Head Smut (Corn)

Sphacelotheca reiliana

Ear of corn infected with Head Smut
Ear of corn infected with Head Smut. Photo courtesy of William M. Brown Jr. Licensed under CC BY 3.0.
Head smut is a systemic corn disease that causes the ears or tassels to be replaced with dark brown to black galls.
Head smut infects the corn seedling and remains in the system throughout the life of the plant. It is caused by the telespores of a soil-borne pathogen that is able to survive in the ground for several ears. The spores attack the roots of the seedlings and invade the meristemic tissue.
There are no symptoms of head smut until the plant enters its reproductive phase. The initial symptom is that of large galls with creamy white membranes that appear on the tassels or ears. As the galls mature, they become dark brown or black and burst open to release masses of dark spores. It's easily differentiated from common smut through the mostly intact strands of vascular bundles that are contained within the membranes. It gives the spore masses a stringy appearance.
Corn tassel infected by head smut
Corn tassel infected by head smut. Photo courtesy of R.L. Croissant. Licensed under CC BY 3.0.
Spores of head smut are known to survive in soil for quite some time, and the level of infection in a field is directly related to the concentration of spores in the soil. Spores can be seedborne, but this isn't considered to be a big source of infections.
Be sure to plant resistant hybrids or fast-emerging plants. It can also be helpful to plant early when temperatures do not allow spore germination. If infected ears or tassels are found, remove and burn them to avoid the spread of more spores.
Rather than a protective fungicide seed treatment, it would be more advisable to treat seeds with a systemic fungicide. It is also helpful to have a well-maintained, well-balanced soil composition. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.