ergot

noun

1. the hard, reddish-brown or black, grainlike masses (sclerotia) of certain parasitic fungi (esp. genus Claviceps) that replace the kernels of rye or other cereal plants
2. the disease in which this occurs; specif., the disease of rye caused by a species (Claviceps purpurea) of this fungus
3. the dried sclerotia of the rye fungus from which several alkaloids are extracted that have the ability to contract blood vessels and smooth muscle tissue
4. any of these alkaloids

Origin: Fr < OFr argot, a rooster's spur, hence (from the shape) the disease growth in the plant