Ergot

Claviceps purpurea is an ascomycete in the Clavicipitaceae, the family that also contains Cordyceps relatives and the grass endophytes. It is best known for the sclerotium it forms, called an ergot, which takes the place of a single grain in the ear of rye and many other grasses.

The ergot itself is not a fruiting body but a dense survival structure of compacted fungal tissue. It is hard, curved like a small horn or claw, purple-black to dark violet-brown on the outside and white to pale pink within, and it usually projects noticeably beyond the glumes of the floret it has replaced. Rye is the classic host because its florets stay open longest, but the fungus occurs on a great many wild and cultivated grasses.

What makes the life cycle striking is the second act. After a winter on the ground, each ergot germinates to produce several miniature fruiting bodies on slender stalks, each ending in a rounded head like a tiny pink or purplish drumstick or pinhead mushroom, a couple of centimetres tall at most. These are the sexual stromata from which ascospores are shot into the air.

Sclerotia in the ear

Ergots are elongate, cylindrical to slightly tapered and usually curved, from a few millimetres up to 3 cm or more on rye and generally smaller on other grasses. The surface is matt purple-black, dark violet or nearly black, often with fine longitudinal cracks or a whitish bloom. They are hard and cannot be crushed between the fingers, and a cut across one shows a firm white to pinkish interior with a thin dark rind. They typically protrude well beyond the glumes, making an infected ear look spiky and irregular.

Honeydew stage

Before the sclerotium hardens, the colonised floret exudes sticky, sugary droplets, pale yellow to amber and sometimes shining on the glumes, carrying vast numbers of tiny one-celled conidia. Dust and insects often stick to it and the surface may darken with sooty moulds.

Stromata

Overwintered sclerotia lying on or just under the soil produce one to about twenty stalked stromata in spring. Each has a thin, flexuous, pale to purplish stalk 5-25 mm long topped by a globose head 1-2 mm across, pinkish-buff to purple-brown, dotted with the pinprick openings of embedded perithecia.

Spores

No spore print applies. Ascospores are extremely long, thread-like, hyaline and single-celled, discharged from the perithecia in the stromatal heads; conidia from the honeydew are small, oval and colourless.

The fungus colonises the open flowers of grasses, with rye the most familiar host and wheat, barley, oats and triticale also used. It is at least as common on wild grasses in rough grassland, roadside verges, marsh edges, dune slacks and woodland rides, on genera such as Lolium, Festuca, Dactylis, Alopecurus, Elymus, Ammophila and Phalaris, where the dark horns are often easier to spot than in a crop.

Sclerotia mature through summer and fall to the ground or are carried in threshed grain. Germination in spring needs a period of cold followed by mild, moist conditions, and stromata appear when the earliest grasses are flowering. Cool, wet weather at flowering favours colonisation because it prolongs the period the florets stay open. The species is widespread throughout temperate regions of Europe, Asia, the Americas, Africa and Australasia.

Claviceps purpurea is an obligate parasite of grass ovaries. Airborne ascospores land on a receptive floret and the hypha grows into the ovary along the same route a pollen tube would take, replacing the developing seed with fungal tissue. The fungus then produces honeydew laden with conidia, which insects attracted to the sugar transfer to other open florets, giving a rapid secondary spread through a flowering stand.

As the season progresses the colonised ovary hardens into the sclerotium, a nutrient-rich resting body that survives winter on the soil surface. Its germination into stalked stromata the following spring completes the annual cycle, timed to coincide with grass flowering. Ecologically the fungus is a seed parasite that reduces the reproductive output of grass populations and, because it depends on open florets, is most successful on cross-pollinated grasses such as rye and many wild species.

Common bunt (Tilletia caries) also replaces the grain but produces a soft grey ball, still within the glumes, that crushes to brown powder; ergots are hard, dark, protruding and cannot be crushed. Loose smut (Ustilago) leaves a bare rachis after its powdery spore mass blows away. Blackened kernels caused by sooty moulds are surface growths on an otherwise normal grain.

The spring stromata can be mistaken for tiny stalked fungi in the litter. Ascomycetes such as Cordyceps relatives and Tolypocladium also produce club-shaped stalked heads but arise from buried insects or from Elaphomyces truffles rather than from a hard dark grain-shaped body. Digging carefully to expose the object at the base of the stalk settles the identification: an ergot stroma always rises from a purple-black sclerotium shaped like the grass floret it replaced.