Assessment Notes

Justification

Suillus suilloides is a rare sequestrate to semi-sequestrate suilloid bolete. Currently it is known from 11 collections from 11 localities, from the Sierra Nevada, Cascade and Cascade Range of California, with a single sites in the western Siskiyou Range in California and the Cascade Range in southern Oregon. 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. Given the small number of known records, lack of recent collections, and decline in its assumed habitat, this appears to be a rare species. Even multiplying the known number of localities by 200 times, each with up to four mature individuals, only brings the estimated population to 8,800 mature individuals. As this is a hypogeous species, its spores are dispersed by small mammals resulting in the species occurring in small, genetically isolated subpopulations, of fewer than 1,000 mature individuals. It is assessed as Vulnerable.

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Geographic range

Suillus suilloides is a rare sequestrate to semi-sequestrate suilloid bolete. Currently known from 11 collections from 11 sites, from the Sierra Nevada, Cascade and Cascade Range of California, with single sites in the western Siskiyou Range in California, and the Cascade Range in southern Oregon.

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Population and Trends

The species is only known from 11 collections from 11 sites; 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. The lack of recent collections may in part be due to a reduction in survey effort, but that likely does not fully explain the situation. Given the limited number of known records, lack of recent collections, and decline in its assumed habitat, this appears to be a rare species. Even multiplying the known number of localities by 200 times, each with up to four mature individuals, only brings the estimated population size to 8,800 mature individuals. As this is a hypogeous species its spores are dispersed by small mammals resulting in the species occurring in small, genetically isolated subpopulations of fewer than 1,000 mature individuals.

Population Trend: Decreasing

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Habitat and Ecology

It was recorded as occurring solitarily or scattered, completely buried in duff or soil. It is an ectomycorrhizal forming species, likely associated with five-needle pines and has been found 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 distances than wind-dispersed macrofungi. Suillus suilloides remains a poorly known species as only 11 collections have been made, and most of the records 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.

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Threats

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). Additionally, 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.

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Conservation Actions

Continued work on breeding blister rust resistant pines, and control of blister rust spread is needed to maintain its host tree. Protecting known sites from logging and other large scale disturbance is important. Targeted surveys are needed at historic sites and other likely sites to better document its population size and trends as well as the species tree associations and habitat requirements.

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Use and Trade

No use/trade is known.

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Source and Citation

Mueller, G.M. 2025. Suillus suilloides. The IUCN Red List of Threatened Species 2025: e.T273227395A273244884. Accessed on 24 November 2025.

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