E-Flora BC: Electronic Atlas of the Flora of British Columbia

Summary:
Features include the development on plants of Ericaceae of thickened gall-like areas on leaves, which turn red on the upper surface and on the underside become covered with a white spore-bearing mat: the plants lose their leaves, bear less fruit and often die. The fungus remains in the stems and rhizomes of the plant during the winter. A true fruiting body is not produced. It affects a variety of plants including Vaccinium species (blueberry, cranberry, whortleberry etc.), Arctyostaphylos, Menziesia, Rhododendron, and Arbutus menziesii, at least in the widest sense of the name. Modern authors have defined the species more narrowly. Breitenbach''s description is for European collections on Vaccinium vitis-idaea. Savile''s concept includes the following synonyms: Exobasidium azaleae Peck, E. discoideum Ellis, E. andromedae Peck, E. karstenii Sacc. & Trotter, E. cassandrae Peck, E. ledi P. Karst., E. oxycocci Rostr., E. agauriae P. Henn., E. parvifolii Hotson, and E. angustisporum Linder. The online Species Fungorum, accessed March 30, 2012, maintained these as separate. Savile also combines Exobasidium arctostaphyli Harkn. as a var. arctostaphyli of E. vaccinii, with spores 12.5-16.5(20) x 3.3-5.0 and basidia 2-3(4)-spored, and assigns some BC collections.

Numerous collections appear under this name from BC at Pacific Forestry Center, and Savile examined collections from BC, AB, NL, NS, NU, ON, QC, CA, MA, NJ (type of E. discoideum), Finland (type of E. karstenii), and Germany.

Fruiting body:
attacks leaves, usually causing the infected places to become thickened and gall-like, and to turn bright red with yellowish border on the upper surface of the leaf, these places on the underside covered with white farinose spore-bearing surface, sometimes the whole ends of the shoots are involved and show abnormally increased growth, (Breitenbach), systemic, "producing shoot galls or witches''-brooms", or localized, "producing bud galls, leaf galls, or unhypertrophied leaf spots", (Savile); parasitic "in leaves, young shoots, and flowers of various ericaceous hosts, and stimulating the infected parts to the production of leaf, shoot, or flower galls" that bear the fruiting on their surface; on leaves growing flat on the underside, scurfy or felty and compact, grayish, the upper side of the leaf usually somewhat reddish, (Burt)

Microscopic:
spores 11-16 x 2.5-4.5 microns, cylindric, some bent, smooth, inamyloid, colorless, with droplets, with one to six septa when mature, some budding conidia; conidia 5-9 x 1-1.5 microns; basidia 50-60 x 5.5-6 microns, 4-5-spored, slenderly clavate, sinuous, without basal clamp connection; cystidia not seen; hyphae 1-2 microns wide, sparse, septa without clamp connections, (Breitenbach), spores 8.0-14.5 x 2.0-3.2 microns, nearly cylindric, "slightly to moderately curved, sometimes moderately hooked at base", (0)1(3)-septate at maturity, conidia often abundant, about 4.0-10.0 x 0.7-1.5 microns; basidia (2)3-6(8)-spored, projecting 13-27(30) microns beyond cuticle, 3.0-6.0(7.0) microns wide, sterigmata 2.0-4.5 x 1.0-1.7 microns, (Savile, on V. vitis-idaea), spores 10-20 x 2.5-5 microns but usually about 12-18 x 3-3.5 microns, simple or 1-sepate, becoming more septate in germinating; conidia simple 6-9 x 1-1.5 microns; basidia usually 4-spored; the grayish mat consisting of more or less interwoven and branched hyphae then bear conidia and give a maximum thickness ranging up to 60-70 microns, (Burt)