Major Groups > Boletes > Red-Capped, Blue-Staining Boletes


Key to 38 Red-Capped, Blue-Staining Boletes in North America  

by Michael Kuo

Many of the boletes keyed below are notoriously difficult to identify--and, in many cases, poorly understood and poorly defined to begin with. European names and concepts have been used for some of the mushrooms; specific mycorrhizal association (which we now know to be often important in determining species) has traditionally been all but ignored by North American mycologists; erudite microscopic features have been under-emphasized in some areas, over-emphasized in other areas, and sometimes simply misunderstood due to insufficient sampling; species have been named or re-interpreted based on single collections; and, more recently, mistakes in field guides and online treatments have helped to take already-murky waters and turn them into a bona fide soup of bolete muck. Muck, muck, muck. So if you have collected a red-capped, blue-staining bolete and you are bound and determined to "put a name on it," you should know from the onset that your efforts may be more involved with psychology than mycology and that, if science is your goal, simply documenting and preserving your collection, and sending it to a herbarium, might help to enable a contemporary, DNA-based study of these beautiful but confusing mushrooms.

If you do decide to put on your boots and go bolete mucking, you will need to have made a collection in which both immature and mature specimens are represented, you will need to test chemical reactions (ammonia, KOH, and iron salts) on the cap surfaces and sliced flesh of your specimens--and you will definitely be using your microscope. The morphology of mature spores is one feature you will be assessing, but figuring out the structure of the pileipellis (the outer "skin" of the cap surface) is often essential. The pileipellis should be sectioned from the youngest, least mature specimen you can find, since the initial structure often begins to collapse and become unrecognizable in older specimens. Use the bolete equivalent of the Roman aqueduct section, mounted in KOH, and do not crush the mount too hard since doing so can disrupt the structure of the cells. Be prepared to assess the general arrangement of the hyphae (trichoderm, palisadoderm, and cutis are the common arrangements among these boletes), to measure the width of the pileipellis elements, and to assess the shape of the "terminal cells" (the cells at the ends of the elements)--and be prepared to be frustrated as you attempt to interpret what North American bolete authors (especially Smith & Thiers, 1971) could possibly have meant in their pileipellis descriptions, since they used terms inconsistently and neglected to illustrate the feature in virtually every case. If you want to be thorough you should also mount everything in Melzer's reagent--but I have real doubts about whether this will actually help you much, despite Smith & Thiers; in my experience amyloid, fleeting-amyloid, and kind-of-amyloid tissue reactions appear to be occasional and variable among many of the species in this group, and not necessarily indicative of speciation.

I hope this key helps to clarify some of the confusion--but I am not Mycology Superman and I may, of course, be simply mucking things up even more. For the casual bolete observer, the high points of my treatment include the following: I have elevated Boletus harrisonii to the level of most-collected little red blue stainer, and taken Xerocomellus rubellus and Boletus campestris off of their pedestals; I have updated the western "butter boletes" to reflect the recent publication of Arora and Frank (2014), which defined new species; I have sent the "curry odor" character, in the larger, eastern species, to the trash bin (see the discussion on the page for Boletus bicolor); I have attempted to clarify concepts for Boletus sensibilis and Boletus pseudosensibilis, which are often confused (especially online); and I have described and illustrated a few species (Boletellus pseudochrysenteroides, Boletus carminipes, Boletus cokeri, Boletus pallidoroseus, Boletus subfraternus) that are rarely treated.

1.Pore surface maroon, red, orange, or dark brown--or yellow when young, becoming red or orange with maturity.

1.Pore surface not maroon, red, orange, or dark brown in any stage of development.

2.Found in western North America (from the Rocky Mountains westward).

2.Found east of the Rocky Mountains.

3.Stem reticulate, at least over upper portion; mature stem usually at least 2 cm thick; known primarily from California, Oregon, Washington, and Idaho.

3.Stem not reticulate; mature stem 2 cm thick or not; variously distributed in western North America.

4.Mature cap with appressed hairy scales; associated with conifers.

4.Cap without hairy scales; variously associated.

5.Cap red to pink; spores generally 11-15 µ long; associated with oaks.
Butyriboletus querciregius

5.Cap purplish red; spores generally 13-17 µ long; usually associated with conifers.

6.Appearing from May to July; montane.
Butyriboletus primiregius

6.Appearing from September to December; coastal and montane.

7.Bright yellow partial veil present (especially notable when young).

7.Partial veil absent.

8.Stem 3 cm or more thick at maturity, reddish above and yellow toward the base.
Boletus smithii
at MykoWeb

8.Stem usually under 2 cm thick, reddish at the base and red to yellow above.

