Italian chef Antonio Carluccio says it is delicious with spaghetti and red chilli. But to gardeners it is a menace that threatens their hedges, roses and rhododendrons.

The parasitic and apparently tasty honey fungus not only divides opinions; it is also widely seen as the largest living organism on Earth.

More precisely, a specific honey fungus measuring 2.4 miles (3.8 km) across in the Blue Mountains in Oregon is thought to be the largest living organism on Earth.

Several species of fungi belong to the Armillaria genus, which is popularly known as honey fungus. They colonise and kill a variety of trees and woody plants.

The large clumps of yellow-brown mushrooms that appear above ground are the fruiting bodies of much larger organisms. They consist mainly of black bootlace-like rhizomorphs that spread out below surface in search of new hosts, and underground networks of tubular filaments called mycelia.

However, it is only in recent years that scientists have discovered quite how large these can get.

Tree killers

In 1998 a team from the US Forest Service set out to investigate the cause of large tree die-offs in the Malheur National Forest in east Oregon.

They identified affected areas in aerial photographs and collected root samples from 112 dead and dying trees, mostly firs. Tests showed all but four of the trees had been infected with the honey fungus Armillaria solidipes (previously known as Armillaria ostoyae).

When mycelia from genetically identical A. solidipes meet, they can fuse to form one individual. The researchers harnessed this ability, growing fungi samples in pairs in petri dishes. By observing which ones fused and which ones rejected each other, they found that 61 of the trees had been struck down by the same clonal colony – individuals with identical genetic make-up that all originated from one organism.

The most widely-spaced were 2.4 miles (3.8 km) apart. The team calculated that the A. solidipes covered an area of 3.7 sq miles (9.6 sq km), and was somewhere between 1,900 and 8,650 years old.

At the time, the largest known organism was a fungus of the same species discovered in 1992 in south-west Washington, which stretches over 2.5 sq miles (6.5 sq km).

Biologists have long debated what constitutes an individual organism. The record-breaking A. solidipes clonal colony passes the test based on a definition of being made up of genetically identical cells that can communicate, and that have a common purpose or can at least coordinate themselves.