Recent taxonomic discoveries suggest a host-symbiont co-migration of Russula subsect. Castanopsidum with Fagaceae in the Americas.

Three members of ectomycorrhizal fungi belonging to Russula subsection Castanopsidum are described from neotropical montane forests dominated by species of Fagaceae in Panama and Colombia. Russula esperanzae sp. nov. from Colombia differs in the field from other members of the subsection by slender longitudinally striated red stipes as well as characteristics of spores and basidia. Russula atroamethystina subsp. atroamethystina, originally described from Costa Rica, is reported for the first time from Panama. Closely related and morphologically similar Colombian collections are described as R. atroamethystina subsp. guacahayoensis subsp. nov. and differ from the typical subspecies by longer hymenial cystidia as well as molecular sequence data. The distribution of this newly described subspecies is restricted to Fagaceae forests east of the Isthmus of Panama. This suggests an allopatric diversification, likely driven by the absence of Fagaceae in the tropical lowland forest of the Darién Gap. Russula esperanzae is exclusively associated with Trigonobalanus excelsa, a tree species of Fagaceae endemic to Colombia. The role of its association with this tree in the evolutionary history of the species is discussed. Based on molecular timing, species of the subsection Castanopsidum started to evolve approximately 15.1 M years ago (mya) and display an amphi-pacific disjunct distribution with an Asian clade that split from the American clade around 11 mya. Citation: Manz C, Ortiz-Suárez A, Adamčíková K, Looney B, Noffsinger CR, Caboň M, Hampe F, Piepenbring M, Hofmann T, Buyck B, Parra-Aldana CA, Adamčík S, Corrales A (2025). Recent taxonomic discoveries suggest a host-symbiont co-migration of Russula subsect. Castanopsidum with Fagaceae in the Americas. Fungal Systematics and Evolution 16: 181-195. doi: 10.3114/fuse.2025.16.10.