 Suillus americanus is very similar in appearance to Suillus sibiricus ( distributed in western North America and western and central Asia ) but the latter species associates with Pinus monticola and Pinus flexilis rather than Pinus strobus . One field guide suggests that Suillus sibiricus has a thicker stem than S. americanus , brown spots on the cap , and is a darker , more dingy yellow . Molecular phylogenetics analysis has shown , however , that specimens of S. sibricus collected from China and western North America , as well as S. americanus from eastern North America , are most likely " a single circumboreal taxon " . 
