Poplar Fieldcap

Cyclocybe aegerita, long known as Agrocybe aegerita or Agrocybe cylindracea, is the classic wood-inhabiting fieldcap of warm temperate regions, fruiting in tight bundles from the trunks, wounds, and stumps of poplars. It has been grown on logs and supplemented sawdust for a very long time in southern Europe, where the Italian name pioppino, meaning little poplar mushroom, reflects its favoured host.

The genus Cyclocybe was separated from Agrocybe to hold a group of larger, wood-inhabiting species with tough stems and persistent membranous rings, leaving the smaller soil-dwelling fieldcaps in Agrocybe proper. Both sit in the family Strophariaceae and share dull brown to tobacco-brown spore prints and attached gills. Cyclocybe aegerita is the best known member of its genus.

The fungus is easy to spot because it fruits high on standing trunks in dense, tiered clumps, sometimes several metres above the ground, on trees that are otherwise alive. Old caps develop a distinctive network of fine cracks across the surface as the cuticle dries and splits, giving them an aged, crazed appearance quite unlike the smooth young buttons alongside them.

Cap

The cap is 4-10 cm across and occasionally to 15 cm, hemispherical then convex and finally broadly flattened, sometimes with a low umbo. The colour is warm tan, ochre, chestnut, or dull brown, usually darkest at the centre and paling to whitish or cream at the margin, and darker overall in wet weather. The surface is dry and smooth at first, then characteristically wrinkled and finely cracked into a shallow crazed network as it ages, especially in dry conditions. The margin may be faintly striate and often carries veil fragments when young.

Gills

The gills are adnate, sometimes with a short decurrent tooth, and are crowded and fairly broad. They begin whitish to pale buff, then turn dull clay, and finally a dark cinnamon or tobacco brown as spores mature, with paler, minutely fringed edges.

Stipe

The stipe is 5-15 cm long and 0.5-1.5 cm thick, whitish to pale cream, browning towards the base, distinctly tough and fibrous, and often curved or eccentric where clumps push out from a vertical trunk. It carries a persistent, fairly high, membranous white ring that is soon dusted and stained tobacco brown by falling spores. The base is frequently fused with neighbours and may have fine white mycelial fibres.

Flesh and spore print

The flesh is white and firm in the cap, tougher and more fibrous in the stem, and does not change colour on cutting. The odour is pleasant and mealy or faintly of flour. The spore print is dull tobacco brown, lacking any rusty or purple tint.

The fungus fruits in dense clusters on living and dead broadleaf trees, most consistently on poplars including black poplar, and regularly on willow, elder, elm, ash, fig, and mulberry. It emerges from bark wounds, pruning cuts, forks, hollow trunks, and stumps, and can appear well above head height on standing trees. It is common along riverbanks, in old orchards, in avenues of poplar, and in parkland.

It favours warm conditions and its natural centre of distribution is the Mediterranean and warm temperate Europe, extending across warmer parts of Asia and into North America and Australasia. The season is unusually long, with fruiting possible from spring through summer and autumn wherever there is warmth and moisture, and it will fruit repeatedly from the same wound over successive years.

Cyclocybe aegerita is a saprotroph and a weak parasite. It usually enters a living tree through a wound or dead branch stub and then decays the heartwood, causing a white rot in which lignin and cellulose are both broken down and the affected timber is left pale and soft. Because it is largely confined to already dead heartwood, an infected poplar or willow may continue growing for many years while being progressively hollowed.

That hollowing role has ecological value, producing the cavities in old riverside trees that are used by nesting birds, bats, and invertebrates. On dead wood and stumps the same mycelium continues purely as a decomposer. The fungus fruits opportunistically whenever temperature and moisture allow rather than in a single narrow autumn window, which is why the same trunk wound can produce several flushes in a year.

The Sheathed Woodtuft, Kuehneromyces mutabilis, forms similar clusters on wood and also has a ring, but it is smaller and more slender, its cap is strongly hygrophanous with a distinctly two-toned appearance as it dries from the centre outwards, and its stem below the ring is covered in coarse dark recurved scales, whereas the stem of C. aegerita is smooth and pale.

Galerina marginata is smaller and thinner with a smooth stem, a hygrophanous tawny cap, and a rusty rather than tobacco-brown spore print. Pholiota species on the same substrates have viscid or conspicuously scaly caps and rusty brown prints. Agrocybe praecox and its relatives are similar in colour but are smaller, grow from soil, mulch, and grassy ground rather than from trunks, and are far less tough in the stem. Hypholoma species cluster on wood too but give a purple-brown spore print and leave only a cobwebby ring zone.