Smuts

1. a small flake of soot or other dirt or a mark left by one.

   all those black smuts from the engine

2. a fungal disease of cereals in which parts of the ear change to black powder.

   a few bad crop years with smut and drought and frost

   fungal infections such as the smuts

3. obscene or lascivious talk, writing, or pictures.

   porn, in his view, is far from being harmless smut

late Middle English (in the sense ‘defile, corrupt, make obscene’): related to German schmutzen ; compare with smudge¹. The noun dates from the mid 17th century.