Claviceps purpurea is a member of the Ascomycota, order Hypocreales (Clavicipitaceae). The fungus produces three dissimilar morphological stages; a conidial and a sclerotial stage occur on wheat, an ascogenous stage develops from sclerotia in soil.
The most characteristic symptoms of ergot are purple to black, horn-like sclerotia (ergots) protruding from the glumes on maturing heads. Sclerotia replace one to several seeds in cereal ears; the size of sclerotia increases with the size of the host seed, decreases with the number of sclerotia per head and is up to 10 times larger than seeds. Sclerotia of C. purpurea are elongated (2-40 mm), cylindrical with rounded ends, straight to sharply curved. The rind is hard and wrinkled or with longitudinal ridges (on rye); the interior is gray to white and parenchymatous.
The sclerotial stage of the fungus is preceded by a commonly ignored honeydew stage at flowering. Infected inflorescences exude a sugary sap that accumulates in sticky, yellowish droplets. Before sclerotia develop, infected ovaries swell and become stroma-like. Their surface is often covered with a layer of dense conidiophores.
Sclerotia of C. purpurea contain a number of ergoline alkaloids (derivatives of lysergic acid) toxic to humans and animals. Ergotism may occur through consumption of ergot-contaminated feed or grain products or during grazing of infested pastures. Ergolines are stable during processing, cooking or baking.
At maturity, sclerotia drop to the ground or are among the harvested seed.
Ergot is favored by wet, cool weather that accompanies and prolongs the flowering period.