Common Rust is caused by the fungus Puccinia sorghi. Its spores are carried north on air currents during the growing season rather than surviving the winter on corn debris in northern climates. Infection is favoured by cool, humid weather and heavy dew. Cinnamon-brown pustules develop on both sides of the leaves and release powdery spores that spread to other corn plants. Severe infections can cause leaves to yellow and die prematurely, but infections appearing late in the season usually cause little damage.

How to Fix It
Choose resistant varieties: Resistant or tolerant sweet-corn varieties are the most effective defence against Common Rust.
Don't panic over late infections: Rust appearing after the ears are well developed is unlikely to cause significant crop loss.
Keep leaves dry: Water at soil level when possible. If overhead watering is necessary, water around midday so leaves dry quickly.
Plant on time: Earlier plantings may escape the higher spore levels and favourable disease conditions that occur later in the season.
Don't rotate just for rust: Common Rust does not overwinter on corn debris in northern climates, so crop rotation and removing corn residue will not control this disease.