Originally described as Gastroboletus suilloides based on a collection made at Mammoth Mountain, California, USA (Thiers & Trappe 1969). It was transferred to the genus Gastrosuillus (Thiers 1989), and then subsumed into Suillus based on polygenetic research (Kretzer & Bruns 1997).

Kretzer & Bruns (1997) and Nguyen et.al. (2017) showed that there is very little to no genetic difference between the three ‘gastroid’ Suillus described from California; S. suilloides, S. amaranthi, and S. umbrinus, and all should be consider synonymous.

Synonyms:
Gastroboletus suilloides Thiers
Gastrosuillus suilloides (Thiers) Thiers
Gastrosuillus amaranthi Thiers
Suillus amaranthi (Thiers) Kretzer & T.D. Bruns
Gastrosuillus umbrinus Trappe & Castellano
Suillus umbrinus (Trappe & Castellano) W. Klofac

Suillus suilloides is a rare sequestrate to semi-sequestrate suilloid bolete

Currently known from 11 collections from 11 locations, from the Sierra Nevada, Cascade and Klamath ranges in California, with a single record from southern Oregon Cascade Range.

Believed to be associated with five-needle pines. Five-needle pines are suffering decline from the introduced fungal pathogen White Pine Blister Rust (Cronartium ribicola), and subsequent attacks by the Mountain Pine Beetle (Dendroctonus ponderosae).

Sierra Nevada and Cascade Range of California, with a single sites in the western Siskiyou Range in California, and the Cascade Range in southern Oregon.

Only known from 11 collections from 11 locations; and only a single collection has been made in the past thirty years. Suitable habitat has declined due to a combination of mountain pine beetle outbreaks, drought stress of pines, and forest fires.

Solitary or scattered, completely buried in duff or soil. Ectomycorrhizal, likely associated with five-needle pines. Fruiting from late spring into fall (till the end of November). This species is dependent on mycophagy (primarily eaten by small mammals) for spore dispersal; which in general go shorter distance than in wind-dispersed macrofungi. 

Suillus suilloides remains a poorly known species, only 11 collections have been made, and most lack habitat notes. It is believed to associate with Sugar Pine (Pinus lambertiana), and possibly other five-needle pines, but more research is needed to detail habitat restraints.

Many five-needle pine species are declining due to the introduced fungal pathogen White Pine Blister Rust (Cronartium ribicola), and subsequent attacks by the Mountain Pine Beetle (Dendroctonus ponderosae).

Prolonged droughts and decades of fire suppression have drastically altered western montane forests, leading to thicker, denser, Abies dominated forests. As a result, hotter, stand replacing fires (rather than patchwork and understory burns) are commonplace, altering appropriate habitat drastically, and making it ill-suited for this species.

Continued work on breeding blister rust resistant pines, and control of blister rust spread. Protect known locations from logging and other large scale disturbance.

Extent of range. Tree association and habitat requirements of this species. Targeted surveys at historic locations.

None known.

Castellano, M.A., Smith, J.E., O’Dell, T., Cázares, E. and Nugent, S. 1999. Handbook to Strategy 1 Fungal Species in the Northwest Forest Plan. U. S. Department of Agriculture, Forest Service, Pacific Northwest Research Station: Portland, OR. 195 p.

Kretzer, A. and Bruns, T.D. 1997. Molecular revisitation of the genus Gastrosuillus. Mycologia 89: 586–589.

Nguyen, N., Vellinga, E.C., Bruns, T.D. and Kennedy, P. 2017. Phylogenetic assessment of global Suillus ITS sequences supports morphologically defined species and reveals synonymous and undescribed taxa. Mycologia 108: 1216–1228.

Siegel, N., Vellinga, E.C., Schwarz, C., Castellano, M.A. and Ikeda, D. 2019. A Field Guide to the Rare Fungi of California’s National Forests. Bookmobile: Minneapolis, MN. 313 p.

Thiers, H.D. 1975. California Mushrooms – A Field Guide to the Boletes. Hafner Press.

Thiers, H.D. 1989. Gastroboletus revisited. Memoirs of the New York Botanical Garden 49: 355–359.

Thiers, H.D. and Trappe, J.M. 1969. Studies in the genus Gastroboletus. Brittonia 21: 249–251.