Fly over northeastern Oregon and you will see rings of dead conifers punched through the canopy, like cigarette burns in a carpet. Foresters catalogued those rings for years as separate outbreaks of root disease. Sampling eventually showed they were bruises left by one body underneath.
That body is a honey fungus, Armillaria ostoyae, and this one covers 965 hectares of the Malheur National Forest in the Blue Mountains. Call it 9.65 square kilometres, or 2,385 acres, or roughly 1,665 football fields, the comparison Scientific American reached for. Almost all of it sits underground, and the only part visible from the surface is the trees it is killing.
How you tell one fungus from its neighbour
The number comes from fieldwork done in 1998 and 1999 by a team of researchers including Brennan Ferguson and USDA Forest Service scientist Catherine Parks, published in the Canadian Journal of Forest Research in 2003. They took 112 samples from the roots of dead and ailing conifers across about 16,100 hectares of dry mixed-conifer forest, then had to work out how many separate fungi they were holding.
Their method, somatic incompatibility, is less forbidding than the name. Grow two samples towards each other in a dish and watch what happens at the border. Strangers stop, refuse to mix and lay down a dark line between them. Cells from the same individual merge and carry on as though nothing happened.
Sixty-one of the samples merged.
Those 61 came from trees up to 3,810 metres apart. Four other genetically distinct individuals turned up in the same patch of forest at 20, 95, 195 and 260 hectares, which makes the big one unusual even by the standards of its own species.
The age is arithmetic, not a birth certificate
So how do you put an age on something with no rings and no birth certificate?
Age here comes from division: halve the maximum width to get a radius, then divide by how fast the fungus is known to creep through soil. Published spread rates for A. ostoyae run from 0.22 metres a year to about a metre, so the team ran the sum three ways and got 1,900 years, 2,200 years and 8,650 years. The widely repeated figure of 2,400 years sits near the cautious end of that spread. Ferguson and his co-authors called their own approach conservative, since lopsided growth from the original infection point would push the real age higher.
Whatever the true number, this is something that has been copying itself for millennia with barely any drift. Geneticists who sequenced a 2,500-year-old Armillaria individual in Michigan for Proceedings of the Royal Society B found its mutation rate to be extraordinarily low. That is one study of one fungus in a related species, so treat it as a hint rather than a law. James Anderson, who led it, has pointed out the contrast with cancer, which mutates furiously and destabilises itself. This thing does the opposite and just keeps going.
What you can actually see
Very little is the honest answer. Forest pathologist Mike McWilliams walked Atlas Obscura around the site and pointed out what to look for. He chips the bark off an infected fir with a Pulaski, exposing cream-coloured fans of fungal tissue pressed flat against the wood. Below ground it travels as rhizomorphs, black cords like bootlaces, feeling from one root system to the next. In autumn it sometimes fruits, and clumps of honey mushrooms appear at the base of dying trees. They are edible, though the Oregon Encyclopedia warns they upset some stomachs and need thorough cooking.
Whether it deserves to be called a villain
Grand fir and Douglas-fir made up 89 per cent of the infected trees sampled. Both species crowded into these forests over the past century, as fire suppression and selective logging pushed out the ponderosa pine and western larch that once dominated. McWilliams reckons that shift helps explain the size. Ferguson’s team read it differently, arguing that the fungus predates any human fire policy by thousands of years and that burning does little to an established individual. What grew, on their account, was the visibility of the damage rather than the organism causing it.
Either way there is no removing it. Root disease of this kind has no cure at the stand scale, and the standing advice for foresters is to favour the species that shrug it off and thin out the ones that do not.
McWilliams has a caveat he likes to repeat, which is that this is the largest documented organism and there is almost certainly a bigger one somewhere nobody has thought to sample. Hard to argue. The record only exists because a handful of people walked into one specific forest with sampling kits, a GPS unit and the patience to grow 112 wood chips in petri dishes.