Exobasidium vaccinii (Fuckel) Woronin   
'Cowberry Redleaf'

Kingdom: Fungi
Phylum: Basidiomycota
Subphylum: Ustilaginomycotina
Class: Exobasidiomycetes
Order: Exobasidiales

British distribution: Distribution of the host — frequent in the North and West.
World distribution: Europe, Asia, Greenland, North America.

Exobasidium vaccinii
Exobasidium vaccinii
Exobasidium vaccinii on Cowberry (Vaccinium vitis-idaea), showing infected and uninfected plants, near Schiehallion, Perthshire, August 2009.

A host-specific fungal parasite on Cowberry (Vaccinium vitis-idaea).

Exobasidium is a genus of plant-parasitic fungi, mostly on members of the heather family (Ericaceae), causing usually pink, red or purple leaf-deformations and galls. No fruiting body as such is formed, the hymenium existing as a white, felt-like layer on leaf undersides or gall surfaces.

E. vaccinii causes distortions of individual leaves, the infected portions becoming bright red, thickened and enlarged, the enlarged areas becoming markedly concave. The lower surface becomes white-felted with the mycelium. It is often frequent on the host in moorland areas and is much the most common Exobasidium species on this host.

This account follows current understanding of the genus and of this species. Previously, the name has been misapplied to Exobasidium on other hosts, including E. myrtilli on Bilberry (Vaccinium myrtillus) and E. japonica on azaleas and rhododendrons (Rhododendron spp.).

The genus was revised and monographed by Nannfeldt (1981) and a valuable key to Scandinavian species, arranged by host plant, was posthumously given by Nannfeldt in Hansen & Knudsen (1997). This includes almost all British species and others that might be found here.

Occurring on Vaccinium vitis-idaea are:
E. vaccinii, as described above, forming localised infections on individual leaves, only sometimes infecting entire shoot tips, which then become enlarged, basidiospores 11–19 × 2.5–4.5 µm, some budding off conidia 8–11 × 0.8-1.2 µm (Nannfeldt, 1981), 5–9 × 1-1.5 µm (Breitenbach & Kränzlin, 1986), variably septate; basidia with 4 or more sterigmata;
E. juelianum, causes systemic infections that affect entire shoots, which become dwarfed and pinkish red, hymenium on the undersides of the leaves, basidiospores 9–14 × 2.5–4 µm, some budding off conidia 9–10 × 0.8–1.5 µm (Nannfeldt, 1981), 4–5 × 1 µm (Breitenbach & Kränzlin, (1986), variably septate; basidia with 4 or more sterigmata; rare, mainly in the Scottish Highlands;
E. splendidum, causes systemic infections that affect entire shoots, which become strongly red, hymenium on the undersides of the leaves, basidiospores (15–)20–27 × 6–11.5 µm, (figures from Nannfeldt, 1981 & Hansen & Knudsen, 1986), non-septate, conidia unknown; basidia with just 2 sterigmata; a single authentic British record from the Scottish Highlands (FRDBI and NBN).
Note the discrepancy in reported conidial measurements; regrettably I do not have measurements for the photographed material.

Other British Exobasidium species in Britain include E. myrtilli on Bilberry (Vaccinium myrtillus), E. rostrupii and V. oxycocci on cranberries (Vaccinium oxycoccus, V. microcarpum), E. karstenii on Bog Rosemary (Andromeda polifolia), E. sydowianum on Bearberry (Arctostaphylos uva-ursi), E. japonicum and E. rhododendri on garden Rhododendron spp., and E. camelliae on garden Camellia spp. (family Theaceae). Useful sets of photographs are provided in Ryman & Holmåsen (1984) and Breitenbach & Kränzlin (1986).

•   Breitenbach, J., & Kränzlin, F. (1986). Fungi of Switzerland, vol. 2: non-gilled fungi: Heterobasidiomycetes, Aphyllophorales, Gasteromycetes, Verlag Mykologia, Lucerne.
•   Hansen, L., & Knudsen, H., (eds.) (1997). Nordic macromycetes, vol. 3: heterobasidioid, aphyllophoroid and gastromycetoid basidiomycetes. Nordsvamp, Copenhagen.
•   Nannfeldt, J.A., (1981). Exobasidium, a taxonomic reassessment applied to the European species. Symbolae Botanicae Upsalienses 23(2): 1–72.
•   Ryman, S., & Holmåsen, I., (1984). Svampar. En fälthandbok. Interpublishing, Stockholm.

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