9.Stem usually about as long as the cap is wide, red from the bottom to about the midportion, pinched at base; cap almost always brownish red to reddish brown.

9.Stem often taller than the cap width, red nearly to the apex, rarely pinched at base; cap usually a shade of brown, sometimes becoming reddish with age.

10.Spores truncated at one end; pore surface usually bruising promptly and prominently.

10.Spores not truncated; pore surface bruising slowly and sometimes faintly.

11.Partial veil present, covering the pore surface when young and later clinging to the cap margin.

11.Partial veil absent.

12.Cap fairly smooth and bald; partial veil bright yellow; spores smooth; widely distributed in eastern North America.

12.Cap covered with prominent hairy scales; partial veil whitish to pinkish; spores ribbed; in North America known from the Gulf Coast and Mexico.

13.Stem prominently reticulate, overall or over at least the upper third.

13.Stem not reticulate, or merely finely reticulate at the apex.

14.Taste bitter; cap usually becoming brownish with age; spores 9-12 µ long.
Caloboletus peckii
in Smith & Thiers, 1971

14.Taste mild to sour or lemony; cap remaining red or becoming brownish; spores variously sized.

15.Cap purplish pink to purplish red; stem mostly yellow (occasionally with reddish shades near the base).
Boletus roseopurpureus

15.Cap bright red to brownish red; stem red over the basal portion or nearly overall.

16.Cap bright red, remaining so into maturity; pileipellis at least partially a trichoderm.

16.Cap rusty red, soon fading to brownish; pileipellis always a cutis.
Boletus pseudopeckii
in Smith & Thiers, 1971

17.Cap and stem brownish red ("bay red"); pore surface very pale yellow; blue staining weak; ammonia on cap often green or olive; under conifers or growing from well rotted conifer wood.

17.Not with the above combination of characters.

18.Spore print dark olive gray to brown; spores ribbed.

18.Spore print olive brown; spores not ribbed.

19.Mushroom at maturity small to medium in size: cap 2-6 cm across, stem 5-10 (-15) mm thick.

19.Mushroom at maturity medium to large in size: mature cap 5-20 cm across, stem (10-) 15-35 mm thick.

20.Flesh in stem base, when sliced, with tiny bright orange to bright red dots (see illustration; use a hand lens).

20.Orange to red dots absent from flesh in stem base.

21.Fresh cap deep, bright red; stem surface usually featuring red dots and points; cap often becoming moderately to prominently cracked and mosaic-like with age; dried specimens retaining red shades; pileipellis a palisadoderm with septate, barrel-shaped to subglobose cells that are hyaline to faintly yellowish in KOH.

21.Fresh cap brick red; stem surface usually bald or slightly red-pruinose but without red dots and points; cap not becoming cracked, or only slightly so with age; dried specimens becoming brown; pileipellis a trichoderm of cylindric, tubular cells that are bright yellow in KOH.

22.Young cap usually a shade of brown (including reddish brown), sometimes becoming red with age; cap surface usually becoming prominently cracked and mosaic-like, with reddish flesh in the cracks; mature stem often taller than the cap width.

22.Cap color varying but generally more red than above when young; cap surface not normally becoming mosaic-like (and, if so, not showing reddish flesh in the cracks); stem-cap proportions varying.

23.Spores truncated at one end; pore surface usually bruising promptly and prominently.

23.Spores not truncated; pore surface bruising slowly and sometimes faintly.

24.Fresh cap reddish brown to purplish brown; known from eastern Texas in association with water oak and loblolly pine; spores 7-10 µ long.
Boletus lewisii

24.Fresh cap red; variously distributed and associated; spores varying in length.

25.Mushroom identifier willing to accept that, while some of the North American names for species in the rubellus group will eventually have to be applied as a matter of taxonomic necessity, the current species concepts in the group are demonstrably unreliable (and occasionally silly), and a very thorough contemporary study of many well documented collections will be required before any names can be applied with scientific confidence.
Little red-capped, blue-staining bolete

25.Mushroom identifier either wants to explore outdated, unreliable and occasionally silly species concepts--or wants to cling to an imagined past in which "there was no DNA and names made sense, damnit." (Microscopic examination of a scalp section from the disc of a young cap required in several cases.)

26.Mature cap tiny (about 2 cm across at maturity); stem red except for extreme apex; sliced flesh staining blue, then slowly light red; spores 8.5-11 µ long.

26.Mature cap 2-6 cm across; stem yellow, orangish, or red; sliced flesh staining blue, then remaining bluish, or fading back to yellow--or not staining at all; spores varying in length.

