Toxic Fungi of Western North America

by Thomas J. Duffy, MD

GI reactions due to fungi usually considered edible

Most mushroom “poisonings” cause no more than GI distress: abdominal cramping, diarrhea, nausea and vomiting. Although a number of toxins are known to cause only GI symptoms, most of these are poorly understood. There are too many GI toxic fungi to describe here and other sources should be consulted. There is a description of additional GI toxic species appended to this text.

The great variation in the presence of any symptoms between people eating the same collection for a meal suggests that individual susceptibility is important. The leading American mycologist of the 20th century, Alexander H. Smith, picked specimens of Lepista irina, an edible mushroom, from a 'fairy ring' of this species in 1958 and checked several specimens under the microscope. 'Fairy rings' of mushrooms, often with 15-40 individuals, grow outward as nutrients in the central area are used up by the fungi. All such mushrooms are believed to be the fruiting bodies of the same mass of mycelia. Therefore, the mushrooms from such a collection should be nearly identical chemically. Although students at the University of Michigan had been picking them for food and Dr. Smith had done so for several years, this time everyone partaking of his collection enjoyed them, except for a husband and wife who woke up in the middle of the night with gastrointestinal symptoms. The most likely assumption was individual idiosyncrasy. (188)

The true Laetiporus sulphureus does not occur along the Pacific Coast. The Laetiporus that grows on Eucalyptus and probably on oaks is Laetiporus gilbertsonii. (188a) The Laetiporus growing on conifers is Laetiporus conifericola. Both species are edible and highly sought after. Each can cause occasional idiosyncrasies. The San Francisco Toxicology Committee in 1975 surveyed its members along with some amateur mycologists in the PNW. (177) Examination of the 158 reports of fungal poisoning surprisingly revealed that seven persons had had almost identical reactions to Laetiporus--nausea and often vomiting within 5-45 minutes of ingestion of the sulfur shelf. Symptoms lasted a few minutes to 2 ½ hours. Numerous other cases surfaced later, especially with raw or nearly raw specimens. There also was initially thought to be a difference in symptoms dependant on the various substrates. It seemed that Laetiporus “sulphureus” growing on eucalyptus more frequently caused symptoms, but that thesis was later in question as more cases produced by Laetiporus “sulphureus” growing on oak and conifers were reported. It remains possible that Laetiporus gilbertsonii more often causes symptoms than Laetiporus conifericola.

Laetiporus gilbertsonii
Laetiporus gilbertsonii photo © Fred Stevens

About the same time, members of MSSF also recorded two rare, but dramatic and presumably idiosyncratic reactions to chanterelles. These two initial cases were similar and unusual in that nausea and short-lived vomiting began almost immediately after ingestion. The first victim vomited after a small plate of European chanterelles (Cantharellus cibarius). This unpleasant experience was repeated when he ate a supposedly mushroom-free soup in Switzerland. The cook admitted that a small amount of chanterelles had been added to the soup for extra flavor. The second poisoning was similar, but occurred in Western North America.

Most of the chanterelles along the northern coast are Cantharellus formosus (not the European Cantharellus cibarius) and Cantharellus cibarius var. roseocanus. (189) There are a number of unnamed species in Western North America, including the Cantharellus most common in the San Francisco Bay area. There appear to be at least five chanterelle species in California and more in Oregon, Washington and Idaho. (189a) These chanterelles usually fruit in moist areas, but may be found in dry areas as well occasionally. When this occurs, they are usually found later in the year. In the dry San Joaquin Valley east of the coastal mountains in California, they sometimes fruit in 3-4 year cycles. (190)

Armillariella mellea, the honey mushroom, which produces occasional gastrointestinal symptoms both in North America and in Europe, is now known to be several species--of which the most common in California is Armillariella ostoyae with a light orangish tan to medium brown cap. Armillariella gallica, which has a smooth cap and more bulbous base is more common from the PNW to Colorado, than it is in California. There is some uncertainty as to whether occasional GI toxicity is more related to cooking or individual sensitivity to one or both of these Armillariella species. Many Europeans blanch Armillariella mellea in boiling water before eating or pickling.

Armillariella mellea
Armillariella mellea photo © Michael Wood

Another genus that may cause GI upsets is Leccinum, a member of the family Boletaceae (boletes) that has scaly stipes and is most commonly found under birch. Although most cases of reported Leccinum toxicity come from Colorado, this fact is likely an artifact of active reporting and the fact that Colorado has so much Leccinum in association with birch. Most problems appear to be with poorly cooked or raw Leccinum. Dr. Dennis Desjardin and Marilyn Shaw believe that, as in the blue-staining Boletus,there may be a GI irritant present at very low levels. (91)

Morels should not be eaten raw, but they are occasionally served that way in salads. The first report came from Canada and the morels were eaten with alcohol. (191) Gastrointestinal symptoms occurred in 77 out of 483 persons at a Vancouver Banquet. (82)

Leucoagaricus leucothites (Lepiota naucina), a white-capped mushroom with white to faintly pinkish gills is a good edible. Some field guides refer to a grey “variant” that may cause gastrointestinal upset. This mushroom is actually a separate species, Leucoagaricus barsii (= L. pinguipes), and it causes occasional GI distress.