Suillus granulatus, also known as The Weeping Bolete or Granulated Bolete, is a medium to large member of the Suillaceae family. It is characterized by its orangish sticky cap, non-bruising grayish-yellow tubes with small round pores on its underside that discharge milky droplets in young specimens, and a slender stem, the upper part of which is strongly dotted with brownish-pink glands. This species is also very closely associated with 2-needled pine trees like the Scots Pine. [i]
S. granulatus was first described by Carl Linnaeus in 1753 as Boletus granulatus. Later on, in 1796, the French naturalist Henri François Anne de Roussel moved it to the Suillus genus and assigned it its current name.
The name Suillus is derived from the Latin word “suilla,” which means “pertaining to swine” or “pig-like.” The specific epithet granulatus comes from the Latin word “granulum,” which means “a small grain,” referring to the granular texture of the upper surface of the mushroom’s stem. Therefore, the name Suillus granulatus can be interpreted to mean “pig-like mushroom with a granular cap.”
Lastly, based on recent molecular and morphological data the classification of Suillus granulatus and related species has been changed. Previously, and according to most field guides in prints, it was thought that Suillus granulatus and closely related species such as Suillus lactifluus and Suillus weaverae were the same. However, it is now accepted that North American specimens described as Suillus granulatus are most likely Suillus weaverae or Suillus lactifluus, which are stand-alone species. Furthermore, the latter species are often described as associated with 5-needled white pine trees. This is uncharacteristic for Suillus granulatus as it only grows in association with 2-needles red pine trees. Lastly, to bring more confusion into the fold, Suillus granulatus does not appear to be an endemic species of North America, but its specimens can be found growing in North America, probably as an imported species. In the end, a more in-depth analysis of the prevalence and range of Suillus granulatus in North America is needed.
Cap: The cap measure 7-15 centimeters in diameter. The cap is initially convex, becoming broadly convex or nearly flat with age. The margins of the cap are inrolled in young specimens, older specimens have spread margins that may be irregularly lobed or sinuous at times. The color of the cap is quite variable, it ranges from a yellow to cinnamon-brown to an orange color with interspersed darker, reddish brown areas in age as the surface color breaks up. The surface of the cap is smooth, bald, and covered in a greasy, sticky material.
Flesh: The flesh is white to light yellow and soft. The flesh does not change color when bruised or cut.
Pores: S. granulatus is a bolete, as such it does not have gills but rather a hymenium, or a surface consisting mainly of spore-bearing structures. The hymenium of this species is a dull yellow color consisting of small, slightly angular pores which measure approximately 5 mm deep and 1 mm wide. Young S. granulatus mushrooms secrete milky droplets from their small pores. When mature, the hymenium turns a more dingy greenish-brownish color.
Stem: This stem of this species measures approximately 5 centimeters long and 1.5 centimeters thick. It is cylindric throughout with mild tapering downward. While young, the stem is solid but with age it frequently becomes solid. The stem is whitish to pale yellow in color, and towards its apex, it has granular surface. The granular surface is formed from the exudation of milky sap that dries and hardens.
Spores: The spores of S. granulatus measure 6-9 by 3-4 µm in size. They are spindle-shaped to elliptical to narrowly inequilateral. The surface of the spores is smooth. The spores are hyaline to brown in potassium hydroxide (KOH).
Spore Print: The spore print of this species is brown to dull cinnamon or ochre.
Smell: This species does not have a distinctive odor.
Flavor and Edibility: S. granulatus is edible, but bland and not desired by most. Many individuals are disdained by its soft flesh and slimy texture as well as its rather unpleasant, subnauseous taste. Although it’s worth mentioning that some sources list the edibility of this mushroom as “good”.
Habitat: This species has a mycorrhizal association with 2-needled pine trees, most commonly the Scots Pine (Pinus sylvestris). It is found growing alone or in dense gregarious groups on mainly rich, fertile or calcareous soil in areas where pine occurs naturally or have been planted.
Range: This species is widespread and very common in Europe, Asia, Australia, and other parts of the world. As mentioned above, specimens of this mushroom may also be found in continental North America but it is not endemic but rather an introduced species with red pines.
Fruiting season: The fruiting season last from late June to November. In warmer climates, S. granulatus can fruit throughout the winter. This species often fruits in very large numbers.
This species is rather easy to recognize if one pays attention to its distinctive features, which include the granulations on its stem, the slimy yellow-orange cap, the absence of a veil, a hymenium that secrets milky droplets in young fungi, and the close association with 2-needles pines. Nonetheless, there are several mushroom species that resemble Suillus granulatus in appearance. Some of the common look-alike species include:
- S. grevillei – this species has a similar cap, but there is a very distinct ring zone on its stem and its pores are much larger and angular.
- S. luteus – this species has a distinctive partial veil and ring. Furthermore, it lacks the milky droplets on the pores and the granules on the stem.
- S. brevipes – this species has a shorter stem and does not ooze milky droplets from the hymenium.
- S. collinitus – this species has a darker brown colored cap as well as some dark radial fibrillae and pink basal mycelium.
- S. bellinii – this species has a paler, almost whitish, cap as well as the tubules that are slightly decurrent on the stem.
- S. mediterraneensis – this species has an olive-brown ochraceous cap. Its flesh is more markedly yellow.
- S. flavogranulatus – this species has a darker yellowish to dingy ochre cap at maturity as well as larger pores.
- S. kaibabensis – this species has a paler cinnamon or yellowish cap.
- S. wasatchicus – this species has reddish pores when young that become yellowish in age.
- S. punctatipes – this species is larger and thicker-stemmed with larger yellow pores that are often somewhat radially elongated and/or decurrent. It also has a variably colored cap that can range from orange-brown to pinkish-purple.
- S. monticolus – this species has a swollen or bulbous stem.
- S. albidipes – this species has a cottony roll of tissue around its cap margin when young.
Toxicity, Safety & Side Effects[vii]
Although edible, Suillus granulatus is known to cause gastrointestinal upset and contact dermatitis in some individuals.
To avoid gastrointestinal upset, it is recommended that the cap of the mushroom and the tube layer be removed prior to cooking and consumption.
Contact dermatitis is thought to be caused by the milky sap discharged from the pore undersurface. To avoid it individuals may want to wear gloves when handling specimens of this mushroom.
[i] McKnight, K. H., & McKnight, V. B. (1987). Granulated Bolete. In Peterson Field Guide to Mushrooms (pp. 115-116). Houghton Mifflin
[ii] Kuo, M. (2022, February). Suillus granulatus.
[iii] Davis, R. M., Sommer, R., & Menge, J. A. (2012). Suillus granulatus. In Field Guide to Mushrooms of Western North America (pp. 331-332). University of California Press.
[iv] First Nature. Suillus granulatus (L.) Roussel – Weeping Bolete.
[v] Lincoff, G. (1981). Dotted-stalk Suillus. In National Audubon Society Field Guide to North American mushrooms (pp. 584) Knopf.
[vi] D. Arora (1986) Granulated Slippery Jack In Mushrooms Demystified: A Comprehensive Guide to the Fleshy Fungi, Ten Speed Press, Berkeley (p. 501-502)
[vii] McKnight, K. H., & McKnight, V. B. (1987). Granulated Bolete. In Peterson Field Guide to Mushrooms (pp. 115-116). Houghton Mifflin