Common rust of corn
Common rust is caused by the fungus Puccinia sorghi and occurs every growing season. It is seldom a concern in hybrid corn. Rust pustules usually first appear in late June. Early symptoms of common rust are chlorotic flecks on the leaf surface. These soon develop into powdery, brick-red pustules as the spores break through the leaf surface. Pustules are oval or elongated, about 1/8 inch long, and scattered sparsely or clustered together. The leaf tissue around the pustules may become yellow or die, leaving lesions of dead tissue. The lesions sometimes form a band across the leaf and entire leaves will die if severely infected. As the pustules age, the red spores turn black, so the pustules appear black, and continue to erupt through the leaf surface. Husks, leaf sheaths, and stalks also may be infected. Pustules of common rust with brick-red spores. Image: A. Sisson
Common rust foliar symptoms. Image: D. Mueller
Common rust disease cycle.
The fungus survives the winter as spores in subtropical and tropical regions; spores are carried long distances by wind and eventually reach the Midwest. Rust development is favored by high humidity with night temperatures of 65-70°F and moderate daytime temperatures. The disease is usually more severe on seed corn.
Resistant hybrids and inbreds are available. Foliar fungicides labeled for common rust are available.