What You Should Know
Stropharia caerulea is an inedible species of agaric fungus in the family Strophariaceae. It is a common species found in Europe and North America, where it grows as a saprophyte in meadows, roadsides, hedgerows, gardens, and woodchip mulch.
The fruit bodies of Stropharia caerulea feature a greenish-blue cap sparsely covered in white flecks of the veil at the margin, and a sticky, glutinous surface texture. Gills on the cap underside have an adnate or sinuate attachment to the stipe. They are initially pale purplish-brown, becoming darker brown in age as the spores mature. The greenish stipe is covered in white scales up to a thin, transient ring.
Other names: Blue Roundhead.
Stropharia caerulea Mushroom Identification
Saprobic, growing alone or gregariously; usually found in gardens, landscaping areas, and waste places; North American distribution uncertain.
2–4 cm; bell-shaped at first, becoming broadly bell-shaped or nearly convex; very slimy when fresh; bald; when young and fresh, dark greenish-blue; sometimes fading to yellowish-green or developing yellowish areas and spots; the margin hung with whitish partial veil remnants, especially when young.
Broadly attached to the stem; close; short-gills frequent; whitish at first, becoming purplish gray to purple-brown; edges colored like the faces.
3–5 cm long; 5–10 mm thick; equal above a slightly swollen base; sticky when fresh; with a faint ring zone (usually lacking a well-developed ring); pale above, colored like the cap below; basal mycelium white.
White, or in the lower stem colored like the cap; unchanging when sliced.
Spores 7–9 x 4.5–5.5 µm; ellipsoid; smooth; pale brown in KOH; yellowish-brown in Melzer's; with a very tiny pore. Chryso-cheilocystidia abundant; 30–40 x 10–12.5 µm; widely fusoid; hyaline in KOH; thin-walled; with yellowish-refractive inclusions. Chryso-pleurocystidia scattered; often scarcely projecting; 20–30 x 7.5–10 µm; subclavate to widely fusoid; hyaline and thin-walled; with yellowish-refractive inclusions. Pileipellis a thick ixocutis of hyaline to golden, poorly defined elements.
Stropharia caerulea Look-Alikes
Is darker blue-green and its cap scales are persistent; it has reddish-brown gills with white edges. This species is much less common than Stropharia caerulea.
Is also blue-green but does not have a slimy cap with scales; it has a strong odor of aniseed.
Is an uncommon species that grows in meadows. It has a more slender form than S. caerulea, a soft, spongy stipe, and flesh with an odor similar to fresh pepper. Microscopically, it has a dense palisade of slender, capitate (with a spherical tip) non-staining cheilocystidia on the gill edge. This gives the gills of young, fresh fruit bodies a whitish edge, a feature that is absent from S. caerulea.
Stropharia caerulea Taxonomy and Etymology
Although this blue mushroom has been known to science for more than two centuries, its separation from Stropharia aeruginosa had not been clearly defined until, in 1979, the German mycologist Hanns Kreisel (b. 1931) published a paper in Sydowia (an international Mycological journal produced in Austria), which established its currently-accepted scientific name Stropharia caerulea.
Long before Kreisel's work on this and related species, the British mycologist James Bolton had described the Blue Roundhead in 1788 and given it the binomial scientific name Agaricus politus.
Stropharia, the genus name, comes from the Greek word strophos meaning a belt, and it is a reference to the stem rings of fungi in this generic grouping. The specific epithet caerulea means blue, and often it refers to a deep blue rather than the blue-green coloring of the Blue Roundhead.
The specific epithet caerulea is Latin for "blue". The mushroom is commonly known as the blue roundhead. French mycologist Régis Courtecuisse has called the mushroom "verdigris agaric", but numerous other authors use this name to refer instead to Stropharia aeruginosa.
Photo 1 - Author: Jerzy Opioła (Creative Commons Attribution-Share Alike 3.0 Unported)
Photo 2 - Author: Michel Langeveld (Creative Commons Attribution-Share Alike 4.0 International)
Photo 3 - Author: Lamiot (Creative Commons Attribution-Share Alike 4.0 International)
Photo 4 - Author: Ben Krul (Public Domain)