Ergot sclerotia on rye, illustration. The fungus ergot (Claviceps purpurea) infects cereal crops and grasses, causing a reduced yield. During infection, the ergot induces the plant cells to divide and enlarge, resulting in a large mass (black), called the sclerotium, which allows the fungus to survive adverse conditions. Ingestion of ergot causes poisoning (ergotism) in animals and humans, which can be fatal. Rye (Secale cereale) is a cereal crop grown extensively for use by humans and to feed animals.