D rex vs Armillaria: Giant Fungi Compared

Researchers compare D rex and Armillaria by measuring different things: D rex is judged by its above-ground fruiting body (the mushroom), while Armillaria is judged by its underground mycelial network. The former is a single, massive mushroom; the latter is a clonal colony that spreads through tree roots. This article explains how scientists measure, verify, and compare these two very different kinds of giant fungi.

Background: What Are D rex and Armillaria?

D rex (full name Dendromyces rex, though some sources use Fomitopsis rex) is a polypore mushroom discovered in 2010 in Yunnan, China. Its fruiting body can grow to about 10 meters long, weigh up to 500 kg, and produce billions of spores. It is the largest known individual mushroom by physical size.

Armillaria is a genus of honey mushrooms. The famous specimen in Oregon’s Malheur National Forest, nicknamed the “Humongous Fungus,” is a single clonal colony of Armillaria ostoyae estimated to cover about 3.7 square kilometers (965 hectares). It is not a single mushroom but a vast network of mycelium (root-like threads) that spreads through tree roots, with individual mushrooms appearing above ground only seasonally.

How Researchers Measure Each Fungus

Scientists use fundamentally different methods for the two because of their different growth forms.

• D rex: Measured directly. Researchers physically measure the length, width, and weight of the fruiting body. They also count spores and analyze DNA to confirm it is one organism.
• Armillaria: Measured indirectly. Researchers take soil and root samples across a forest, isolate the fungus, and compare DNA fingerprints. If samples share identical DNA, they belong to the same clone. The area is then mapped by plotting the sample locations.

For Armillaria, the “size” is the area of the mycelial network, not the visible mushrooms. For D rex, the “size” is the volume or mass of the single mushroom.

Key Differences in Growth and Ecology

The two fungi occupy different ecological niches and grow in different ways.

• Lifespan: D rex is annual – it grows, fruits, and dies within a year. Armillaria is perennial; the Oregon colony is estimated to be thousands of years old (some estimates range from 2,500 to 8,000 years).
• Reproduction: D rex reproduces via spores released from its large fruiting body. Armillaria spreads primarily through rhizomorphs (root-like structures) that grow from tree root to tree root, though it also produces spores.
• Habitat: D rex grows on the ground in subtropical forests, often near oaks. Armillaria is a parasite/pathogen that kills trees, so it thrives in temperate forests with dense tree cover.
• Size metric: D rex is the largest mushroom; Armillaria is the largest organism by area.

Why the Comparison Is Tricky

Comparing the two is like comparing a single apple to an entire orchard. The D rex mushroom is a discrete, visible object. The Armillaria colony is mostly invisible, spread over kilometers, and only a tiny fraction (the mushrooms) appears above ground. Researchers must decide what “largest” means:

• Largest by mass? D rex wins among mushrooms, but the Armillaria mycelium’s total mass is unknown and likely much larger.
• Largest by area? Armillaria wins by orders of magnitude.
• Largest by age? Armillaria wins.

Another complication: D rex is a single organism in the sense that all cells are connected. Armillaria clones are also genetically identical, but the mycelium may be fragmented – some researchers debate whether a broken clone still counts as one organism. DNA analysis helps, but physical continuity is rarely proven.

Common Misconceptions

A frequent error is assuming Armillaria is a giant mushroom you can see. It is not – the visible mushrooms are small, about 5–15 cm tall. The “giant” part is underground. Another misconception is that D rex is the largest organism ever. It is the largest mushroom, but the largest single organism is a clonal aspen tree (Populus tremuloides) in Utah, nicknamed “Pando,” which covers about 43 hectares and weighs thousands of tons. The Armillaria colony covers more area than Pando, but Pando has more biomass.

Also, some reports claim Armillaria covers 3.7 square miles (9.6 km²), but the original estimate was 3.7 square kilometers (about 1.4 square miles). The larger figure is a common unit mix-up.

How to Compare Them Yourself

For D rex, measure the footprint of the mushroom (roughly a rectangle or ellipse). For Armillaria, use the reported area of the colony. Then ask:

• What is the total area?
• What is the estimated mass?
• How old is it?
• Is it a single connected organism?

For Armillaria, the area is the key number. For D rex, the mass and volume are more meaningful because its shape is not circular.

Frequently Asked Questions

Which is bigger, D rex or Armillaria?
By area, Armillaria is far bigger – the Oregon colony covers ~3.7 km², while D rex is about 10 m long. By mass, D rex (up to 500 kg) is the largest single mushroom, but the Armillaria mycelium’s total mass is unknown and probably much larger.

Is D rex a real species?
Yes, it was formally described in 2011 by Chinese mycologists. Its classification has been debated – some place it in Fomitopsis – but it is a valid, distinct species.

How old is the Humongous Fungus?
Estimates vary, but genetic and growth-rate studies suggest the Oregon Armillaria is between 2,500 and 8,000 years old. The exact age is hard to pin down because the fungus grows slowly and fragments over time.

Can you eat D rex or Armillaria?
Armillaria species are edible when cooked, but some people have allergic reactions. D rex is not known to be edible – it is a woody polypore, and no culinary use is documented.

Why is D rex called a “giant fungus” if Armillaria is bigger?
Because D rex is a single mushroom. The term “fungus” can mean the whole organism (including mycelium) or just the fruiting body. In popular media, “giant fungus” usually refers to the visible mushroom, so D rex wins that title.

How do scientists confirm an Armillaria colony is one organism?
They collect samples from many points, extract DNA, and compare genetic markers. If all samples have identical multilocus genotypes (and sometimes the same mating type), they conclude the samples come from the same clonal individual. They also look for physical connections via rhizomorphs, though this is rarely possible over large areas.

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