Parasitic and Pathogenic Fungi

Parasitic and pathogenic fungi form symbiotic relationships with other living organisms, similarly to mycorrhizal fungi; however, they benefit to the detriment (and sometimes death) of their host. Many species affect trees and other plants. Others attack mushrooms, sometimes rendering the host species unrecognizable. Even members of the animal kingdom (including humans) are susceptible to parasitic fungal infection. Some parasitic fungi form mushrooms, while many do not.

A pine branch with a section of the branch swollen and cracking.
A swollen cankerous branch of a whitebark pine tree infected with Cronartium ribicola.

NPS/B. Fallon Photo

Parasitic fungi can be an incredibly destructive force, in some cases disrupting industrial logging operations and forest management efforts (see Armillaria spp.). At Mount Rainier, a fungal disease called white pine blister rust (Cronartium ribicola) threatens high-elevation whitebark pine (Pinus albicaulis). White pine blister rust was introduced to North America in the early twentieth century. It causes branch swelling, branch death, and cankers from which orange blisters (spore masses) emerge. Infected trees usually die, sometimes within just a few years of infection.

Interestingly, this fungus relies on two different types of hosts to complete its life cycle. Spores distributed by infected whitebark pine trees do not go on to infect other trees. Instead, they infect smaller plants, such as gooseberry, currant, and even Indian paintbrush. Within a year or so of colonizing these smaller plants, the fungus then produces a different type of spore that is capable of infecting white pines.

A bright red lobed mushroom growing out of muddy ground.
Hypomyces lactifluourum, the lobster mushroom, parasitizing a mushroom in the Russula genus. Notice the ridges that indicate where the gills of the Russula would normally be.

NPS/B. Garcia Photo

In other cases, parasitism in the fungal kingdom can look very different. A locally common fungus, Hypmyces lactifluorum parasitizes non-descript gilled mushrooms in the Russula genus, radically transforming their fruit bodies. While the unaffected host species are rarely eaten and difficult to identify, once parasitized, they are considered to be a choice edible that is widely sold and consumed in the US under the common name lobster claw. Most years, these bright orange “mushrooms” are a common sight in Mount Rainier’s lowland forests.

A cluster of wavy, white mushroom growths.
Sparassis radicata, the cauliflower fungus.

NPS/B. Garcia Photo

Sparassis radicata

Common Name: Cauliflower fungus
Description: White to cream, forming large clusters of growth resembling cauliflower or egg noodles. The densely packed branches arise from a central base. This western species forms a prominent rooting base.
Ecology: Saprobic and parasitic on conifers in western North America. Grows on living and dead trees. Causes brown rot, especially at the base of trees.

A circular fungus with rings of yellow, brown and blue-black.
A beautiful specimen of Phaeolus schweinitzii, the dyer's polypore.

NPS Photo

Phaeolus schweinitzii

Common Name: Dyer’s polypore
Description: Irregular, concentric lobes arising from a common basal stem structure. Extremely variable in color and appearance; often with banded zones of differing color and texture. Black, brown, rusty-brown, yellow-brown, olive, and/or reddish. Outer edge often bright yellow, orange, or whitish. Growing from the ground (from buried wood) or at the base of trees.
Ecology: Saprobic and parasitic, causing brown rot in the root systems and lower sections of conifers.
Notes: True to its name, the dyer’s polypore is sought after for dye making purposes. It yields several beautiful colors (oranges, yellows, golds and greens) depending on the specimen age, dying material, and mordant used.

A patch of mushrooms with small brown caps and thick stems growing against a tree.
Arillaria species growing in the late fall under a Douglas-fir.

NPS/B. Garcia Photo

The Genus Armillaria

Common Name: Honey mushroom
Description: Highly variable, but generally cinnamon brown, yellow-brown, or tan caps covered in fibrils. Whitish gills and a prominent ring on the stalk. Stalk flesh stringy, stuffed with a cottony pith. Growing in small to large groups, mostly in the fall.
Ecology: Pathogenic and parasitic on both hardwoods and conifers. Causes white rot and spreads readily from tree to tree. The mycelial network of an individual Armillaria ostoyae in the Malheur National Forest in Oregon composes the world’s largest known organism!
Notes: Beware of dated field guides that describe all honey mushrooms as edible! Recent studies have confirmed that several similar looking Armillaria species exist in the West, many of which cause severe gastrointestinal distress (read: nausea, vomiting, diarrhea) in many people. The mycelium of some Armillaria species glows in the dark when actively growing.

The stem and underside of a mushroom covered in yellow mold-like coating.
Hypomyces luteovirens parasitizing an unidentified Russula species.

NPS/B. Garcia Photo

Hypomyces luteovirens

Description: Appears as a powdery, yellowish, mold-like coating on several species of Russula. This parasite often visibly affects the gills and stem of the mushroom, leaving the upper surface of the cap more or less unchanged. The interior flesh of mushrooms infected by H. luteovirens tends to be quite firm and riddled with yellow pits not normally present in Russulas.
Ecology: Parasitic on various Russula species. H. luteovirens is related to the lobster mushroom, but is not known to be edible. Fairly common at Mount Rainier in the late summer and early fall.

A hand holds a large bright orange mushroom with a wavy, flared top.
Hypomyces lactifluorum parasitizing a gilled Russula.

NPS/B. Garcia Photo

Hypomyces lactifluorum

Common Name: Lobster mushroom, lobster claw
Description: Medium to large orange fruit body without gills. Variable in shape, but often irregular, wavy, concave cap often with upturned edges. Hard (when fresh), generally covered with minute pimples. Internal flesh white. Older specimens often visibly coated in white spores.
Ecology: Parasitic on gilled mushrooms in the Russula and Lactarius genera. Infected specimens are transformed in size, structure, color, texture, and smell. It is often impossible to identify the host mushroom with any certainty.
Notes: It is unclear whether this mushroom gets its name from the claw-like appearance of the fruit body or its strong, seafood-like smell. These mushrooms are common in Mount Rainier’s lowland forests mid-summer through fall.


Last updated: April 21, 2021

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