Smut is a fungal disease of corn that may infect any aboveground portion of the plant including leaves, stems and ears though infections on ears are the most obvious. Immature galls are white and spongy and appear to be covered with a greenish-white membrane.
Galls later become brown with dark powdery spores.
Galls form wherever spores land but do not move systemically through the plant. Spores overwinter in the soil and debris. Spores may remain viable for 5 to 7 years. Leaf galls remain small but those on the ears or stems can become rather large and will release large numbers of spores when they rupture. This disease is favored by hot, dry weather and tends to be worse on plants injured by hail, cultivation or insects and on soils that have had heavy applications of nitrogen fertilizer or manure.