual morph or developing separately, conspicuous. First white tissue (central column) forming within the bark, becoming surrounded by sterile yellow margin and narrow discs rupturing bark epidermis, followed by the production of conidia in olivaceous to black chambers containing black conidial masses translucent though bark. Conidiomata 0.9–3.2 mm diam., subconical or pulvinate, more or less circular in outline, scattered or crowded.

Covering discs 0.3–1.2 mm long, slit-like to circular, flat to convex, shiny white to yellowish, becoming obscured by dark olivaceous brown to black conidial deposits forming patches to 2.7 mm diam., sometimes confluent to 1 cm; discs and pulvinate or conical columns beneath, consisting of dense textura intricata of hyaline hyphae and numerous colourless crystals, becoming brittle with age. Conidiophores emerging around the central column from a pseudoparenchymatous base, filiform, branched near the base and usually 1–3 fold asymmetrically at higher levels, first hyaline, turning brown from their tips; terminal conidiogenous cells (11.5–)18–33(–42.5) × (2–) 2.5–3.5(–4.5) µm (n = 47), more or less cylindrical, with up to 5 or 6 annellations, densely arranged, repetitive, producing α-conidia. Conidia (10.5–)12.5–15(–17.5) × (6.2–)7.2–8.5(–9.5) µm, l/w (1.3–)1.6–2(–2.7) (n = 260), oval, ellipsoid or subglobose, 1-celled, dark brown, thick-walled, smooth, with a few drops and a small scar. No β-conidia detected.

Culture: Colony on CMD at 16 °C forming irregular white and brown to ochre zones partly covered by aerial hyphae or hyaline, undifferentiated, forming brown spots and irregularly disposed conidiomata; on MEA at room temperature first white, later with broad white and brown zones with undulating margin and conidiomata forming mostly on the outer margin. Odour indistinct to fruity.

Melanconis stilbostoma occurs frequently on Betula spp. on the northern Hemisphere in Asia, Europe and North America (Barr 1978; Fan et al. 2016, 2018; Kobayashi 1970; Sogonov et al. 2008).

(all on twigs of Betula pendula except where noted): Austria, Kärnten, Gallizien, near Wildensteiner Wasserfall, grid square 9453/3, 11 Jul 2007, W. Jaklitsch (WU 31896); St. Margareten im Rosental, village area, grid square 9452/4, 27 May 1992, W. Jaklitsch (WU 15266); Trieblach, below Cihuc, grid square 9452/2, 14 Apr 2001, W. Jaklitsch W.J. 1740 (WU 31895, BPI 872036; culture A.R. 3637); Wograda, grid square 9452/3, 27 May 1997, W. Jaklitsch W.J. 1080 (WU 31894); same area and host, 31 May 2000, W. Jaklitsch W.J. 1474 (BPI 871332); Zabrde, grid square 9452/4, 7 Aug 1993, W. Jaklitsch (WU 15191); Niederösterreich, Aspangberg-St. Peter, Außerneuwald, Höllergraben, grid square 8462/1, 24 May 2015, G. Koller (WU 36779); Edlitz, Königsberg, grid square 8562/2, 14 Jul 2007, W. Jaklitsch W.J. 3125 (specimen lost; culture MS = CBS 121894); Friedersbach, S and SO from the village, grid square 7457/2, 19 Aug 2001, W. Jaklitsch W.J. 1775 (BPI 872038; culture A.R. 3725); Neunkirchen, Gloggnitz, Saloder, village area, grid square 8361/2, 10 May 2015, G. Koller (WU 36752); Grimmenstein, between Eben and the Kulmriegel, grid square 8362/4, 14 May 2015, G. Koller (WU 36812); Thaures, grid square 7156/1, 21 Sep 1997, W. Jaklitsch W.J. 1109 (WU 37048); Weidlingbach, grid square 7763/1, 27 Jun 1999, W. Jaklitsch W.J. 1329 (WU 37049); Oberösterreich, Schärding, Raab, Rothmayrberg, Rothmayr, 10 Mar 2012, H. Voglmayr (WU 38241); St. Willibald, Großer Salletwald, at the road B 129 to Peuerbach, grid square 7648/1, 31 Dec 2011, H. Voglmayr (WU 31899); Vienna, Alsergrund, at the hospital AKH, grid square 7764/3, 23 Jul 1993, W. Jaklitsch (WU 15537); Favoriten, Rothneusiedl, grid square 7864/3, 4 Sep 1993, W. Jaklitsch (WU 15758); ibidem, 22 Jan 1994, W. Jaklitsch (WU 15559). Czech Republic, Bohemia, Malonty, Hodonický potok, grid square 7253/3, 25 Sep 2003, W. Jaklitsch W.J. 2427 (WU 31898). Germany, no collection data (type material B 700016528 and B 700016529 of Melanconium betulinum from B). Italy, Sicily, Etna, SW Linguaglossa, near I Due Monti, on Betula aetnensis, 18 Jun 2016, H. Voglmayr & W. Jaklitsch (WU 37047; culture D258). Japan, Nagano, Karuizawa, Mt. Asama, on Betula platyphylla Sukachev var. japonica (Miq.) Hara, 21 Sep 1965, T. Kobayashi (TFM FPH2710; culture MAFF 410225 = M3-9 = ME12). Poland, Narewka, NE Nowa Lewkowo, 27 Jul 2015, H. Voglmayr (WU 35970; culture D143).

