Corn smut, also known as common smut, is a common disease of sweet corn caused by the fungus Ustilago maydis.This disease rarely kills corn plants, but usually causes the production of smaller ears. Corn smut commonly develops on the tassels, ears, leaves, and shoots of corn as a gall.rystal Floyd. Photo by M. Grabowski. Corn smut, also known as common smut, is a common disease of sweet corn caused by the fungus Ustilago maydis. This disease rarely kills corn plants, but usually causes the production of smaller ears.