As is typical for Kentucky, symptoms of common rust can be found on corn leaves in many fields at this time of the season.

Rust diseases appear as tiny, rust-colored pustules on leaves. Two rusts can attack corn in Kentucky: common rust and southern rust.

Rust pustules are circular to oval, golden-brown to cinnamon brown, and up to 1/8" long. Pustules of common rust are widely scattered on leaves and are common on both upper and lower leaf surfaces. Pustules of southern rust tend to occur in dense congregations on upper leaf surfaces, with few on lower surface. For a figure showing a side-by-side comparison of the two rusts, see page 37 of the Extension publicationID-139: A Comprehensive Guide to Corn Management in Kentucky. Leaves turn yellow and dry up when severely infected by rusts.

Spores of both fungi are carried on springtime winds from southern areas of the U.S. Common rust is active during cool (60˚F to 75˚F), humid weather; southern rust is most active during warm (80˚F), humid conditions. Both fungi infect leaves when spores are present and leaf surfaces are wet. Both are potentially more severe in late plantings. Greatest yield loss occurs in susceptible hybrids when outbreaks begin during early grain fill.

Common rust rarely causes economical loss in field corn in Kentucky, because of good resistance levels in commonly grown hybrids. In contrast, an aggressive outbreak of southern rust in late-planted crops may reduce yield and stalk strength in a grain crop and quickly desiccate silage corn.