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.
Not formally assessed. Only known in Great Britain from two sites, both recently discovered. However, Vaccinium uliginosum appears to be very poorly investigated as a host, and Exobasidium pachysporum could therefore be very under-recorded.
Exobasidium expansum and E. vaccinii-uliginosi also occur on Vaccinium uliginosum, but attack either the entire plant or large parts of it, rather than causing discrete leaf spots as seen in E. pachysporum infections. E. vaccinii-uliginosi can also be clearly differentiated by its much larger basidiospores of 16-23(-28) x 6.5-9.0 µm.
Exobasidium pachysporum is an obligate parasite and pathogen of Vaccinium uliginosum, and is found only on this host.
In Great Britain, known only from two sites, from England, Cumberland VC70 (Kershope Forest, 2008); and Scotland, East Inverness-shire VC96 (Chalamain Gap, near Cairngorm Mountain, 2013 and 2015).