Taxonomic and phylogenetic evidence reveal two new Russula species (Russulaceae, Russulales) from northern Thailand

Russula is the most diverse genus within the family Russulaceae and has an ectomycorrhizal association with many host trees. During the monsoon season of 2022, five Russula specimens were collected in northern Thailand. After morphological and molecular analyses, the five specimens were determined to belong to Russula subgenus Heterophyllidia. A single specimen belonged to R. bellissima. Four other collected specimens were found distinct from all other known species. We have identified these four specimens as two new species, namely R. pseudomodesta and R. sribuabanensis. Russula pseudomodesta belongs to the subsection Modestinae, while R. sribuabanensis belongs to the subsection Virescentinae. The phylogenetic placement of two new species within Russula has been confirmed based on sequences of the nuclear internal transcribed spacer (nrITS) region and a large subunit (nrLSU) of the nuclear ribosomal RNA (nrRNA). Comprehensive morphological descriptions, field photographs, and comparisons of two new species with other morphologically and phylogenetically closely related species are provided.