Thousands of years ago, two microscopic spores spawned and created a monster. It grew — up to three feet a year — sending out dark, gnarly, threadlike organs called rhizomorphs that explored the subterranean darkness, foraging for food. Now it’s a nebulous body, a tangled mat beneath the Oregon soil that occupies an area the size of three Central Parks and may weigh as much as 5,000 African elephants.

Its scientific name is Armillaria ostoyae, but you can call it The Humongous Fungus. It’s the largest known terrestrial organism on the planet, according to the United States Forest Service. It’s also a deadly forest pathogen.

Although none (that we know of) are as big, there are many others in the Armillaria genus. These fungi cause root rot disease in plants in forests, parks, orchards and vineyards across North America, Europe and Asia. What sets them apart from other fungi is those stringy rhizomorphs that find weak trees, colonize their roots, kill and eat them.

An international group of scientists led by Hungarian researchers now have completed the first step to defeating these fungal demogorgons. In a genetic analysis published Monday in the journal Nature Ecology and Evolution, they uncovered tactics that help Armillaria develop their unusual rhizomorphs, grow so big and get so good at killing host plants.

The biologists compared the genomes of four Armillaria species to 22 related fungi and looked for unique or special proteins and gene families. They also compared the rhizomorphs to their fruiting bodies, called honey mushrooms, at different stages of development.

They found that Armillaria fungi had expanded their genomes by copying certain chunks of it, which meant additional capacity for making new rules of behavior. In a way, the fungus had more genes involved in making it really good at being bad. For example, the team found extra proteins for killing cells and gobbling up the glue that holds plant cell walls together.

They also found special tactics for hiding: Armillaria can reabsorb substances lost in the soil that alert trees of their presence and avoid the tree’s full defenses. This lets them kill trees undetected and start leeching nutrients from the dead stuff before competing bacteria and fungi arrived. And if and when competitors do show up, Armillaria can create a chemical environment intolerable for them.

They also found that rhizomorphs and fruiting bodies shared most genes, suggesting to the researchers that at some point the organs hijacked an ancient tool kit for making those cartoonish fruiting bodies we generally call mushrooms. But specific genetic traits in the rhizomorphs — a red-sensing photoreceptor and pheromones related to mating — may help them seek out their victims, the researchers speculated.

However they find their hosts, pathogenic Armillaria are ancient. They arrived an estimated 21 million years ago and stuck around for a reason.

In healthy, natural, diverse forests, the fungi absorb carbon and kill only the weakest trees, like gangly, underground forest rangers. But in monocultures made up of genetically identical trees, stresses like drought can cause all the trees to fall weak simultaneously.

“This is how Armillaria can wipe out entire forests,” said György Sipos, a biologist at the University of Sopron in Hungary who led the study with László Nagy, of the Hungarian Academy of Sciences.

While you’re unlikely to spot a fungal monster like the Humongous Fungus lurking beneath your soil, honey mushrooms can sometimes appear on the surface clumped together around tree trunks. And beneath the bark, you may find the indicators of infection that look like dark shoe strings and white paint. You can try to remove the stump, but there’s not much you can do from there because rhizomorphs go deep and live long.

On the brighter side, you may get to witness foxfire, the glow of rotting wood that results from some infections.

A version of this article appears in print on , on Page D2 of the New York edition with the headline: Beating the Spread: The Fungus Among Us? In Oregon at Least, It Is Quite Humongous. Order Reprints | Today’s Paper | Subscribe