Exobasidium expansum  

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.

Map of known distribution in Great Britain and Ireland.