Summary:
Suillus flavidus is recognized in North America by 1) a tan to yellowish to olive-brown, umbonate, viscid to glutinous cap, 2) pale yellow flesh that stains dingy cinnamon, 3) irregular pores that are often compound and often radially arranged, and are yellowish to olive yellow bruising dingy pinkish cinnamon, 4) a slim, whitish to pale yellow stem with glandular dots, 5) viscid annulus, and 6) growth under pines. |Authors have debated for many years whether Suillus umbonatus, described from Utah, is a synonym of Suillus flavidus of Europe. Moser(12) thought so and provided a detailed analysis. Here is the opinion of Nguyen(3) published in 2016, based on their molecular studies (with Latin names italicized), "The /flavidus clade contained specimens with names such as S. umbonatus E.A. Dick & Snell, S. megaporinus Snell & E.A. Dick, S. flavidus (Fr.: Fr.) J. Presl, and S. helenae. Members of this clade exhibit the largest diameter of pores in the genus (up to 1cm). The name S. flavidus has been used for European specimens, whereas the name S. umbonatus is applied to the western North American specimens, and S. megaporinus is applied only to specimens collected in the Sierra Nevada of California. The holotype sequence of S. helenae (named for Helen Smith, Alexander Smith’s wife) also fits within the group and was described from a single collection in Oregon. If holotypes of the aforementioned species fit in this clade, S. flavidus is the oldest name and should be used for this species." |They do include 6 collections identified as Suillus umbonatus in the /flavidus clade (3 from CO, CO, CA). |The online Index Species, and MycoBank, accessed February 18, 2018, both maintained them as separate entries, but this is just a reflection of the fact that the holotype of Suillus umbonatus has not been sequenced. |The balance of the evidence is that what we have been identifying in the Pacific Northwest as Suillus umbonatus should now be called Suillus flavidus (Pacific Northwest material does fit the description of Suillus umbonatus.) |On the subject of Suillus helenae, the molecular work of Nguyen(3) shows that the holotype of Suillus helenae from OR falls within the clade that included Suillus flavidus and Suillus umbonatus, which would indicate synonymy of Suillus helenae with Suillus flavidus under the latter name. Suillus helenae has been differentiated by lacking the partial veil and annulus of Suillus flavidus (Thiers(12) original description, Arora(1), Bessette(3)). Thiers(12) say (with Latin names italicized), "It has the stature and large pores characteristic of Suillus umbonatus Dick & Snell or S. flavidus (Fr.) Singer but both of these species have a veil and no such structure was present on any of the basidiocarps of this collection. The cuticular structure of S. helenae is also distinct in that there is no apparent gelatinization of the hyphae in KOH and, furthermore, these hyphae are relatively large and appear incrusted. The discovery of this species marks the first example apparently representing an evolutionary line branching from the S. umbonatus-S. flavidus complex."

Chemical Reactions:
cap surface stains reddish brown with KOH, (Bessette)

Odor:
none (Smith)

Taste:
slightly sour (Smith)

Microscopic:
spores 7-9(10) x 4-4.5 microns, narrowly elliptic to nearly oblong, smooth, yellowish in KOH, darker yellowish to pale tawny in Melzer''s reagent; basidia 4-spored, 20-26 x 5-7 microns, clavate, colorless or nearly so; pleurocystidia 28-46 x 5-8 microns, cylindric, clavate, or narrowed at apex, content colorless to yellowish brown, with some amorphous debris adhering to outer surface, scattered or present in bundles with brown amorphous pigment around base of bundle, cheilocystidia similar; cap epicutis a thin layer of gelatinous to subgelatinous hyphae 3-6 microns wide and yellowish in KOH, the ends often ascending to form an obscure turf; caulocystidia "in bundles surrounded by brown pigment and similar to cheilocystidia, and unincrusted septate (near base) crooked filaments 30-50 x 4-6 microns also present in a dense to lax turf"; clamp connections absent, (Smith), spores 7-11 x 3.5-4.5 microns, elliptic to spindle-shaped, smooth, (Arora)

Spore Deposit:
dull cinnamon (Smith), olive-brown to dull cinnamon-brown, (Arora)

Notes:
S. flavidus has been found in WA, OR, ID, CA, and UT, (Smith), and was reported from BC (in Redhead). Suillus umbonatus also occurs in CA (Thiers(1), Arora(1)). Nguyen(3) include collections from CO and CA at least.

Synonyms and Alternate Names:
Ditiola shopei Coker
Guepiniopsis alpina (Tracy & Earle) Brasf.