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.
Not formally assessed. Records are scattered throughout the British Isles, but there have only been four sightings since 2000. Its status needs review.
Found in living and overwintered leaves of Betula species, including B. nana as well as the more arborescent species. Atopospora betulina is an obligate and probably non-culturable parasite. It forms tar spots on living leaves in the summer months, within which the conidiomata and immature ascomata are formed, and overwinters on fallen leaves before the ascomata mature and ascospores are released.
Reported from England (Cambridgeshire, Durham, W Gloucestershire, N Hampshire, Herefordshire, W Kent, E Norfolk, W Norfolk, Northumberland, Shropshire, S Somerset, Surrey, Warwickshire, Worcestershire, MW Yorkshire, SW Yorkshire) and Scotland (Easterness, Moray, Easter Ross, Wester Ross) and Wales (Glamorgan, Merioneth).
There are no indications that the species causes damage to the host plant, although presumably some reduction in growth must occur due to diversion of energy resources into the fungal mycelium. There is no evidence of a mutualistic relationship.