Butyriboletus regius

The species was first described and illustrated by Julius Vincenz von Krombholz in 1832. Common names for the mushroom include the red-capped butter bolete and the royal bolete.

Butyriboletus regius was formerly classified as a member of the section Appendiculati of the genus Boletus. Molecular analysis demonstrated that this and related "butter bolete" species, including Boletus appendiculatus, are phylogenetically distinct from Boletus, and the new genus Butyriboletus was created to contain them.

Harry D. Thiers described a similar mushroom from California as B. regius, though it is not the same species.{{Citation needed|date=June 2025}}

File:2008-12-09 Boletus regius 1 15136.jpg Thiers, collected in California]]

The caps are convex or flat, reaching a diameter of {{convert|6–20|cm|abbr=on|frac=4}} wide.{{Cite book |last=Arora |first=David |author-link=David Arora |title=Mushrooms Demystified: A Comprehensive Guide to the Fleshy Fungi |url=https://archive.org/details/arora-david-mushrooms-demystified-a-comprehensive-guide-to-the-fleshy-fungi-ten-speed-press-1986/page/526/mode/2up |publisher=Ten Speed Press |isbn=978-0-89815-170-1 |location=Berkeley, CA |year=1986 |orig-date=1979 |edition=2nd |pages=526}} The cap surface is pink to red, occasionally with hints of yellow or brown, more so around the margin. Initially velvety to slightly tomentose (hairy) when young, these minute hairs tend to slough off with age, and the cap develops wrinkles and pits. The cap flesh is yellow, usually slowly and erratically bruising blue in North American specimens. In Europe, it does not stain when exposed to air or stains weakly.

The pores on the underside of the cap are angular and measure about 1–2 per millimetre. The color of the pore surface is bright yellow to start, but eventually darkens somewhat, and will stain blue with damage. The depth of the tubes comprising the pores extends to {{convert|0.8–2.5|cm|1|abbr=on|frac=4}}.

The stem measures {{convert|5–14|cm|abbr=on|frac=4}} long and {{convert|2.5–6|cm|abbr=on|frac=4}} thick, typically with a thick, bulbous base. It is solid (i.e., not hollow), and a bright yellow color, often with reddish tones, particularly near the base of the stem. The stem surface can be covered with fine yellow reticulations either throughout its length, or just on the upper portion. The spore print is olive-brown. The smooth, hyaline (translucent) spores are roughly elliptical to somewhat fusoid (wider in the middle and tapering toward the ends) to more or less cylindrical, and have dimensions of 12–17 by 4–5 μm.

Chemical tests can be used to help identify B. regius in the field. The cap cuticle will stain a pale purple color if FeSO₄ is applied; this same test will turn the flesh grayish.

Butyriboletus regius is an ectomycorrhizal species with a broad host range, associating with oak and conifers, especially fir. The fruit bodies grow singly, scattered, or grouped together. In North America, they usually appear from August to November, although they also appear between May and June. The North American distribution includes the Pacific Northwest states of California, Oregon, and Washington, where its frequency of occurrence ranges from "rare to locally abundant". It is rare in Europe, appearing on the Regional Red List of several countries and being considered endangered in the Czech Republic. The species has also been recorded from China.

Both European and California species are considered choice edibles, but the latter can be maggot-infested.

{{Portal|Fungi}}

• List of North American boletes

{{Reflist|colwidth=30em|refs=

{{cite journal |vauthors=Arora D, Frank JL |title=Clarifying the butter Boletes: a new genus, Butyriboletus, is established to accommodate Boletus sect. Appendiculati, and six new species are described |journal=Mycologia |year=2014 |volume=106 |issue=3 |pages=464–80 |doi=10.3852/13-052 |pmid=24871600|s2cid=207708824 }}

{{cite web| title=Boletus regius Krombh.| publisher=Boletales.com| url=http://boletales.com/genera/boletus/b-regius/| first=Boris| last=Assyov| access-date=2011-04-06}}

