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U.S. Army Corps of Engineers botanists discovered a new site for the rare Pruitt’s Amanita mushroom at Fern Ridge Reservoir west of Eugene, Ore., Nov. 20, 2012.

U.S. Army Corps of Engineers botanists discovered a new site for the rare Pruitt’s Amanita mushroom at Fern Ridge Reservoir west of Eugene, Ore., Nov. 20, 2012. (Photo by Rhiannon Thomas)

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U.S. Army Corps of Engineers botanists discovered a new site for the rare Pruitt’s Amanita mushroom at Fern Ridge Reservoir west of Eugene, Ore., Nov. 20.

U.S. Army Corps of Engineers botanists discovered a new site for the rare Pruitt’s Amanita mushroom at Fern Ridge Reservoir west of Eugene, Ore., Nov. 20. (Photo by Rhiannon Thomas)

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Posted 12/11/2012

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By Rhiannon Thomas
Terrestrial Stewardship Section, Willamette Valley Project


U.S. Army Corps of Engineers botanists discovered a new site for the rare Pruitt’s Amanita mushroom at Fern Ridge Reservoir west of Eugene, Ore., Nov. 20.

Soils and botany student intern Leanna Van Slambrook spotted some white mushrooms popping out of the charred, soggy ground on the southwest side of the reservoir after a prescribed burn and remembered that a rare Amanita had been found after a burn a few years back.  She and Rhiannon Thomas, a botanist for the Corps’ Willamette Valley Project, photographed and collected specimens for identification.  Amanita experts confirmed the mushroom’s identification a during a later site visit. 

No scientific description of the species has yet been published.  It is the same species as David Arora’s Anonymous Amanita, (name also unpublished) a California species described in his book, “Mushrooms Demystified.”  The Corps’ Fern Ridge Project is the only known location for the mushroom outside of California.

Pruitt’s Amanita was first discovered at Fern Ridge Reservoir in 1975 by Eugene naturalist Hal Pruitt, in a field of planted Sudan grass.  The mushrooms at the reservoir are unusual because they grow in seasonally flooded sites.  On Pruitt’s first visit to the site, they found hundreds of the mushrooms floating in the water.  Pruitt’s Amanita was seen at this site again in 1976 and 1977 but was not spotted again until 1998, when a Corps botanist discovered a new site about a mile away in native wet prairie.

A large fruiting was seen in another native wet prairie near the first site after a 2002 prescribed burn.  This led to speculation that the species may fruit heavily after disturbance such as fire. 

All three Oregon sites are on land already protected as a Research Natural Area by the Corps’ Willamette Valley Project, in addition to being within the State of Oregon’s Fern Ridge Wildlife Area.

Historically, Willamette Valley wet and upland prairie habitat was maintained by fire.  Its suppression threatens prairies and many species dependent on them, including several listed under the Endangered Species Act.  Scientific studies have demonstrated the benefits of prescribed burning to species like Bradshaw’s lomatium, Kincaid’s lupine and Fender’s blue butterfly.

The Corps’ prescribed fire program at Fern Ridge Reservoir is intended to return fire to the prairie habitats we manage.  Since 1988, the Corps and its partners – with the U.S. Bureau of Land Management’s Eugene District as burn planner and leader – have executed 16 prescribed fires of up to 100 acres.

For more information about the Corps’ environmental efforts at Fern Ridge Reservoir, visit the Portland District website at

http://www.nwp.usace.army.mil/Locations/WillametteValley/FernRidge.aspx.

Fern Ridge