Mushrooming


Around theWorld Exobasidium and the World of Rhododendrons


Britt A. Bunyard H


igh on an alpine slopes, where rhododendrons cling to thin soils and the wind carries the scent of resin and stone, you may notice a leaf that looks strangely


luminous—smooth, swollen, and blushed with yellow or rose. Tese curious growths are not botanical whimsy. Tey are the handiwork of Exobasidium, a lineage of fungi so understated that most hikers never realize they’ve walked past a basidiomycete in full reproductive glory. Members of the Exobasidiaceae are obligate biotrophic fungi


that parasitize vascular plants, especially in the Ericaceae (the family of blueberries, rhododendrons, azaleas, manzanitas, and madrone). Tey are unusual among basidiomycetes in that they produce no macroscopic fruiting bodies whatsoever. Teir basidia form directly on infected host tissues, emerging as a thin hymenial layer on the surface of a gall or chlorotic patch. Te spores themselves are ballistosporic, each bearing an abaxially oriented hilar appendage—a microscopic flourish that taxonomists rely on in the absence of any visible fruiting body. Te family comprises four genera—Exobasidium, Austrobasidium, Laurobasidium, andMuribasidiospora—with far more than 50 species distributed globally, though most favor temperate climates. (Te genus Endobasidium is still of uncertain taxonomic placement.) Identification depends on a combination of host species, microscopy, and host symptoms, since the fungi themselves remain nearly invisible. Life cycles in the Exobasidiaceae are tightly host-coupled.


Some species are strict specialists; others are more catholic in their tastes. Phylogenetic studies suggest a long history of co-speciation with host lineages, punctuated by occasional host jumps and ecological radiations. Teir obligate parasitism and lack of free-living stages make them exquisite models for studying fungal biodiversity, coevolution, and plant pathology. Exobasidium rhododendri, the species most familiar to alpine botanists, infiltrates the leaves of Rhododendron ferrugineum and R. hirsutum and rewrites the plant’s developmental program. Te result is a gall: a smooth, inflated, often brilliantly colored structure caused by hypertrophy and hyperplasia of host cells. Te fungus grows intercellularly, siphoning nutrients while keeping the host alive. Te plant responds with a defensive overgrowth that walls off the infection, creating a microenvironment where the fungus can mature and produce its basidia. It is parasitism without carnage—an elegant, slow-motion negotiation between fungus and shrub.


Mad Honey


Rhododendrons have another story—one that has overshadowed Exobasidium in popular imagination but is worth mentioning for context. Certain species, especially Rhododendron ponticum and R. luteum of the Black Sea region, produce nectar rich in grayanotoxins. Bees convert this into the infamous “mad honey,” capable of causing nausea, dizziness, slowed heart rate, and hallucinations. The earliest written account of mad honey intoxication appears in Xenophon’s Anabasis, a fourth century BCE narrative chronicling the retreat of the Greek mercenary army known as the Ten Thousand. Near the ancient city of Trebizond—modern Trabzon, Turkey—the soldiers consumed local honeycombs and were promptly incapacitated, an early and vivid description of grayanotoxin poisoning. Pliny the Elder later retold the story, and the legend grew. But this is a tale of chemistry, not fungi. Alpine rhododendrons of Italy—R. ferrugineum and R. hirsutum—produce safe, toxin-free honey.


Azalea gall (pinxter apple) on Rhododendron periclymenoides. Photo courtesy of Kathie Hodge, Cornell University.


Te influence of Exobasidium extends beyond botany. In


coastal British Columbia, Indigenous groups—including the Heiltsuk, Henaaksiala, and Southern Tsimshian— recognized the pale, earlike galls induced by Exobasidium on Rhododendron menziesii (formerlyMenziesia ferruginea) as “ghost’s ears.” Children foraged them as treats; adults wove them into stories. Tsimshian oral tradition tells of Raven tricking Squirrel to hoard the ghost’s ears, linking the fungus to themes of cunning and scarcity. Te Heiltsuk termpspiyu yisulual and the Tsimshian ts’imu·k both refer to these fungal galls—an exquisite example of how morphology, memory, and metaphor intertwine. In these narratives, Exobasidium is not a pathogen but a presence: a quiet inhabitant of the forest whose transformations carry meaning.


Summer 2026 FUNGI Volume 19:2 17


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