The question of the world’s rarest mushroom presents a unique challenge because the fungal kingdom remains largely unknown compared to plants and animals. Mycologists estimate the existence of millions of fungal species, yet only a small fraction have been formally described, making any claim of absolute rarity difficult to verify. The bulk of a fungus exists as a hidden, thread-like network called mycelium, meaning the visible mushroom, or fruiting body, only appears under specific, often brief, environmental conditions.
Defining Rarity in the Fungal Kingdom
Mycologists determine a fungus’s scarcity using criteria that differ significantly from those used for other organisms. One primary indicator is endemism, which describes a species found naturally in only one small, distinct geographic area. If a species is restricted to a single valley or forest patch, its population is inherently small and vulnerable.
Another significant factor is specific substrate dependence, where a fungus requires a single, precise source of nutrients to fruit. This might mean growing only on the roots of one type of ancient tree or on wood decaying at a particular stage. Rarity is also measured by the number of documented observations; a species seen only once or twice globally, or one with a massive gap between sightings, is considered exceptionally scarce.
The Leading Contender for the Rarest Mushroom
The mushroom often cited as a leading contender for the world’s rarest is the Texas Star (Chorioactis geaster), also known as the Devil’s Cigar. This fungus exhibits a highly unusual and restricted distribution, found almost exclusively in specific regions of Texas and, inexplicably, in a few isolated areas of Kyushu, Japan. This pattern is known as a disjunct distribution, underscoring its extreme environmental specificity.
The Texas Star begins as a dark, cigar-shaped capsule covered in dense, velvety brown hairs. When conditions are right, the fungus undergoes a dramatic transformation, splitting open into a star-like, four-to-seven-pointed ray structure. This opening process releases a distinct, audible hissing sound as the spores are rapidly expelled into the air. The interior of the open star is a pale, creamy white, contrasting sharply with the dark exterior.
Its existence is tied to decaying cedar elm stumps and other specific hardwood debris in the southern United States and Japan. The unusual separation of its populations across two continents, with no known intervening habitats, suggests either an ancient, relict distribution or a dependence on an undiscovered, globally sparse symbiotic partner. The limited number of reliable sightings over the past century solidifies its standing as one of the most elusive organisms.
Other Notable Examples of Scarce Fungi
Fungal scarcity can arise from many different pressures, as illustrated by other highly uncommon species. The Crown-tipped Coral Mushroom (Artomyces pyxidatus), for instance, was once assumed to be locally extinct in parts of the United Kingdom. Its rarity in these regions stems from a lack of fruiting, potentially due to subtle environmental changes or a very long, unpredictable life cycle.
The New Zealand Cortinarius species provides another example. Some fungi in this genus are rarely collected because they are tied to specific, declining host trees, such as the Nothofagus (southern beech) forests. When a fungus relies on an endangered or diminishing ecosystem, its survival and observation become inherently difficult.
Finally, the Italian species (Boletus lupinus) represents potential extinction. This striking bolete has not been reliably observed since 1983, despite multiple targeted expeditions to its known habitat in the Apennine beech forests. Its disappearance highlights how habitat loss and climate shifts can cause a rapid decline in geographically restricted species.