It is caused by the fungus Claviceps purpurea, The pathogen produces a structure called a sclerotium that grows in place of the rye kernel. This sclerotium contains poisons that are very harmful to man and animals. The sclerotium is harvested with the rye grain. The grain is ground into flour, made into bread and is eaten by people. Eating the contaminated bread results in a disease called Ergotism.