Ergot (Claviceps purpurea) is a fungal disease that affects wheat, barley, oats, rye, triticale and a wide range of grasses, particularly black-grass (Alopecurus myosuroides). It attacks the ear at flowering and replaces a few spikelets with hard, black sclerotia.

Although ergot has very little effect on crop yields in sufficient concentration it has a number of harmful, biological effects on humans and animals due to the presence of toxic alkaloids.

Ergot is more commonly found in rye and spring wheat, and in those varieties which have a more open flowering habit. It is fairly easily managed by the chain since affected grains are easy to see (being black) and also lighter than healthy grain. Therefore cleaning equipment on farms and in grain stores is effective in removing them. Flour millers reject grain contaminated by ergot, and the normal tolerance in international trade is 0.001%.