Shared ectomycorrhizal fungi between a herbaceous perennial (Helianthemum bicknellii) and oak (Quercus) seedlings

•  Ectomycorrhizal infection of Quercus seedlings can be low at a distance from established ectomycorrhizal vegetation. Here we investigate whether Helianthemum bicknellii, a herbaceous ectomycorrhizal perennial of prairies and oak savannas, creates patches of elevated ectomycorrhizal infection of Quercus seedlings. •  We performed two studies. First, ectomycorrhizas of H. bicknellii were compared with ectomycorrhizas of Quercus spp. Second, soil bioassays were conducted with Quercus macrocarpa seedlings grown in soils from near H. bicknellii; near established Quercus spp.; or distant from ectomycorrhizal vegetation. •  Eight species associated with H. bicknellii were identified: Cenococcum geophilum, Russula aff. amoenolens, an unknown Pezizalean fungus, Laccaria laccata, Tomentella sp., Lactarius mutabilis, Russula aff. pectinatoides, and Cortinarius sp. Internal transcribed spacer restriction fragment length polymorphism (ITS RFLP) patterns of all species except Cortinarius sp. matched to ectomycorrhiza from Quercus. In the bioassay, ectomycorrhizal infection was higher in near-Helianthemum soils than in distant soils, but lower than in near-Quercus soils. •  These results demonstrate that H. bicknellii, a common herbaceous plant of oak savannas, shares ectomycorrhizal partners with Quercus and provides patches of increased ectomycorrhizal infection of Quercus seedlings.