Contact:  Sherri Richardson Dodge

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PORTLAND, Ore. July 24, 2000. It is as large as 1,665 football fields combined. It is 3.4 square miles in size and covers 2,200 acres of land in the Blue Mountains of eastern Oregon and the Malheur National Forest near Prairie City. It is a recently discovered clone of Armillaria ostoyae, the tree-killing fungus that causes Armillaria root disease.

It is also the most humongous fungus or clone of Armillaria ostoyae found to date. Dr. Catherine Parks, a scientist with the USDA Forest Service’s Pacific Northwest Research Station, led the research in discovering the fungus in cooperation with Oregon State University. The fungus is calculated to be about 2400 years old, although it could be two to three times older. 

“We have discovered an individual, or clone, of this fungus can occupy immense areas of the forest and live for thousands of years,” said Parks. “This fungus lives in a below-ground habitat, spreading very slowly outward from tree to tree along roots or by growth through the soil using special shoestring-like structures called rhizomorphs.”

“The fungus is visible,” says Parks, “in the clusters of golden-colored mushrooms occasionally seen in the fall on the forest floor that represent just the tip of the iceberg in regard to its true size and impact upon the forest.”

The fungus plays a role important to forest ecosystem processes. “Root disease causes gaps in the forest canopy,” Parks explains. “The gaps can be replaced by brushy hardwood species or young conifers increasing the diversity of forest age and tree types. When trees die, snags are created that are used as shelter and breeding areas for wildlife. Tree mortality from root disease also contributes to nutrient recycling and increased productivity potential of the site.”

Research implications include:

·       The existence of many small, Armillaria root disease centers in the same locale within mixed-conifer forests should raise suspicions that one clone of the fungus may actually encompass the entire area. Not all trees will be dying at the same time because the disease expresses itself differentially with factors such as host condition and availability.

·       Root disease can affect the desired composition and structure of a future forest and thereby interfere with natural resource management goals.

·       To minimize tree mortality near the fungus, managers with timber production goals may want to use less susceptible tree species such as western larch and ponderosa pine  during planting, and harvest susceptible hosts such as Douglas-fir and true fir during thinning.


Earlier findings of large fungi include about 37-acres of Armillaria bulbosa discovered in 1992 in a hardwood forest in Michigan and a clone of Armillaria ostoyae infecting ponderosa pine in eastern Washington that is estimated to be 1,500-acres in size. Clonal studies on other species of Armillaria in eastern North America, Europe, and Australia were found to be no more than a few tens of acres in size at the most.

Collaborators on the Parks study include Brennan Ferguson, Oregon State University; Dr. Tina Dreisbach, PNW Research Station, Forest Service; Dr. Greg Filip, Oregon State University; and Craig Schmitt, Wallowa-Whitman National Forest, Forest Service.