American Parasol
Leucoagaricus americanus
Also called reddening lepiota, american lepiota, reddening parasol
A stout parasol with concentric reddish-brown scales over a white cap and a spindle-shaped stem, whose flesh and surfaces stain wine-red to reddish-brown wherever they are handled. It fruits in clusters on wood chips, sawdust and old stumps rather than in undisturbed forest soil.
Overview
Leucoagaricus americanus is one of the larger lepiotoid fungi of eastern North America, and for much of the twentieth century it was shuffled between Lepiota and Macrolepiota before molecular work settled it in Leucoagaricus, within the family Agaricaceae. Like other members of that group it has free gills, a white spore print, a membranous ring that slides freely along the stem, and a fruiting body that grows from decaying plant material rather than from a partnership with tree roots.
What sets the species apart from its relatives is its habit of reddening. Cut flesh, bruised gills and the handled stem surface all shift through wine-red and rust to a dull brown, and old fruiting bodies often stand in the wood chips looking uniformly reddish-brown as though scorched. Combined with a stem that swells in the middle and tapers at both ends, this gives a silhouette quite different from the tall, straight-stemmed woodland parasols.
The fungus has spread widely on landscaping mulch and is now a familiar sight in city parks, nursery beds and along wood-chipped paths. Because it recycles coarse woody debris it tends to appear in the same beds year after year until the substrate is exhausted, often in dense clumps of several fruiting bodies fused at the base.
How to identify it
Cap
5-15 cm across, at first an ovoid to almost cylindrical egg with a solid brown surface, then expanding to convex and finally flat with a broad, low umbo. As the cap widens the brown cuticle tears into flat, concentrically arranged reddish-brown to cinnamon scales, densest at the centre and progressively smaller and sparser toward the margin, exposing the white to cream flesh beneath. The margin may split radially in dry weather and often shows faint striations in age.
Gills
Free from the stem and separated from it by a narrow collar, crowded, broad, white to pale cream at first, slowly discolouring reddish then dull brown where bruised or in age. Gill edges are minutely fringed under a hand lens.
Stipe
6-15 cm tall and 1-2 cm thick at the apex, conspicuously swollen or spindle-shaped through the middle and tapering to a narrowed, often rooting base. The surface is white and smooth, becoming yellowish then reddish-brown from the base upward as the fruiting body ages or is handled. A thick, white, membranous ring sits high on the stem and can usually be slid up and down once mature.
Flesh & spore print
Flesh white and firm, turning distinctly reddish to wine-red within a minute or two of being cut, then slowly darkening to brownish. The spore print is white, and the spores are small, smooth and ellipsoid with an apical germ pore.
Where and when it grows
This is a saprotrophic species of disturbed and enriched ground. It fruits on hardwood chips, sawdust heaps, old stumps and their buried root systems, compost, and the mulched beds of gardens, parks and roadsides. It is frequently caespitose, with several stems fused into a tight clump, and can also appear scattered across a wood-chipped path.
The main range is eastern and central North America, extending west with landscaping mulch, and closely similar reddening material has been recorded in Europe and elsewhere where wood chips are used. Fruiting runs from early summer into autumn, peaking after warm rain, and a given mulch bed may produce flushes for several consecutive years before the wood is fully broken down.
Ecological role
Leucoagaricus americanus is a decomposer, not a root symbiont. Its mycelium colonises coarse lignocellulose - chipped bark, sapwood and buried stump wood - and breaks it down into humus, which is why the species is so strongly tied to mulch beds, sawdust dumps and stump grindings. Colonies frequently produce dense white mycelial mats through the upper layer of chips.
By accelerating the breakdown of woody landscaping waste it plays a part in turning what is essentially forestry by-product into soil organic matter. Fruiting is triggered by rainfall on a warm substrate, and the free gills release white spores that scatter from the exposed underside of the flat mature cap.
Similar species
The Shaggy Parasol, Chlorophyllum rhacodes, also has a scaly cap and reddening flesh, but its scales are coarse, shaggy and upturned rather than flat and concentric, its ring is thick with a distinctly double edge, and its stem base is an abrupt club or bulb rather than a spindle swollen at mid-height. Chlorophyllum molybdites is comparable in size and grows in lawns, but its mature gills turn dull green and it drops a green spore print.
Macrolepiota procera is taller and more slender, with a stem patterned in dark snakeskin-like bands and flesh that does not redden appreciably. Leucoagaricus leucothites is smaller, with a smooth silky white cap lacking scales and a simple bulbous stem base with no spindle shape and no reddening.