Corn rust is caused by the parasitic fungus Puccinia sorghi, which produces golden brown to cinnamon-brown colored pustules at various places on the stems and leaves of the corn plant. As the fungal infection matures, black spores develop that are carried by air currents to other plants.

Thriving in cool temperatures and humid conditions, P. sorghi requires only a few hours of dew to complete an infection. Older tissue is generally rust resistant; therefore, the spores usually attach to newer, more tender leaves.

Infection occurs when a spore falls onto a leaf or stalk and produces a germ tube. The thread-like tube grows along the host surface until it finds an opening, usually entering through a natural pathway such as a microscopic air pore, called a stomate. Branching out within the tissue, the tube forms a network of filaments and sends bulbous protrusions called haustoria to invade cells and absorb nutrients stored in the living cytoplasm. After a few days, the fungus reproduces by generating spores at the center of the infection site. The spore masses erupt through both sides of the leaf, producing visible brownish-red pustules that mature into the characteristic black spores of corn rust.

Corn rust is generally not considered a serious threat to this major grain crop because potential for infestation can be diminished by use of one of several rust resistant corn hybrids.