Suillus weaverae    (H.H. Sm. & Shaffer) Kretzer & T.D. Bruns

A Species of Fungus 

MN Status:
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Fuscoboletinus weaverae, Suillus granulatus ssp. snellii

  Basis for Listing

Suillus weaverae is endemic to Minnesota, not yet reported from anywhere else in the world. This species, named in honor of its collector, has been collected at three locations in Minnesota. Suillus weaverae is susceptible to human disturbance through habitat alteration and pollution, and it was listed as an endangered species in Minnesota in 1996.


Mushroom caps of S. weaverae are 35-70 mm (1.4-2.8 in.) broad, rounded, glabrous (no hairs) and have a viscid (sticky) surface. When young, the cap color is vinaceous buff in the center and pale reddish cinnamon on the margin. Later, caps become mottled with pinkish cinnamon in the center and a darker margin. The flesh stains yellow to orange around larval channels. Odor is mild and the taste pleasant. As a member of the Boletaceae family, this fungal species has tubes (pores) rather than gills, which are 1.5-4 mm (0.06-0.16 in.) long. Tubes are white when young, becoming yellowish over time, and do not stain. Tube mouths are angular or irregular in shape, somewhat radially arranged but not elongated. Tube mouth color when young is pale to light yellow, becoming more orange yellow in age, and developing vinaceous brown glandular dots. Stems are 40-80 mm (1.6-3.2 in.) long, 9-15 mm (0.4-0.6 in.) thick at the apex, usually tapering downward or swollen above the middle. Stem color is tan with vinaceous brown glandular dots. Sometimes the surface and interior of the base stain light to moderate yellow.

White to bright yellow mycelium, the filamentous vegetative portion of the fungus, occurs at the base and in surrounding soil. The veil, the covering formed around the button-shaped young fruitbodies, is fibrillose (covered with silk-like fibers) to cottony. Veil color is white to pale cream. The veil sheathes the lower stem and tubes in young fruitbodies, remaining as a sheath on the lower stem or disappearing, but not forming a ring. It may leave patches on the cap margin. Spore print is light yellowish brown to light reddish brown when fresh, grayish to purplish brown when dry. Spores are 6.7-8 µm x 2.7-3.2 µm, narrowly oblong, and smooth. The distinctive features of S. weaverae are the glabrous and viscid cap, the yellow tubes when mature, the glandular dotted stem, and the brownish drab spore print color. Because of the spore print color, the species was originally placed in the genus Fuscoboletinus. This genus has now been merged with Suillus based on recent molecular studies.


Suillus weaverae has been found in humus in sandy soil of mixed deciduous-coniferous forest containing Quercus spp. (oak), Populus spp. (poplars), Betula spp. (birch), Pinus banksiana (jack pine), P. resinosa (red pine), P. strobus (white pine), and Larix laricina (tamarack). It is most likely associated with P. resinosa and P. banksiana.

  Biology / Life History

Fully grown fungi disperse spores into the air, which plant themselves in the ground and grow into hyphae, or fungal filaments. Hyphae from two different fungal organisms meet, fuse, and form a mycelium, the filamentous vegetative portion of a fungus (excludes the fruiting/reproductive phase of the life cycle). A small, button-shaped fungus forms, pushes above the ground, and obtains a covering called a veil. When the veil ruptures, an adult fungus grows. Mushroom fruitbodies can be produced in the summer and fall from the perennial underground mycelium. Dispersal is via wind borne spores or locally by extension of the mycelium between neighboring trees.

Suillus weaverae is a presumed ectomycorrhizal fungus that is a beneficial root symbiont, most likely associated with P. resinosa and/or P. banksiana. The fungus forms on root surfaces and is able to absorb water and nutrients, benefiting the plant. In turn, the plant supplies the fungus with energy-rich sugars manufactured through photosynthesis. Suillus weaverae is closely related to Suillus brevipes, S. luteus and S. pseudobrevipes, all of which are associates of plants in the Pinus genus.

  Conservation / Management

Three S. weaverae populations are currently known to occur in the state. The habitat is vulnerable to disturbance from human activity including habitat alteration or loss. Ectomycorrhizal fungi in general are sensitive to nitrogen deposition and other forms of pollution that affect the soil. Mycorrhizal fungi on sandy soils are especially susceptible to air and water pollution.

  Best Time to Search

The best time to search for S. weaverae is during its fruiting season. Based on related species, this would likely extend from July through early October in Minnesota.

  Conservation Efforts in Minnesota

In 1996, the Minnesota DNR funded a study to characterize the diversity and abundance of ectotrophic mycorrhizal fungi in old-growth and young northern hardwood-conifer forests. Additional surveys are needed to search for other potential populations and to monitor the known location of this species.

  References and Additional Information

Nguyen, N. H., E. C. Vellinga, T. D. Bruns, and P. G. Kennedy. 2016. Phylogenetic assessment of global Suillus ITS sequences supports morphologically defined species and reveals synonymous and undescribed taxa. Mycologia 108(6):1216-1228.

Smith, A. H., and R. L. Shaffer. 1965. A new species of the Boletaceae. The Michigan Botanist 4:27-30.

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