{{cite book| last=Thiers| first=Harry D.| author-link=Harry D. Thiers| year=1975| title=California Mushrooms: A Field Guide to the Boletes| publisher=Hafner Press| location=New York, New York| url=http://www.mykoweb.com/boletes/index.html}}

{{cite web|url=http://www.mykoweb.com/CAF/species/Boletus_regius.html |title=Boletus regius |publisher=California Fungi |first1=Michael |last1=Wood |first2=Fred |last2=Stevens |access-date=2011-04-06 |url-status=dead |archive-url=https://web.archive.org/web/20110606051140/http://www.mykoweb.com/CAF/species/Boletus_regius.html |archive-date=2011-06-06 }}

{{cite book |title=North American Boletes |vauthors=Bessette AE, Roody WC, Bessette AR |year=2000 |publisher=Syracuse University Press |location=Syracuse, New York |page=147 |isbn=978-0-8156-0588-1}}

{{cite web |url=http://www.fungi4schools.org/Reprints/ENGLISH_NAMES.pdf |title=List of recommended English Names for Fungi in the UK |publisher=British Mycological Society |year=2003 |access-date=2012-09-03 |archive-url=https://web.archive.org/web/20110716083053/http://www.fungi4schools.org/Reprints/ENGLISH_NAMES.pdf# |archive-date=2011-07-16 |url-status=dead }}

{{cite journal |author=Chiu W.F. |title=The boletes of Yunnan |journal=Mycologia |year=1948 |volume=40 |issue=2 |pages=199–231 (see p. 224) |jstor=3755085 |url=http://www.cybertruffle.org.uk/cyberliber/59350/0040/002/0224.htm |doi=10.2307/3755085|url-access=subscription }}

{{cite book |author=Ellis J.P. |title=Fungi without Gills (Hymenomycetes and Gasteromycetes): An Identification Handbook| url=https://books.google.com/books?id=vowdIZ7GqD4C&pg=PA46 |year=1990 |publisher=Springer |isbn=978-0-412-36970-4 |pages=46}}

{{cite book |author=Koune J.-P. |title=Threatened Mushrooms in Europe |url=https://books.google.com/books?id=PL94uDj8yBEC&pg=PA29 |year=2001 |publisher=Council of Europe |isbn=978-92-871-4666-3 |page=29}}

{{cite journal |author=Mikšik M. |title=Rare and protected species of boletes of the Czech Republic |journal=Field Mycology |year=2012 |volume=13 |issue=1 |pages=8–16 |doi=10.1016/j.fldmyc.2011.12.003|doi-access=free }}

{{cite book |vauthors=Smith SE, Read DJ |title=Mycorrhizal Symbiosis |url=https://books.google.com/books?id=qLciOJaG0C4C&pg=PA198 |year=2008 |publisher=Academic Press |isbn=978-0-12-370526-6 |page=198}}

{{cite book |last=Snell |first=Walter |last2=Dick |first2=Esther A. |author-link1=Wally Snell |title=The Boleti of Northeastern North America |year=1970 |publisher=J. Cramer |location=Lehre, Germany |pages=74–5 |isbn=978-0854860166}}

{{cite book |author=von Krombholz J.V. |title=Naturgetreue Abblidungen und Beschreibungen der essbaren, schädlichen und verdächtigen Schwämme |trans-title=Realistic Illustrations and Descriptions of Edible, Harmful, and Suspicious Fungi |year=1832 |volume=2 |location=Prague |publisher=In Commission in der J. G. Calve'schen Buchhandlung, 1831–1846 |page=3, t. 7 |url=https://www.biodiversitylibrary.org/page/15574368 |language=German}}

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• {{IndexFungorum|803323}}

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regius

Category:Edible fungi

Category:Fungi described in 1832

Category:Fungi of China

Category:Fungi of Europe

Category:Fungi of North America

Category:Fungus species