Parasitic on leaves of a deciduous tree. Sexual state: Ascomata 65–100 µm high, 70−115 µm wide ( = 76 × 93 µm, n = 5), epiphyllous, subcuticular, black, immersed to erumpent, scattered, uniloculate without a distinct ostiole. Peridium 9−20 µm (= 14 µm, n = 15) wide, comprising several layers, dark brown to hyaline cells of textura angularis. Hamathecium lacking pseudoparaphyses. Asci 35−45 × 9−14 µm ( = 40.5 × 11.5 µm, n = 20), 8-spored, sometimes less than 8, bitunicate, broadly cylindrical, ellipsoidal, short pedicellate or sessile, apically rounded with an ocular chamber. Ascospores 11−19 × 3−6 µm ( = 13.6 × 4.4 µm, n = 20), overlapping bi-triseriate, hyaline, aseptate, ellipsoidal to fusiform, sometimes with a thin mucilaginous sheath. Asexual state: aureobasidium-like; Colonies white to black. Conidiogenous cells intercalary or terminal, forming blastic conodia, denticulate or with minute scar. Conidia aseptate, rarely 1-septate, hyaline, smooth, elliptic-oblong. Secondary conidia and endoconidia are sometimes produced (asexual morph description follows Sivanesan 1984).
Material examined: UK, Cambridgeshire: Cambridge, Gog magog Hills, on Fagus sylvatica (Fagaceae), 18 May 1964, H.J. Hudson (IMI 110819, holotype).
Fig. 1 Columnosphaeria fagi (IMI 110819, holotype). a Herbarium material b, c Ascostromata on the host surface. d Section through ascostroma. e Peridium. f-h Bitunicate asci i-l Ascospores. Scale bars: d = 25 µm, f-h = 20 µm, i-l = 5 µm.