Atopospora betulina  

Stromata developing in upper surface of living leaves, 0.5–2.0 mm diam and 120–220 µm tall, irregularly shaped but typically ± circular, shallowly domed or ± flat-topped, subcuticular, shining black, scattered or gregarious, composed of ± vertically arranged thick-walled polygonal cells, dark brown in the central part and with 2-3 more strongly pigmented layers of cells at the surface.

Anamorph: Conidiomata to 200 µm diam, acervular, subcuticular, formed in the upper layer of the stroma, flattened or flattened-pulvinate, the upper wall composed of a thin layer of black fungal cells with the host cuticle outside, rupturing irregularly to release the conidia. Lower wall composed of mid brown prismatic cells, not well differentiated from the stromatic tissue. Conidiophores arranged in a palisade-like layer, poorly developed, ± cylindrical, ± straight, sometimes septate, brown, each producing a single conidiogenous cell. Conidiogenous cells 8–10 × 4–5 µm, ± cylindrical, holoblastic, proliferation uncertain, pale brown, sometimes verruculose towards the apex. Conidia 15–23 × 8–9 µm, irregularly curved and with an attenuated apex, the apex truncate, with a median or slightly submedian septum, pale brown, smooth or faintly verruculose, without a gelatinous sheath or appendages.

Teleomorph: Ascomata 50–95 µm diam, initiating as locules in a single layer within the stroma but apparently not maturing until after winter, to ca 20 within each stroma, visible from the surface as shallowly domed to hemispherical bulges. Interascal tissue composed of filiform septate pseudoparaphyses 2–3 µm in diam, at least sometimes deliquescent at maturity. Asci 35–60 × 9–15 µm, cylindric-clavate to clavate, thick-walled and fissitunicate, 8-spored. Ascospores arranged biseriately, 10–15 × 4–6 µm, cylindric-ellipsoidal, the ends rounded, becoming yellowish or olivaceous brown, not or hardly constricted at the supramedian septum, thin- and smooth-walled, without a gelatinous sheath or appendages.