Final answer:
The fungus named Claviceps purpurea causes ergot of rye, leading to a condition known as ergotism which has gangrenous and convulsive forms.
Step-by-step explanation:
The organism that causes ergot of rye is a fungus called Claviceps purpurea. This fungus grows on rye and other grains, producing a mycotoxin known as ergot toxin. This harmful alkaloid is behind the disorder known as ergotism, which manifests in two forms: gangrenous and convulsive. The gangrenous ergotism results from vasoconstriction leading to loss of blood flow and gangrene, while convulsive ergotism is marked by effects on the central nervous system such as mania and hallucinations. A notable incident of gangrenous ergotism struck Eastern Europe in the 5th century AD caused by the consumption of rye contaminated with C. purpurea.