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The discovery of the world's largest fungus - up to 8,500 years old and carperting nearly 10 square kilometres of forest floor - has raised questions about what constitutes an individual organism.

A study of a tree-killing fungus in rugged northeast Oregon, USA, found that a single individual covered an area equivalent to about 1,600 football fields, according to a report in the current issue of the Canadian Journal of Forest Research.

"The fact that an organism like this has been growing in the forest for thousands of years really expands our view of the forest ecosystem and how it works," said Dr Catherine Parks, a pathologist at the U.S. Department of Agriculture who led the study. "From a broad scientific view, it challenges what we think of as an individual organism."

The fungus is the most outstanding known individual of the Armillaria ostoyae species, which grows in high-latitude northern hemisphere forests and causes large production losses due to root disease. It lives in the soil and spreads mainly along tree roots by shoestring-like threads called rhizomorphs. Apart from dead and dying trees, its only surface evidence are its fruiting bodies, known commonly as honey mushrooms.

The researchers discovered the giant fungus in the Malheur National Forest, some 590,000 hectares of rugged high-desert grasslands, pine forests and alpine lakes. Elevations range from 1,200 to 2,750 metres, the highest point being the Strawberry Mountain range that passes from east to west and through the forest.

The single organism discovered has yielded new insights into a fungus' role in forest ecology. It had been thought that Armillaria fungi grew in distinct clusters within forests, visible from the air by ring-shaped patches of dead trees.

But when the researchers collected samples of fungus from 9.65 square km of discontinuous dead patches in the Oregon forest and grew them together in laboratory Petri dishes, they did not react to each other as they would to alien individuals.

"The technique is actually very simple and makes use of this fungus's own ability to distinguish one individual from another," Parks said.

The results confirmed the identical genetic make-up of all the samples. The researchers were surprised that such well-separated clusters of fungus represented the spread of a single individual. They estimated its age at somewhere between 2,000 and 8,500 years.

"It's one organism that began as a microscopic spore and then grew vegetatively, like a plant," she said. "If you could take away the soil and look at it, it's just one big heap of fungus with all of these filaments that go out under the surface."

Forest managers had thought that the deliberate suppression of wildfires worsened the spread of the fungus: "But because this fungus is thousands of years old, and grew long before fire systems were influenced by man, this isn't the case. It also means that fire does not naturally control this disease."

The researchers now believe the fungus is part of the natural cycle of renewal and decline within forests and that it is often present in areas with little obvious tree damage.

Forest managers may be more cautious about using selective tree-cutting aimed at controlling fungal spread: "After you cut an infected tree, the entire root system can be colonised by the fungus, which then increases the disease potential around that area."

Planting species less susceptible to the fungus - such as western larch, western white pine and ponderosa pine - and harvesting susceptible individuals during thinning would reduce the fungus's impact on forest yields, she said.

Bob Beale - ABC Science Online