Toxic Fungi of Western North America

by Thomas J. Duffy, MD

Ecology and general anatomy of fungi

A mushroom is the fruiting body of a fungus; just as a berry is the fruiting body of a green plant, bush or tree. The vegetative portion of the fungus that gives rise to mushrooms is a tangle of long cellular threads called mycelia. The substrate on which a mushroom grows may be bare soil, humus, dung, decayed wood, living trees or other organic material.

Most of our commercially sold mushrooms are decomposers, such as the white to brown varieties of Agaricus bisporus most commonly seen in grocery stores. Other mushrooms are parasitic, most conspicuously on trees, although often on previously damaged ones. Many fungi also combine with plant rootlets in a mutually beneficial fashion called mycorrhizal.

Since fungi do not have chlorophyll to provide energy for their growth and reproduction, mushrooms may associate with green-plant rootlets to advantageously swap the chemicals of life. The mycelia of such mushrooms closely wrap themselves around and/or penetrate the fine rootlets of trees and other green plants to form a relationship called mycorrhizal. In such an association, there is a bi-directional movement of chemical compounds in which water, mineral salts and simple nitrogenous substances flow to the green plant and simple carbon compounds along with more complicated carbohydrates flow to the mushroom. The structures themselves are called mycorrhizae.

Most of the mushrooms in our forests have winding mycorrhizal attachments to the fine rootlets of trees that are superficial in that they do not penetrate the cells of the rootlets; such mycorrhizal structures are called “ectotrophic mycorrhizae”. In contrast, those mycorrhizae that penetrate cells by one of several known structures are called “endotrophic mycorrhizae”--a type of mycorrhiza typical of non-woody green plants and some plants of the heath family such as blueberries. The vast majority of all plant species, perhaps as high as 95%, belong to genera that characteristically form mycorrhizae with fungi.

Trees, grown with their native mycorrhizal fungi, grow much more quickly in forests or garden plots than those trees grown without their fungal partners. Part of this benefit is due to an increase in the surface area of the fungal-associated roots and their better absorption of subsurface water and minerals; part of the benefit is due to active chemical transfer by the mycorrhizae.

Some of the more ancient genera of trees, including sequoia (Sequoiadendron, Sequoia and Metasequoia) do not form ectomycorrhizae. All the large ectomycorhizal mushrooms found in Redwood forests are in association with other trees or bushes, such as manzanita, oak, tanoak and madrone. Sequoiadendron gigantea and Sequoia sempervirens form endomycorrhizal structures—spores, branching hyphae and balloon-like vesicles—but all on a microscopic scale requiring special stains.

The fruiting pattern of all higher fungi is exceedingly complex and much more variable than that of green plants. Known factors include moisture, drainage, temperature, pollution, time of the last fruiting, changes in associated plants and soil disturbance. High winds rapidly dry out what would otherwise be a good crop. Fog along the coast primes many species that fruit in the fall and some fungi get enough fog drip to repeatedly fruit before the fall rains. Snow bank fungi depend on the depth of snow, rapidity of snowmelt and the amount of spring rainfall among many factors.

Mycelia can lie unproductive for many years. A retirement community (Rossmoor) near San Francisco had a hillside on which morels had not previously been found: a period close to 25 years. After disturbing the soil for new landscaping, wood chips were placed over the ensuing berm. The following spring in 1996 hundreds of morels sprang up. No morels have been seen since that time. Such an isolated fruiting may be due to pre-existing mycelia in the ground that respond to the stimulus of uncovering, followed by the placement of a moisture barrier (in the form of woodchips).

Many mushrooms do not at all look like the common store mushroom. There are club, coral, cauliflower, cup and other forms in a spectacular array of colors—white, yellow, gray, brown, orange, red, violet, but very rarely, green. However, our most familiar mushrooms have a cap, vertical plates (gills) under the cap and a "stalk"—referred to here as a stipe. The mushroom's spores are usually ejected from the sides of these variably spaced gills, which may be free from the stipe or attached in several ways to it. In a variant of this tendency to form gills, there may be only “folds” or ridges as in chanterelles; occasionally even this structural feature is reduced to smooth or pimpled surfaces.

A ring or annulus may be present on the stipe as the remnant of an embryonic partial veil that was attached between the cap edge and the stipe. A universal veil, when present, sheathes the entire mushroom in its embryonic stage and leaves a cup or other traces of tissue near the base of the stipe (volva) and often “warts” or “patches” on its cap. A universal veil is always present in the genus Amanita, the genus responsible for most deaths. Such veil tissue may be missed due to weathering of the cap or carelessness in unearthing specimens. The color of the cap is always given as that of the tissue underlying the universal veil.

The gilled fungi like the grocery-store buttons contain the most highly toxic species of the higher fungi. It is ironic that the more bizarre or unfamiliar forms are usually less toxic. The gilled Amanita phalloides (mycorrhizal & non-native) and Amanita ocreata (mycorrhizal & native) have caused almost all of the deaths from poisonous mushrooms in Western North America. Amanita phalloides may have been introduced on the rootlets of imported European cork oak among others. (2)

Pasadena, California, was the site of the first collection of Amanita ocreata. This species, unlike Amanita phalloides, is native to California. It is now found, although less frequently, in Oregon and southern Washington.

All deaths recorded in Western North America have been from gilled fungi, except for a single death reported from Oregon in 1994. The cause of that death was Boletus pulcherrimus Thiers and Halling, a mushroom with tubes instead of gills under the cap. (3) Boletus pulcherrimus Thiers and Halling had been known for years as Boletus eastwoodiae (Murrill) Saccardo and Trotter. The name had to be changed as Ms. Eastwood gave a good description, but mistakenly sent Boletus satanas Lenz to Dr. Murrill by error for the naming as “type” collection. That name “Boletus eastwoodiae (Murrill) Saccardo and Trotter” was therefore dropped, since it did not apply to the specimens, which already had the name Boletus satanas given by Lenz. Drs. Thiers and Halling gathered new material (dried specimens) for a new type collection and named it Boletus pulcherrimus. The older name could not be given back to this species, lest there be confusion as to what actual herbarium material was meant.

The majority of deaths worldwide are due to species containing amatoxins, most commonly in the genus Amanita.