fungal disease of cereal grasses, especially rye, caused by the ascomycete fungus Claviceps purpurea. In an ear of rye infected with ergot, a sweet, yellowish mucus is exuded for a time, followed by a loss of starch as the ear ceases growth. The ovaries then become permeated by the mycelium, a mass of fungal filaments, which in autumn forms the spur-like purple-black sclerotium.

The sclerotium constitutes the source of the drugs ergonovine, which is used in obstetrics to control postpartum hemorrhage, and ergotamine, which is used in treating migraine headaches. After an overdose of medications derived from ergot or after eating flour milled from ergot-infected rye, humans and livestock may develop ergotism, a condition sometimes called St. Anthony's Fire. The symptoms may include convulsions, miscarriages in females, and dry gangrene and may result in death. Ergot is also the source of lysergic acid, from which the powerful hallucinogen lysergic acid diethylamide (LSD) is easily synthesized.