Toxic Fungi of Western North America

by Thomas J. Duffy, MD

“Look-Alike” mushrooms

Most “look-alikes” of toxic and edible fungi only superficially resemble each other, but without close inspection in good light, terrible errors have been made. Deadly examples of mistaken identity include: picking the greenish Amanita phalloides instead of the yellow Amanita (calyptroderma) lanei (Murrill) Sacc. & Trott. with its single thick white patch of universal veil on the cap, picking a young Amanita smithiana Bas instead of Tricholoma magnivelare (Peck) Redhead (“matsutake”) with its marked heft and toughness or picking a white or pallid amatoxic Amanita species instead of the edible Volvariella speciosa with its pink spores. Volvariella speciosa grows in grassy areas and the stiff, thick cup of the volva flares away from the stipe and only collapses on it when old.

Volvariella speciosa also has a fairly distinctive earthy odor with a slippery grey cap when young. Since this species is eaten fairly often in Hawaii, confusion of it with the presumably amatoxic, white to gray-marbled, Amanita marmorata subsp. myrtacearum Miller, Hemmes and Wong (19a) found in the Hawain islands is a possible threat. However, the habitats are usually quite distinct: grass and lawn for Volvariella speciosa and eucalyptus and acacia forests for Amanita marmorata.

Volvariella speciosa
Volvariella speciosa photo © Fred Stevens

Chlorophyllum molybdites, Chlorophyllum (Macrolepiota) rachodes,and Chlorophyllum brunneum are very similar in appearance, having large caps with buff “shingles” (large overlapping scales), etc. Chlorophyllum molybdites causes severe nausea, vomiting and diarrhea; Chlorophyllum rachodes and Chlorophyllum brunneum are good edibles. When thoroughly cooked, Chlorphyllum molybdites may be tolerated by some people, at least in small quantities. It is quite tasty reportedly, but hardly worth risking severe gastrointestinal poisoning. The spores are dull green in contrast to the white spores of Chlorophyllum rachodes and Chlorphyllum brunneum. The three species have scaly caps and smooth stipes, the flesh and surfaces often staining a reddish/orange when bruised or scraped. Chlorophyllum molybdites grows most commonly on lawns in warm areas of the U.S. and Mexico, often in large fairy rings.

Chlorophyllum molybdites
Chlorophyllum molybdites photo © Michael Wood

Puffballs, edible when young and white through the center, have a solid undifferentiated interior, although there may be a slightly distinct sterile base. An Amanita button, when cut in half, will show the embryonic gills and a faint outline of the maturing mushroom, which will later rupture the universal veil that covers it. Puffballs should be cut in half, when collected or prepared for food, not only to distinguish them from Amanita buttons, but also because a yellow, brown, dark purplish or blackish center indicates spore maturation that will probably cause GI distress.

Pot-hunters may confuse the toxic Amanita pantherina sensu auct amer. with brown Agaricus species. Amanita pantherina in the United States varies markedly. What may be called the “pantherina complex” in Western North America describes a number of different species, including Amanita pantherina (DC.:Fr.) Krombh., Amanita pantherina var pantherinoides (Murrill) Dav. T. Jenkins and possibly Amanita multisquamosa Peck. Amanita multisquamosa is primarily an eastern species, but it may additionally occur in the PNW. Other members of the group have not yet been sorted out.