Toxic Fungi of Western North America

by Thomas J. Duffy, MD

Mushrooms known for their GI toxins or irritants

It is often difficult to distinguish GI toxins and irritants from various intolerances. An example of intolerance is Agaricus bisporus and trehalose malabsorption. Young buttons of Agaricus bisporus have more of the complex carbohydrate trehalose. Some people lack the enzyme trehalose to break down this carbohydrate to sugar. (192) The result is diarrhea without vomiting. The lack of trehalose sometimes runs in families. (193)

Other authors have distinguished GI irritants that work locally in the gut (eg. the phenolic compounds in some yellowing species of Agaricus) from GI toxins that work deeper on peripheral nerve receptors or activate central nervous system centers. The CNS vomiting center, for example, may be activated by the terpines in the genus Omphalotus. The toxins of Chlorophyllum molybdites are unknown, but may produce its symptoms by both GI irritation and CNS toxicity.

It has already been noted that raw mushrooms are generally more toxic and some genera should never be eaten raw. Even with "safe" raw edibles, it is best to eat only small amounts uncooked. Relatively safe raw mushrooms include Agaricus bisporus and Agaricus campestris along with Boletus edulis, Boletus aereus and Boletus barrowsii. As noted previously, boletes have tubes instead of gills on the undersurface of the caps. The highly sought-after edulis group (Boletus edulis, Boletus aereus, Boletus barrowsii etc.) is particularly safe. When these boletes are immature, the pore-like mouths of their tubes are closed by a whitish flap.

Ingestion of Agaricus californicus instead of Agaricus campestris produces GI symptoms in many, but not all persons, suggesting personal idiosyncrasy. Back in the 1960’s, the author collected a few “odd Agaricus campestris” on a nearby golf course and had some nausea. They were less felty and more reflective of light, so only a few were eaten. We now recognize these specimens as the often GI symptomatic: Agaricus californicus. This noxious domestic weed is now more common than Agaricus campestris in many California areas. It may also invade groups or fairy rings of Agaricus campestris on lawns. Peck described Agaricus californicus in 1895 from a Pasadena, California collection, but it was not a wide spread nuisance until the last half of the 20th century. (194)

Agaricus californicus
Agaricus californicus photo © Fred Stevens

The anatomical differences between Agaricus campestris and Agaricus californicus are relatively subtle. The Agaricus campestris cap feels and looks more felty to the touch, the ring is exceedingly thin and without definite patches of a secondary veil on the undersurface of the ring. The partial veil tears in a stellate fashion like thin tissue paper and the cap does not turn yellow with 2.5% KOH. Agaricus campestris has a pleasant mushroomy odor and the young gills are pink from the start.

Agaricus californicus frequently has a silky sheen, a more fibrillose cap and the partial veil and ring are much more substantial, the ring having small patches of a double veil on the underside. There usually is a faint phenolic odor to Agaricus californicus, but it may be slight and some people seem unable to smell it. Current disinfectants no longer smell of phenol and the exact odor is best taught by smelling certain other Agaricus species. Crushing and smelling the base of Agaricus (“meleagris”) praeclaresquamosus provides a good example. Sometimes there is very slight yellowing of the carpophore with bruising, but this is not dependable. The gills are whitish in small buttons.

Other gilled fungi commonly producing GI symptoms include other members of the Agaricus group, Xanthodermati: Agaricus hondensis, Agaricus placomyces var. microsporus, Agaricus praeclaresquamosis and Agaricus xanthodermus. These latter Agaricus species are easily distinguished by their yellow staining and unpleasant phenolic odors. Sometimes members of another Agaricus group cause trouble—primarily Agaricus albolutescens and Agaricus silvicola.

Agaricus xanthodermus
Agaricus xanthodermus photo © Michael Wood

Although Clitocybe (Lepista) inversa is listed in most texts as edible, what appears to be the same mushroom has caused poisonings in France. (195) One member of the MSSF has had mild nausea and moderately severe diarrhea from its ingestion on two occasions.

Other offenders are species of Gymnopilus, Hypholoma and Hebeloma (especially Hebeloma crustiliniforme, H. fastibile, H. mesophaeum and H. sinapizans); very acrid species of Lactarius and some species of Lactarius that stain lilac (especially Lactarius glaucescens). (196) The first Hebeloma crustiliniforme poisoning reported from North America was in 1927.197This mushroom is known as "poison pie" in Europe, because of its intense diarrhea. Many Hebeloma species have a radish-like odor; that and brown gills on maturity should cause any such to be bypassed for culinary use.

Hebeloma crustiliniforme
Hebeloma crustiliniforme photo © MIchael Wood

Occasionally, the Russula laurocerasi group, Pholiota species and Phaeolepiota aurea produce GI symptoms. Megacollybia (Tricholomopsis) platyphylla and a number of tricholomas: Tricholoma pardinum, T. pessundatum, T. saponaceum, T. sejunctum, T. sulphureum, T. vaccinum, T. zelleri and possibly T. virgatum are others that often cause GI upsets. Of these, the most vicious offender is Tricholoma pardinum, which may cause prolonged vomiting and diarrhea.

Collectors of coccoli (Amanita lanei) may mistakenly gather a mushroom in the spring that has even brighter yellow to orange caps—the toxic Amanita aprica J. Lindgr. & Tulloss. A frost-like layer of universal tissue tends to persist on the caps. This amanita usually occurs on open banks or along road cuts or paths in Douglas Fir and pine forests at elevations from 600-1850 meters. It is a typical species of the Cascade and Sierra Nevada mountains. This beautiful Amanita produces nausea, vomiting, diarrhea, intestinal cramps and muscle spasms.