27.Basal mycelium white; spores 3-4 µ wide; pileipellis a trichoderm of cylindric, tubular elements.
Boletus rubeus
in Smith & Thiers, 1971

27.Basal mycelium yellow to yellowish; spores 4-6 µ wide; pileipellis a trichoderm to a palisadoderm, elements at least sometimes inflated and barrel-shaped to subglobose.

28.Pores large (1 mm or more across, especially near the stem); pileipellis elements hyaline in KOH.
Boletus fraternus
in Smith & Thiers, 1971

28.Pores small (1-3 per mm); pileipellis elements yellow in KOH.

29.Pileipellis a trichoderm; elements often with amyloid contents, occasionally inflated and reminiscent of sphaerocysts.
Boletus flavorubellus
in Smith & Thiers, 1971

29.Pileipellis a palisadoderm; elements lacking amyloid contents, never sphaerocyst-like.

30.Mature pores elongated and slot-like near the stem; mushroom medium sized (cap 4-9 cm across, stem 0.5-1.5 cm thick); spores 10-13 x 4-5 µ.

30.Pores not normally as above; mushroom medium sized to large; spores varying.

31.Cap rusty red, soon fading to brownish; associated with beech; pileipellis a cutis.
Boletus pseudopeckii
in Smith & Thiers, 1971

31.Cap more red, more persistently, than above; associated with various trees; pileipellis a trichoderm.

32.Sliced cap flesh turning red to reddish as well as blue; cap usually with olive hues (especially when young); distributed from North Carolina to Texas.
Boletus patrioticus

32.Sliced cap flesh not as above; cap with or without olive hues; variously distributed.

33.Ammonia on cap surface flashing strongly blue to green.

33.Ammonia on cap surface not flashing strongly blue to green (but may be slowly olive).

34.Young cap reddish cinnamon, changing quickly to yellow-brown; spores 3-3.5 µ wide; stem mostly yellowish, with a carmine red base.

34.Cap not colored as above; spores 3.5-5 µ wide; stem yellowish above and pink, reddish, or pinkish red below.

35.Fresh, young cap pink to pinkish red; stem base dark rose pink; odor of beef bouillon; pileipellis primarily with long, tubular terminal cells.

35.Fresh cap red to brownish red or reddish brown; stem base yellow or reddish but not rose pink; odor not distinctive; pileipellis with many terminal elements that end in short, septate sections.

36.Most spores under 8.5 µ long.

36.Most spores longer than 8.5 µ.

37.Fresh cap sticky, red to purplish red; pore surface bright yellow; stem yellow above, red below, fairly bald; flesh yellow.
Boletus purpureorubellus

37.Fresh cap dry, red; pore surface grayish olive; stem yellowish to brownish, velvety to hairy; flesh whitish.
Boletus/Gyrodon tennesseensis

38.Stem red from the bottom nearly to the apex (rarely, the upper 1/3 is yellow) throughout development (but basal mycelium may be yellow or white).

38.Stem essentially yellow, but often developing reddish to red colors toward the base or over the bottom half with age, or developing red streaks and spots (or, in one species, with a carmine red base throughout development).

39.Spores amyloid, 14-17 µ long; rare.
Boletus bicoloroides
in Smith & Thiers, 1971

39.Spores inamyloid, 8-12 µ long; common.

40.Cap and stem becoming conspicuously reddish to red- spotted, mottled, and splotched-looking; spores 10-13 µ long.
Boletus rufomaculatus

40.Cap not normally as above; spores varying in length.

41.Ammonia olive on cap surface; stem often becoming ridged longitudinally; generally found in pine-oak woods in the southeastern United States; spores 13-19 µ long.
Boletus rubricitribnus

41.Ammonia not olive on cap surface (but reaction not documented for one species); stem not usually becoming ridged; variously distributed; spores 9-16 µ long.

42.Cap soon developing olive shades; KOH olive on cap surface; pleurocystidia mostly vesiculose to utriform, without narrowed necks; often (but not always) collected under hemlock.
Boletus miniato-olivaceus
in Smith & Thiers, 1971

42.Cap not normally developing olive shades; KOH not olive on cap surface (but reaction not documented for one species); pleurocystidia usually fusoid-ventricose, with narrowed necks; usually collected under oaks and other hardwoods.

43.Cap soon fading to reddish orange or yellow; cap and stem surfaces not bruising; ammonia negative on cap surface.
Boletus miniato-pallescens
in Smith & Thiers, 1971

43.Cap not fading, or fading to reddish cinnamon; cap and stem surfaces bruising instantly dark blue; ammonia yellow on cap surface.

(References used for this page can be found in the reference list for boletes.)

Cite this page as:

Kuo, M. (2014, December). Key to 38 red-capped, blue-staining boletes in North America. Retrieved from the MushroomExpert.Com Web site: