Synonymy of Suillus imitatus, the imitator of two species within the S. caerulescens/ponderosus complex

Identification of three species of Suillus, S. caerulescens, S. ponderosus, and S. imitatus, has always been difficult because of overlapping and non-discrete morphological characters. To solidify the identification of these taxa, we compared the nucleotide sequences from the internal transcribed spacer region (ITS) of the type specimens of S. caerulescens, S. ponderosus, S. imitatus var. imitatus, and S. imitatus var. viridescens with field collected specimens which we identified as S. caerulescens, S. ponderosus, and S. imitatus in northern California. Based on ITS sequence identity and phylogenetic inference, specimens of S. caerulescens and S. ponderosus formed well-supported clades with the holotype of the respective species. However, S. imitatus var. imitatus fits within the S. caerulescens clade and S. imitatus var. viridescens fits within the S. ponderosus clade. Therefore, we synonymize S. imitatus var. imitatus with S. caerulescens and S. imitatus var. viridescens with S. ponderosus, and show that the species can indeed be recognized morphologically based on annulus characteristics.