Smut is a disease of cereals, corn, grasses, onions, and sorghum that can be caused by any of more than 700 species of parasitic fungi. Smuts generally have a negative economic impact on agriculture, because they affect so many food crops. An exception to this is corn smut, which is considered a delicacy in Mexico.

Corn infected with the fungus Ustilago zeae (also known as U. maydis) forms large, swollen, kernel-like globules with soft black flesh covered by a silvery-gray skin. Called huitlacoche (pronounced wee-tlah-KOH-cheh), the native Nahuatl word, this dish is characterized by an inky, mushroom flavor and has apparently been eaten in central Mexico for thousands of years. In the United States, after decades of trying to eradicate corn smut, some farmers are now attempting to grow corn with large corn smut infestations because the fungus is becoming a more popular gourmet food item, drawing much higher prices than healthy corn.

The various smuts are characterized by masses of sooty spores that grow on or inside the plant during a fungus's last stage of growth. Many begin to grow in the plant embryo, feeding on maturing plant tissue through a network of filaments, or hyphae. To reproduce, a blister forms and breaks open, and the black, powdery fungus spores are borne away by air currents.