Exobasidium sydowianum  

A rarely seen, yet conspicuous, fungal parasite and pathogen of Arctostaphylos uva-ursi (bearberry) usually found in high latitudes and/or altitudes. Visible as small yellow-orange-red, rather waxy, thickened/swollen spots on the hosts leaves.

Circumscribed. Leaf-spots small, c. 0.5 cm across, rarely more, in heavy attacks often confluent, not or only moderately thickened, at first bright red above with broad yellowish margin, cuticle soon flaking off, especially from the upper surface, which then turns orange brown and slightly glossy as if greasy, eventually sparsely hymenium-clad beneath. Exceptionally, single annual shoots become wholly diseased with leaves dark purple, simulating such diseased by Exobasidium uvae-ursi. Leaf-spots and diseased leaves with hyphal palisade between epidermis cells also on upper side. Basidia 2—4-sterigmate. Basidiospores (10-)11-17 x 3.0—4.0 µm (M = 12.8—13.8 x 3.6—3.9 µm, fide Sundström), rather plump, musiform (banana-shaped), curved or geniculate (sharply angled), eventually 1-septate. Germination by conidia. Conidia bacilliform (rod-shaped) to subclaviculate.

Description adapted from Nannfeldt, J. A. (1981). Exobasidium, a taxonomic reassessment applied to the European species. Symbolae botanicae Upsalienses 23(2), 1-72.