Figure 1. Southern rust (lower leaf) and common rust (upper) symptoms. Notice the color and the arrangement of pustules. Courtesy: E. Byamukama.
A few corn fields scouted in Brookings and Moody counties were found with southern rust. The presence of southern rust in corn at this time is an indication that the inoculum is present in the state. Southern rust develops from spores blown northwards from southern states. Southern rust can quickly reach severe levels that can reduce corn yields especially if infection takes place when corn is not past the dent growth stage.
Diagnosing Southern Rust
Southern rust, like any other rust, develops into pustules (raised fungal structures) on the leaves (Figure 1) and can be confused with the common rust. Common rust is more abundant and develops on corn throughout the growing season. Southern rust can be differentiated from common rust by the color and the arrangement of the pustules on the leaf. Common rust pustules are dark red in color, while southern rust pustules are orange brown (Figure 1). Southern rust pustules tend to be clustered on the corn leaf whereas common rust pustules may be scattered over the leaf.
The majority of corn fields are past the dent growth stage at this time. These are not likely to impacted by southern rust. However, for late planted or re-planted corn that is not yet past dent, southern rust infection could lead to yield loss. Southern rust is best managed through an application of a fungicide. Most corn hybrids are susceptible to southern rust. For fungicides effective against southern rust, see this publication.