Panaeolus olivaceus

Panaeolus olivaceus is a small dark-spored agaric of lawns and rough grass, superficially so like the ubiquitous Brown Mottlegill that it is regularly overlooked. Its distinguishing marks are subtle: an olive or greyish cast to the drying cap, a tendency for the cap surface to wrinkle finely, and above all a black rather than brown spore print.

The species has been reported under the name Panaeolus castaneifolius and sits within the group of small grassland mottlegills whose limits mycologists have shuffled repeatedly. It is best known from western North America but has been recorded more widely in temperate grassland.

As with all Panaeolus, its characteristic feature is the mottled gill face, produced when spores mature in patches rather than uniformly, so that ripening gills look marbled between grey and black.

Cap

The cap is 1 to 4 cm across, hemispherical to broadly convex or slightly bell-shaped, seldom flat. When moist it is dark greyish brown, often with a distinctly olive or drab tone, and it dries paler to greyish clay or dull tan, sometimes leaving a slightly darker marginal zone. The surface is smooth in youth but frequently develops fine radial wrinkles or a minutely roughened, cracked disc as it dries.

Gills

Gills are adnate, moderately broad and rather close, pale greyish brown at first and becoming heavily mottled with black spots and streaks before turning almost uniformly black. Edges are narrowly whitish and finely uneven.

Stipe

The stipe is slender, 4 to 10 cm tall and 2 to 4 mm thick, straight, hollow and rather fragile, greyish brown to reddish brown, paler and often frosted with fine white pruina toward the apex. There is no ring and no volva, and the base tapers or stays equal rather than forming a bulb.

Flesh and spore print

The flesh is thin, greyish and unchanging when cut, though water-soaked bases can look darker. The spore print is black. Microscopically the spores are smooth, lemon-shaped in face view and slightly roughened in some collections, with a distinct germ pore, and there are conspicuous cystidia on the gill edge.

This mottlegill grows in grassy habitats: lawns, pasture, roadside verges, playing fields, damp meadow turf and occasionally in mossy grass on sandy ground. It favours moderately fertile, moist turf and appears scattered or in small troops, sometimes mixed among the far commoner Brown Mottlegill.

Fruiting runs from spring through to autumn, tracking wet spells and mild temperatures. The species is best documented from the Pacific coastal states of North America but has been reported from other temperate regions, and its true range is probably wider than records suggest because of persistent confusion with related small grassland species.

Panaeolus olivaceus is saprotrophic, drawing nutrients from decaying grass roots, thatch and humus rather than from any living plant. It is not tied to dung in the way that some of its relatives are, though it will grow in ground that has been enriched.

Its mycelium runs through the shallow organic layer of turf and responds quickly to rainfall, producing successive small flushes across a season. Spores are dispersed on air currents from the black gill faces, and the species is easily transported with turf, topsoil and grass clippings, which favours its spread through managed grassland.

The principal look-alike is Panaeolina foenisecii, the Brown Mottlegill, which shares lawns, size and hygrophanous banding but is warmer brown without the olive cast and gives a dark brown spore print from warty spores; P. olivaceus gives a black print. Panaeolus fimicola is greyer and more closely tied to dung and manured pasture, with a smoother cap that lacks the fine wrinkling seen here.

Panaeolus cinctulus is larger and reddish-brown with a bold dark marginal band and prefers compost and mulch. Small Psathyrella species in grass are more fragile, with whiter, more flaring stipes and often veil fragments on the cap margin, and Conocybe species have conical tawny caps with a rusty-brown print rather than a black one.