The question of the world’s biggest mushroom often brings to mind a single, massive cap and stem, yet the reality is far more complex. Fungi represent a distinct biological kingdom, separate from both plants and animals, and their true scale is rarely visible to the human eye. The familiar mushroom that appears above ground is only a temporary, reproductive structure, similar to an apple on a tree. This visible part belies the vast, hidden network that constitutes the actual organism, capable of reaching astonishing sizes. The search for the largest specimen requires looking deep into the soil where the majority of the fungus resides.
Defining Size and Scope in Fungi
Mycologists distinguish between two primary ways to measure the size of a fungus. The first and most significant measure is the size of the mycelial network, which is the main body of the fungus. This network is composed of a dense web of thread-like filaments called hyphae, which absorb nutrients and water from the environment. The mycelium is measured by its total area coverage, estimated biomass, and single genetic identity.
The second measure is the fruiting body, the visible, temporary structure commonly called a mushroom. This structure’s function is solely reproductive, designed to produce and disperse spores. Fruiting bodies are typically measured by their individual weight, cap diameter, and overall height. Understanding this distinction is fundamental, as the world’s largest fungus is defined by its massive mycelial network, while the largest individual mushroom is a separate record.
The True Largest Organism on Earth
The single largest organism on Earth, measured by area and estimated mass, is the fungus Armillaria ostoyae, often referred to as the Honey Mushroom. This immense specimen is located in the Malheur National Forest in eastern Oregon, where it is nicknamed “The Humongous Fungus.” It is a vast, genetically uniform network of mycelium spreading beneath the forest.
Genetic testing determined that this single organism spans approximately 3.5 square miles, or about 2,385 acres. The fungus is estimated to be around 8,000 years old, making it one of the planet’s oldest living organisms. Its total estimated mass reaches up to 35,000 tons.
Armillaria ostoyae maintains this scale through a slow, parasitic growth pattern. It spreads primarily through underground structures called rhizomorphs, which are root-like bundles of hyphae that penetrate tree roots and wood. This fungus is a pathogenic species that causes root rot in trees, sustaining its size by consuming the living tissue of its host forest.
Records for the Largest Above-Ground Structure
The records for the largest individual, visible mushroom are held by different species with impressive physical structures. The heaviest single fungal fruiting body ever documented belongs to the bracket fungus Phellinus ellipsoideus, a species found in southern China. The record-breaking specimen was discovered on Hainan Island, growing under a fallen tree.
This massive, shelf-like bracket measured nearly 10.85 meters (35 feet 7 inches) in length, with an estimated age of about 20 years. Its density and size led to an estimated weight between 400 and 500 kilograms (880 to 1,100 pounds). The perennial nature of this polypore, which adds a new layer of spore-producing material each season, allowed it to achieve immense mass.
In terms of cap diameter and height, certain tropical gilled mushrooms hold the record for the largest classic “mushroom” shape. The species Termitomyces titanicus produces colossal fruiting bodies in West Africa, often found near termite mounds. The cap of this edible mushroom can reach a diameter of up to 1 meter (3 feet), with its stem growing as long as 57 centimeters (22 inches). Other notable giants include the puffball Calvatia gigantea, which can reach 50 centimeters in diameter, and Macrocybe titans from the tropics, featuring caps up to 70 centimeters across.