Common rust (Puccinia sorghi) occurs every growing season in Iowa, but 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 may also be infected.
The fungus survives the winter as spores on corn in the southern United States and central America; 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.
Common rust is best scouted V12 through R4; earlier in seed production fields. Occurs in most Iowa corn fields. Raised, brick-red pustles are diagnostic for common rust.
Foliar fungicides are effective against both common and southern rust, however it is rare for common rust disease severity to be high enough and southern rust to occur early enough in the growing season in Iowa to warrant an application. Resistant hybrids and inbreds are available. Foliar fungicides labeled for common rust are available.
Photo by Darren Mueller