is a disease that affects cereal grains used in brewing—most notably rye, but also wheat and barley. It is caused by the fungus Claviceps purpurea. The fungus produces toxic alkaloids in the affected grain that, when consumed, are poisonous to humans and other animals. Effects of ergot poisoning include convulsions and seizures, vomiting, gastrointestinal distress, gangrene, hallucinations, and often death. Ergot poisoning epidemics have been identified as occurring throughout history, especially in Europe in the Middle Ages, and have been seen in modern times in developing nations suffering from lax oversight of the food supply. However, improved grain cleaning and milling processes have largely eliminated large-scale ergot contamination today. The last recorded epidemic-size outbreak of ergot poisoning occurred in Ethiopia in the late 1970s.

Grain crops are most susceptible to infection in years marked by cold springs and damp, rainy summers; the telltale sign of infection is the presence of dark purplish-black fungal fruiting structures called sclerotia replacing kernels in the grain head just prior to harvest. Reduced yields and significantly reduced grain quality are hallmarks of ergot infection, which, along with the development of poisonous alkaloids, can render the crop a total loss. Rye is the most susceptible cereal grain to ergot infection, although wheat and barley are also affected to an economically significant extent.

Ergot-resistant varieties of rye, wheat, and barley are not available; however, growing other nonsusceptible crops for several years in fields known to have ergot sclerotia in the soil can greatly reduce future infections. Deep plowing can also aid in controlling infection because ergot sclerotia will not germinate when buried under several inches of soil.