Figure 1. High southern rust severity found in some corn fields.
Credit: E. Byamukama
Southern rust was confirmed in Turner, Union, Lincoln, Minnehaha, and Brookings Counties last week. It is most likely that at this time this rust is wide spread in other counties not yet scouted. Southern rust develops under warm and wet weather which is the reason it is considered a late season disease in South Dakota. This rust can quickly reach levels where it can result in reduced corn yields (Figure 1). Southern rust does not survive in South Dakota but develops from spores blown in from southern states.
Diagnosing Southern Rust
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 2). Southern rust pustules tend to be clustered on the corn leaf whereas common rust pustules may be scattered over the leaf. Common rust is frequently found in almost every corn field; however, common rust does not develop to reach yield reducing levels because of good resistance in corn hybrids.
Figure 2. Southern rust (upper) and common rust (lower). Notice the color and the arrangement of pustules. Credit: E. Byamukama
Southern rust is best managed through an application of a fungicide. Most corn hybrids are susceptible to southern rust. However, most of the corn fields scouted last week were past or are approaching the dent growth stage. For corn fields that were replanted due to hail damage, scouting should be done and a fungicide application planned where significant southern rust is developing.