Tree-killing mushroom is largest living thing ever found

Beneath the soil of the Malheur National Forest in eastern Oregon, a fungus that has been slowly weaving its way through the roots of trees for centuries has become the largest living organism ever found.

The Armillaria ostoyae, popularly known as the honey mushroom, started from a single spore too small to see without a microscope and has been spreading its black shoestring filaments called rhizomorphs through the forest for an estimated 2,400 years, killing trees as it grows. It now covers 2,200 acres.

''We ended up having on the landscape this humongous fungus,'' Tina Dreisbach, a botanist and mycologist at the U.S. Forest Service's Pacific Northwest Research Station in Corvallis, Ore., said Friday.

In 1992, another Armillaria ostoyae was found in Washington state covering 1,500 acres near Mount Adams, making it the largest known organism at the time.

''We just decided to go out looking for one bigger than the last claim,'' said Gregory Filip, associate professor of integrated forest protection at Oregon State University and an expert in Armillaria. ''There hasn't been anything measured with any scientific technique that has shown any plant or animal to be larger than this.''

Forest Service scientists are interested in learning to control Armillaria because it kills trees, Filip said, but they also realize the fungus has served a purpose in nature for millions of years.

The outline of the giant fungus, strikingly similar to a mushroom, stretches 3.5 miles across, and it extend an average of three feet into the ground. It covers an area as big as 1,665 football fields. No one has estimated its weight.

The discovery came after Catherine Parks, a scientist at the Pacific Northwest Research Station in La Grande, Ore., in 1998 heard about a big tree die-off from root rot in the forest east of Prairie City, Ore.

Using aerial photos, Parks staked out an area of dying trees and collected root samples from 112.

She identified the fungus through DNA testing. Then, by comparing cultures of the fungus grown from the 112 samples, she determined that 61 were from the same organism, meaning a single fungus had grown bigger than anything anyone had ever described before.

On the surface, the only evidence of the fungus are clumps of golden mushrooms that pop up in the fall with the rain.

''They are edible, but they don't taste the best,'' said Dreisbach. ''I would put lots of butter and garlic on them.''

Digging into the roots of an affected tree, something that looks like white latex paint can be seen. These are mats of mycelium, which draw water and carbohydrates from the tree to feed the fungus and interfere with the tree's absorption of water and nutrients.

The long rhizomorphs that stretch as much as 10 feet into the soil invade tree roots through a combination of pressure and enzyme action.

The huge size of the fungus may be related to the dry climate in eastern Oregon, Dreisbach said Friday. Spores have a hard time establishing new organisms, making room for the old-timers to spread.

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On the Net:

Pacific Northwest Research Station site: http://www.fs.fed.us/pnw/

A site on the mushroom genus Armillaria: http://hiddenforest.co.nz/fungi/family/tricholomataceae/armillaria.h tm