Ergot of rye is a fungal disease caused by species within the genus Claviceps, most notably Claviceps purpurea. This parasitic fungus infects the unfertilized ovaries of rye and other cereal grasses. It replaces the healthy grain with a dark, hard fungal mass called a sclerotium (or ergot body). These sclerotia contain toxic alkaloids that, if consumed by humans or livestock, can lead to a serious condition known as ergotism or "St. Anthony's Fire".

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