What is "ustilago maydis"

Smut \Smut\, n. [Akin to Sw. smuts, Dan. smuds, MHG. smuz, G. schmutz, D. smet a spot or stain, smoddig, smodsig, smodderig, dirty, smodderen to smut; and probably to E. smite. See Smite, v. t., and cf. Smitt, Smutch.]

1. Foul matter, like soot or coal dust; also, a spot or soil made by such matter.

2. (Mining) Bad, soft coal, containing much earthy matter, found in the immediate locality of faults.

3. (Bot.) An affection of cereal grains producing a swelling which is at length resolved into a powdery sooty mass. It is caused by parasitic fungi of the genus Ustilago. Ustilago segetum, or U. Carbo, is the commonest kind; that of Indian corn is Ustilago maydis.

4. Obscene language; ribaldry; obscenity.

   He does not stand upon decency . . . but will talk smut, though a priest and his mother be in the room.
   --Addison.

   Smut mill, a machine for cleansing grain from smut.