Melanconis stilbostoma and its basionym Sphaeria stilbostoma (α papula) were mentioned by Tulasne (1856), but the combination was invalid due to the lack of a generic diagnosis; it was, however, validated in Tulasne and Tulasne (1863). According to Ibai Olariaga, who examined the type in UPS, there are 3 scalps of Betula bark containing many clustered perithecia with black ostiolar necks erumpent through a white disc; neither asci nor spores were found, but brown α-conidia are present abundantly. As the type collection was distributed in Fries’ Scleromyceti Sueciae no. 145, we here lectotypify the species with the copy preserved in UPS, which we epitypify with a recent well-developed collection for which a culture and sequence data are available.

Several asexual morph names have been linked with Melanconis stilbostoma: Melanconium bicolor predates Melanconis stilbostoma, but there is no material extant in B, thus it cannot be checked; also Quercus but not Betula was given as host in the protologue. In addition, Melanconis stilbostoma is a well-known and well-defined name for the generic type of Melanconis. The second name is Melanconium betulinum, which is clearly a later synonym upon our examination of type material. Melanconium elevatum is another synonym. We have, however, not seen type material of this taxon, but the description and illustrations in Corda (1839) are conclusive. Melanconis stilbostoma is a very common fungus on birch throughout the northern hemisphere and likely the most conspicuous species of Melanconis due to the shiny white discs of both morphs, contrasting the dark conidial deposits. In older specimens, the latter may have olivaceous tones, but much less conspicuously than with M. larissae. The latter species differs also in a broad light zone present on its conidia. Melanconis stilbostoma was already cultured by Wehmeyer (1926b) on birch twigs from material, whose ascospore measurements were (13–)15–18 × 5–8 µm, corresponding to those of Barr (1978: 12–18.5 × 6.5–8(–9) µm). Wehmeyer (1941) gave (13–)15–19(–23) × (5–)6–7.5(–9) µm for ascospores, which is in accordance with our measurements ((13.7–)16–19(–23) × (4.7–)6.5–8.5(–9.7) µm); Kobayashi (1970) measured 13–25 × 4–7.5 µm, mostly 15–20 × 5–7 µm and Fan et al. (2016) gave (19–)21.5–23.5(–25) × (6–)7–8 μm, which is slightly larger. Wehmeyer (1941) noted for α-conidia from culture and exsiccata mostly 10–16 × 5.5–7.5 µm and 6.5–12 × 2–2.5 for β-conidia in culture; Barr (1978) found only α-conidia and measured 9–16.5 × 5–7.5 µm, which is in accordance with our observations from Europe (see above). Asian authors gave 9–16.5 × 5–7.5 µm (Kobayashi 1970) and (8.5–)9–14.5(–16) × (4.5–)5–6(–6.5) μm (Fan et al. 2016) for α-conidia, but, in some collections, they also found cylindrical to allantoid, unicellular, hyaline β-conidia, 9–11.5 × 1.5–2.5 µm (Kobayashi 1970) or (9–)10–11(–12.5) × (2–)2.5–3 μm (Fan et al. 2016).