A fungal parasite and pathogen of Vaccinium uliginosum (bog bilberry), infecting branches or entire stems, resulting in slightly stunted or enlarged leaves, pale green to reddish and often mottled above, and with white patches (the fungal fruiting surface, or hymenium) underneath.
Systemic, infecting major branch-complexes or entire stems but apparently never passing through underground organs from one stem to another. Branching only slightly influenced. Leaves mostly slightly diminished, pale green to (in a strong light) reddish, often mottled above, the leaf underside eventually coated with a chalk-white hymenium between leaf veins. Recent infections may be restricted to shoots and sometimes cause enlarged leaves, as seen in E. vaccinii-uliginosi infections. Basidia 2-4-sterigmate. Basidiospores 9-15 x 2.5-4.0 µm, musiform (banana-shaped) to allantoid (sausage-shaped), moderately curved, eventually 1-septate. Basidiospores produce conidia; conidia 5-9 x 1-2 µm, bacciliform (rod-shaped) to elongately ellipsoid.
Description adapted from Nannfeldt, J. A. (1981). Exobasidium, a taxonomic reassessment applied to the European species. Symbolae botanicae Upsalienses 23(2), 1-72.
Not formally assessed. Considered Vulnerable / D2 in the current but unofficial Red Data List of Threatened British Fungi (Evans et al., 2006). Considered a species of principal importance for biodiversity conservation in Scotland, and included in the Scottish Biodiversity List. Known only from two sites in Great Britain, both in Scotland. Not recorded since 1994, and the most recent known population was very small, threatened by construction work, and could potentially have been lost.
Exobasidium pachysporum also occurs on Vaccinium uliginosum in Great Britain, but infections are restricted to discrete and well-defined leaf-spots, rather than being systemic.
E. vaccinii-uliginosi has not been previously recorded in Great Britain, but elsewhere also infects V. uliginosum. Infections are confined to individual shoots, with whole infected leaves moderately to strongly enlarged, only slightly thickened at most, and yellowish-white to bright red above. Shoots infected by E. vaccinii-uliginosi can sometimes resemble those of E. expansum, but the former species possesses considerably larger basidiospores (16-23 x 6.5-9 µm).
Exobasidium expansum is an obligate, biotrophic parasite and pathogen of Vaccinium uliginosum, and is only found on this host.
In Great Britain, only known from four records made within the Cairngorms National Park, two of which are poorly localised but probably in the Cairngorm area (recorded 1961 and 1976); one from Coire Cas near Cairngorm Mountain, East Inverness-shire VC96 (recorded 1994), and one from Glen Shee, East Perthshire VC89 (recorded 1974). The Coire Cas site was reportedly sparsely populated (two infected plants only found after 2 hours of searching), threatened by the construction of the Cairngorm funicular railway between 1999-2001, and this population could potentially have been lost.