Exobasidium pachysporum  

A leaf-spot parasite and pathogen on leaves of Vaccinium uliginosum (bog bilberry).

Leaf-spots well-defined, rarely more than ca. 0.5 cm across, not to moderately thickened, merging together when in heavy infections, upper leaf surface pale pink to wine red and yellow-margined, leaf underside eventually covered with spots of white, powdery hymenium. Basidia cylindric-clavate, 50-55 x 4.5-5.5 µm, with (3-)4 sterigmata, without basal clamp. Basidiospores cylindric-elliptic,  smooth,  hyaline,  some  with  drops or 1-3 septate, 10-16  x  2-6 µm, sometimes also budding off conidia; conidia 6-10 x 1-1.5 µm. Cystidia not seen. Hyphae 1-2 µm across, septa without clamps.

Description adapted from Nannfeldt, J. A. (1981). Exobasidium, a taxonomic re-assessment applied to the European species. Symbolae botanicae Upsalienses 23(2), 1-72; and Breitenbach, J., & Kränzlin, F. (1986). Fungi of Switzerland, Vol. 2. Non gilled fungi-Heterobasidiomycetes, Aphyllophorales, Gasteromycetes. Verlag Mykologia.

Map of known distribution in Great Britain and Ireland.