Ergot Identification Guide

First Impressions

Ergot is the odd one out in any field guide, because in its most visible state there is no cap, no gills and no stalk. What you are looking for is a hard, dark, curved horn sticking out of a grass or cereal flower head, occupying the position where a seed should be.

The finding conditions are unlike mushroom hunting. You are not in a wood; you are walking a rye field margin, a damp meadow, a fen edge, a coastal dune slack or a roadside verge in late summer, scanning seed heads at waist height. From a few paces, an infected head looks as though someone has pushed short black splinters into it at odd angles. On a heavily affected stand you can pick out infected heads from a distance because the dark protrusions break the uniform straw colour of the ripening grasses.

What You Are Actually Looking At

The structure is a sclerotium — a dense, dormant survival mass of fungal tissue, not a spore-producing body. Describe it as you would any other fungal structure:

• Size: typically 2–20 mm long and 1–4 mm thick, but hugely variable with the host; sclerotia on rye are large and conspicuous, those on fine wild grasses can be only a couple of millimetres.
• Shape: elongated, cylindrical to spindle-shaped, usually curved like a claw or a small horn, often blunt at the tip and tapering at the base where it sits in the floret.
• Colour: dark purple-black, blue-black or dull violet-brown on the outside, frequently with a faint greyish or whitish bloom that rubs off.
• Surface: matt, finely cracked or longitudinally furrowed, sometimes with shallow ridges running the length of the body.
• Texture: distinctly hard and firm, not powdery, not spongy. It resists a fingernail and snaps rather than crumbling.
• Position: projecting from between the glumes of a single floret, replacing one grain; the rest of the head can look completely normal.

The Cut Test — The Key Examination

There is no cap flesh to slice, so the equivalent step is to break or cut a sclerotium in half across its width.

The interior should be white to pale pinkish or pale purplish, dense, firm and uniform in texture — no chambers, no powder, no fibres, no larval tunnel. That white-inside, black-outside contrast is the single most decisive field character for ergot and immediately separates it from insect galls, seed-head debris and the powdery smuts.

Two other observations belong here:

• The sclerotium does not change colour appreciably on exposure to air.
• Old sclerotia that have overwintered on the ground are duller, greyer and more deeply cracked, but retain the same firm pale core.

The Earlier Stage: Honeydew

Getting the identification right often means recognising the stage that precedes the black horn. In early to mid summer, when the grasses are flowering, an infected floret exudes a sticky, sugary yellowish fluid — the so-called honeydew stage, produced by the fungus's asexual (sphacelial) phase.

Signs to look for:

• glistening droplets on the flower head, sometimes running down the glumes and drying to a varnish-like crust;
• flies, beetles and other insects clustering on individual flowering heads;
• a sticky feel to the head when touched.

Under a microscope, a smear of honeydew is full of tiny hyaline conidia — the asexual spores that insects and rain splash carry to other flowers. Later in the season, the same floret hardens into the sclerotium.

The Spore-Producing Stage (and Why You Cannot Take a Spore Print)

There is nothing to place gill-side-down. A sclerotium does not release spores, so the standard spore print does not apply. The proper equivalent is to observe germination.

Sclerotia fall to the ground in autumn, overwinter in the soil, and require a period of cold before they germinate in spring. When they do, each one pushes out one to several tiny stromata that look like miniature drumsticks or pins: a slender purple to flesh-brown stalk 5–25 mm tall carrying a spherical head 1–2 mm across. Under a lens the head surface is minutely pimpled, each bump being the opening of an embedded flask (a perithecium) in which asci develop long thread-like ascospores.

You can reproduce this at home for study: collect sclerotia in autumn, half-bury them in damp sand in an open container, leave them outdoors over winter, and check in spring for the drumstick stromata. Confirming that a sclerotium germinates into pin-headed stromata is the definitive identification of the genus.

Hosts, Season and Ecology

Claviceps purpurea is a flower-infecting parasite of grasses, and it is remarkably broad in its host range within the family Poaceae. Classic and frequent hosts include:

• rye, the archetypal host, on which sclerotia grow largest;
• wheat, barley, oats and triticale, less often and with smaller sclerotia;
• a long list of wild grasses — couch, ryegrass, meadow foxtail, cocksfoot, canary grass, purple moor-grass, marram grass on dunes, common reed and many others.

The infection cycle is a good story to hold in your head, because it explains everything you see in the field. Wind-borne ascospores released in spring land on open florets at flowering time; the fungus grows into the ovary and replaces the developing seed; honeydew carries conidia sideways to other flowers through summer; the colonised ovary then hardens into a sclerotium by late summer; the sclerotium drops, overwinters, and germinates the following spring.

Because infection requires open florets, wet and cool weather during the flowering period — which prolongs the time flowers stay open and receptive — produces conspicuously more ergot. Damp meadows, sheltered ditches, humid coastal grassland and shaded field margins are consistently the most productive places to search. Ecologically the fungus is a seed-replacing biotroph: it takes a plant's reproductive output and converts it into fungal survival structures, and its honeydew makes it one of the relatively few fungi that recruits insects as active dispersal agents.

Separating It From the Look-Alikes

Smuts (Ustilago species). These also replace grain, but the infected structure is a soft mass of powdery, sooty-black spores enclosed in a fragile membrane that ruptures at a touch and blackens the fingers. Ergot is hard, does not rupture, and is white inside.

Bunt (Tilletia). The grain keeps its shape but is filled with a dark greasy spore powder; again, powder rather than a firm horn.

Choke (Epichlöe typhina). Another grass parasite, but it forms a white then orange-yellow felty sheath wrapped around the stem and leaf sheath below the head, preventing the head from emerging at all. It never sits in a floret as a black horn.

Insect galls and larval frass. Swollen or discoloured florets caused by midges and other insects may look dark, but cutting them open reveals a hollow chamber, a larva, or granular frass rather than solid white tissue.

Weather-blackened or sprouted grains. Fungal moulds and rain damage can darken grains, but they remain grain-shaped, soft, and are not curved horns protruding well beyond the glumes.

Other Claviceps species. Several relatives infect different grass hosts with smaller, straighter sclerotia; separating them reliably needs the host identity, sclerotium dimensions and microscopy of the germinated stromata.

Field Checklist

• Hard, curved, purple-black horn replacing one grain in a grass or cereal head
• Projects beyond the glumes; rest of the head often normal
• Snaps rather than crumbles; white to pinkish and solid inside
• Sticky yellow honeydew droplets on flowering heads earlier in the season
• Overwintered sclerotia germinate in spring into tiny purple-stalked, pin-headed stromata
• Damp grassland, dune slacks, fen edges, rye fields and verges, late summer
• Not powdery (smuts), not a stem sheath (choke), not hollow (insect galls)