Almond Mushroom Identification Guide

The First Impression

The Almond Mushroom (Agaricus subrufescens) is a mushroom of made ground. The typical find is in a garden bed, a mulched border, the flank of a compost heap, a pile of rotting leaves behind a shed, a woodchipped park path or the warm soil of a greenhouse. It appears in groups and frequently in loose clusters, with several fruit bodies pushing up together through the loose substrate.

The first thing most recorders notice, before any structural detail, is the scent. Bruise the cap edge or break a fragment of flesh and a strong, unmistakable almond-like aroma rises immediately, easily detected at arm's length. Combine that with a fairly tall, pale, finely scaled cap on rich organic substrate in warm weather and you have the beginnings of a determination.

Reading the Cap

Caps run 5-15 cm across and occasionally larger. They start ovate to hemispherical, becoming convex and finally broadly flattened, often with a low, wide umbo and a slightly angular outline.

The surface is dry and matt. The ground colour is whitish to pale straw, and over it lie fine, appressed, silky fibrils and small flat fibrillose scales in warm shades of pinkish-brown, reddish-brown, cinnamon or dull tan. The fibrils are densest and darkest over the disc, thinning and separating radially toward the margin so that more pale ground shows outward. The overall impression is a finely brushed, streaky cap rather than a coarsely shaggy one.

Rub the cap with a thumb: expect a slow yellowish discolouration, not an instant vivid one. Young caps usually carry hanging veil fragments on the margin. A drop of potassium hydroxide on the cap surface produces a yellow reaction, and the Schaeffer cross reaction with aniline and nitric acid is positive, giving an orange-red line at the intersection, a useful supporting character for this part of the genus.

Gills and the Spore Print

Turn the cap. The gills are free, forming a distinct collar around the top of the stipe, extremely crowded, narrow and thin. Their colour sequence is the reason to check several ages of fruit body in a single troop: whitish to very pale grey in the button, then a definite greyish-pink or flesh pink, then dull purplish brown, then very dark brown when the spores are fully ripe.

Take a print from a cap that has just flattened. Cut the stipe flush, place the cap gill-down on white card under a bowl, and leave it four to six hours. The deposit is a heavy dark brown with a purplish cast. Microscopically the spores are small, ellipsoid, smooth and thick-walled, roughly 6-7.5 by 4-5 micrometres.

Stipe, Ring and Base

The stipe is 6-15 cm long and 1-2 cm thick, cylindrical, sometimes slightly enlarged or clavate at the base but never abruptly bulbous and never sitting in a sac. Above the ring it is smooth and white; below it is often finely floccose or scurfy and discolours dingy brownish where handled.

The ring is superior, large, membranous and pendant, hanging as a well-formed skirt. Its underside frequently carries soft cottony patches or floccose scales, and it can tear against the cap margin as the cap expands.

Excavate the base rather than pulling the fruit body free. In this species the base regularly produces conspicuous white mycelial cords, rhizomorphs, that run out into the compost or mulch and bind fragments of the substrate. Finding those cords in loose organic material is a genuinely useful supporting character.

Flesh and Colour Change

Slice a fruit body from cap margin to stipe base. The flesh is white and firm, thicker over the disc than at the rim. Watch the cut surface for a few minutes: expect little change, or at most a slow pale yellowish tinge in the cap flesh and a faint yellowish-ochre flush in the stipe base, later fading to dull brownish. What you must not see is an immediate, saturated chrome-yellow bloom in the base within a second or two of cutting, which belongs to a different group.

Smell the cut surface again. The almond note should be strong and clean, without any inky or chemical undertone.

Substrate, Season and Ecology

Agaricus subrufescens is a saprotroph of rich, well-aerated organic material: compost, leaf mould, manured beds, woodchip mulch, decaying straw and other accumulations of decomposing plant matter. It is a warm-season species, fruiting from mid summer into autumn and continuing later in mild climates, and it is commonest in warm-temperate and subtropical regions and in artificially warm situations such as glasshouses.

Because it colonises made and moved substrates, it turns up in gardens, parks, plant nurseries and around cultivated ground far more often than in undisturbed woodland, and it can appear suddenly in newly mulched beds. Note the substrate carefully; a scaly Agaricus growing in undisturbed forest litter or open pasture is more likely to be something else.

Separating the Look-alikes

Agaricus augustus shares the strong almond scent but is generally larger and stouter, its cap fibrils are distinctly darker brown on a yellowish ground, the yellowing on rubbing is quicker and more pronounced, and it grows in woodland, along shaded paths and beside trees rather than on compost.

Agaricus bohusii grows in dense fused clumps on enriched soil under broadleaved trees, has much darker sepia to blackish-brown appressed scales, reddens slowly in the cut flesh and has only a mild mushroomy odour.

Agaricus sylvaticus and A. langei turn distinctly red where cut, sometimes strongly so, lack the almond scent and are litter species of woodland floors.

Agaricus xanthodermus and A. moelleri are the important morphological contrast: the base of the cut stipe flushes intense chrome-yellow within seconds, and the odour is inky and phenolic rather than almond-like. A. moelleri also has grey-brown fibrils that darken the cap centre almost to blackish.

Agaricus bitorquis grows on hard compacted ground with a smooth white cap and a double ring whose lower part sheathes the stipe with an upturned rim.

Leucoagaricus and Chlorophyllum species occupy the same mulch beds and can look similar in scaling, but their gills stay white or turn greyish-green rather than passing through pink, and their spore prints are white, cream or dull green.

A Working Sequence

Record the substrate and the growth habit first. Check the fine reddish-brown appressed fibrils on a pale ground, the free gills passing pink to dark brown, the pendant skirt-like ring with a cottony underside, and the white mycelial cords at the base. Cut lengthwise, confirm the absence of an instant yellow base flare, and check the almond scent. Then take a dark brown spore print for confirmation.