Cortinarius species are rarely collected for the table in Western North America. There have been no definite Cortinarius orrelanoides poisonings at the time of this publication. Raw Cortinarius species (C. camphoratus & C. purpurascens) have been reported in the West to cause gastrointestinal problems. (122) There seem to be no Dermocybe poisonings reported from our area of concern, although some species of this genus (which Moser segregates from Cortinarius) apparently contain anthroquinones which act as GI irritants. (126)

The acrid species of Russula and Lactarius have GI irritants, whereas the non-acrid species are often edible. Russula emetica (an acrid species native to European sphagnum bogs) is replaced in North America by Russula silvicola. Russula sanguinea, equally acrid, has both a red cap and a stipe flushed rosy to reddish. Russula nigricans in a 1985 Oregon poisoning produced not only GI symptoms, but also dizziness and disorientation. (198)

Entoloma species often produce severe GI toxicity. All of them should be avoided; data on American species is very limited. The spores are pink in mass. Under the microscope, these spores are angular (frequently 5-6 sided); rarely long and longitudinally ridged. The PNW has the toxic Entoloma grande. (199) Several forms of the toxic Entoloma rhodopolium are present in California. Other forms are found in the Pacific Northwest. Entoloma rhodopolium has occasionally also produced muscarinic symptoms. Entoloma lividoalbum is considered toxic in Colorado.

Tricholoma pardinum poisoning often causes such severe vomiting and diarrhea that hospitalization is needed. Sometimes other Tricholoma species with gray, black or reddish-brown caps may cause lesser gastrointestinal distress. Tricholomas with orange or reddish caps found grouped under various trees need to be carefully examined. Some of these, such as Tricholoma populinum, usually found in sandy soil under cottonwood trees are good edibles. However, they may be confused with members of the poisonous Tricholoma pessundatum group: Tricholoma pessundatum, T. manzanitae, T. albobrunneum, T. ustaloides, T. flavobrunneum, T. acerbum and T. ustale. Any of these mushrooms may cause quite severe gastrointestinal symptoms.

Chlorophyllum molybdites is usually found on lawns or in grassy areas (often in huge fairy rings). This large and inviting mushroom has green spores, although buff spores have been rarely seen. Although Chlorophyllum molybdites has been reported safe to eat after boiling, apparently well-cooked meals occasionally cause nausea and vomiting. More than one toxin may be present. A 2–year-old child died after eating Chlorophyllum molybdites in 1900, apparently becoming dehydrated. (202) As noted in XII Miscellaneous Toxins, there have been a few reports of hemorrhage, when this mushroom had been eaten raw. One possible toxin has been reported to be a polymeric protein. (203)

Sceleroderma species, unlike puffballs, usually have shades of black, brown or dark purplish centers even in the early stages of development and, additionally, they have a thick tough outer layer. Scleroderma citrinum and Scleroderma cepa may cause very violent vomiting, diarrhea and prostration. Scleroderma cepa has a nearly white interior. A Tacoma, Washington, woman in 1978 collected what she thought were puffballs, since two of the smaller ones were pure white inside. (204) Sauteéd in butter, they were delicious, but 1½ hours later she suffered vomiting, profuse sweating, prostration and two brief episodes of loss of consciousness. A half hour later, she was nearly back to normal. Dr. Beug at the Evergreen State College noted a small amount of purple in one specimen. Dr. David Hosford at Central Washington State University identified the specimen as Scleroderma areolatum. Dr. Beug then advised that, in addition to a white center for a field description of a puffball, one should add soft and with a texture resembling a marshmallow. Young Scleroderma specimens may also have white centers, but the tissue is then tough or hard.

Scleroderma cepa
Scleroderma cepa photo © Fred Stevens

Morchella species, especially the black specimens, occasionally cause nausea, vomiting and diarrhea. Additional symptoms of fever, headache, sweating and weakness are sometimes reported. (204a)

Although Gomphus floccosus, G. bonari and G. kauffmanii are occasionally eaten without problem, they may cause GI symptoms. Symptoms may occur as late as 8-14 hours after the meal, a latency period which is startlingly different from other GI toxins. At least one toxin, norcaperatic acid, has been detected. Fortunately, these ice-cream cone shaped fungi are easy to identify. Dr. Alexander Smith (who got hay fever-like symptoms from many fungi) reported for Gomphus floccosus that “I have tried it on three different occasions and always experienced considerable (GI) discomfort within eight to fourteen hours”. (205)

Sarcosphaera coronaria is a common montane species that occurs in the spring as an inhabitant of forest duff. Starting out below ground as a hollow sphere with a beautiful lilac tone to its interior, it soon penetrates the duff splitting from the top down as a crown-like cup. Eaten raw, it produces severe vomiting and diarrhea. Although said to be a good edible when thoroughly cooked, this mushroom is best avoided.

Orange or red-pored boletes often cause GI symptoms. Muscarine may also be present in a few. Collectors should generally avoid these boletes in Section Boletus and Subsection Luridi, although Boletus erythropus is edible and very tasty. All of the following have produced GI symptoms: Boletus amygdalinus, B. haematinus, B. luridus, B. satanas and B. pulcherrimus. Boletus luridus also interacts with alcohol. (206) The general advice to avoid all boletes that stain blue regardless of pore mouth coloring proscribes the ingestion of some boletes that are quite tasty in the West such as Boletus appendiculatus. It remains best to avoid those that have orange or red tube mouths that stain blue or are bitter when tasted raw.