Western Giant Puffball
Calvatia booniana
Also called boone's puffball, sagebrush puffball, western giant ball
A very large white puffball of dry western rangeland, its surface broken into a mosaic of flattened polygonal warts. The interior is uniformly white when young and turns to olive-brown powder with age.
Overview
The Western Giant Puffball is one of the largest fungi of arid western North America. Mature fruiting bodies commonly reach 20-50 cm across and occasional specimens exceed 60 cm, sitting on open ground like scattered boulders. Unlike the smooth-skinned giant puffballs of moister climates, the surface of this species is divided into a distinctive mosaic of low, flat, polygonal warts or patches, giving it a tiled or cracked-mud appearance.
Calvatia is a genus in the Agaricaceae, the same broad family that contains the field mushrooms and the parasols. In Calvatia the spore-bearing tissue is enclosed rather than exposed on gills: the entire interior matures into a mass of powder held in a network of thread-like capillitium, and the outer skin weathers away to release it. There is no cap, no gills and no true stalk, only a compact sterile base and a cord anchoring the ball to the soil.
What makes the species easy to recognise is the combination of enormous size, warty tiled surface and habitat. It appears in sagebrush steppe and dry grassland where few other large fungi fruit, often in the same spots year after year as the underground mycelium persists between rains.
How to identify it
Fruiting body
Subglobose to broadly flattened or somewhat cushion-shaped, usually broader than tall, 20-50 cm across and sometimes far larger. The outer skin (exoperidium) is white to cream, ageing buff or dingy tan, and is divided into large flat polygonal warts or scale-like patches separated by shallow grooves. With age the skin weathers, cracks and peels away in sheets.
Gills, pores and stipe
None. There is no cap-and-stem structure at all. The ball attaches to the ground by a narrow point and a stout white rhizomorph cord; the sterile base is small relative to the whole fruiting body, so the ball tends to break free and weather in place.
Flesh and spore mass
The interior (gleba) is pure white, firm, homogeneous and marshmallow-like in young specimens, with no internal chambers, no outline of a developing cap and no darker core. As it matures it passes through greenish-yellow to olive and finally becomes a dry olive-brown to yellow-brown powder. Spores in mass are olive-brown.
Where and when it grows
Grows on soil in open, dry country: sagebrush flats, bunchgrass rangeland, dry meadows, roadside verges, irrigated field margins and washes, from low elevations to the lower montane zone. It is a western North American species, common in the Great Basin, the Columbia Basin, the interior Pacific Northwest, the northern Rockies and adjacent parts of the Southwest and western Canada.
Fruiting is opportunistic and follows soaking rain, so it may appear in late spring, again in summer, or in autumn depending on the year. Fruiting bodies often occur in loose groups, arcs or rings, and the tough weathered husks can persist on open ground for many months after the spores have gone.
Ecological role
The Western Giant Puffball is saprotrophic. Its mycelium lives in the upper soil and decomposes accumulated organic matter, grass litter and root debris, recycling nutrients in habitats where litter breakdown is slow because of low rainfall. It forms no partnership with tree roots, which is consistent with its occurrence in treeless steppe.
Spore release is passive and prolonged. Rather than shooting spores from gills, the fungus matures its entire interior into powder, then relies on the outer skin cracking, weathering and tearing so that wind, rain drops and passing animals puff clouds of spores from the husk over weeks or months.
Similar species
Calvatia gigantea, the true Giant Puffball, reaches similar sizes but has a smooth to finely felted, kid-leather surface without the mosaic of flat polygonal warts, and it favours moister habitats such as pastures, hedge banks and rich woodland edges rather than sagebrush steppe. Calbovista subsculpta is smaller (8-15 cm) and carries raised pyramidal warts each tipped with a darker point, and it grows in montane conifer forest. Calvatia sculpta has tall, sharply pointed pyramidal spines that often lean together, and is a mountain species of the Sierra Nevada and Cascades.
Young unexpanded egg-stage Amanita species can be mistaken for small white balls, but slicing one from top to bottom reveals the silhouette of a miniature cap, gills and stem inside a sac, whereas any puffball is uniformly solid and featureless within.