Southern rust was confirmed in two counties of South Dakota as of last weekend. According to Emmanuel Byamukama, SDSU Extension plant oathology specialist, southern rust is an unusual disease to occur on corn in South Dakota. This provides a warning for other Midwestern corn growers who have not commonly had a problem with southern rust.
"Southern rust is favored by warm (80-90 degrees Fahrenheit) and humid conditions. The occurrence of southern rust is a concern because most of the corn hybrids are susceptible to southern rust, unlike the common rust, for which most dent corn hybrids have resistance genes," Byamukama said.
Byamukama added, because most of the corn is passed dent growth stage, the development of southern rust may not significantly affect grain yield. However, for corn that was planted late and has not passed dent, Byamukama encourages growers to scout and prepare to apply fungicide if southern rust favorable weather conditions continue.
The general recommended threshold for rust severity is 15 percent on a whole plant basis.
"With the warm weather and heavy dew in the mornings, southern rust may develop to higher severities in a short time," Byamukama said. "Growers are encouraged to keep scouting before deciding if a fungicide is needed. By the time heavy pustules are seen on leaves above the ear, it may be too late to apply a fungicide."
The corn plant pathology working group published a list of fungicides that are effective for several fungal pathogens on corn. This list can be downloaded online; http://www.extension.purdue.edu/extmedia/BP/BP-85-W.pdf. Fungicide label should be consulted to crosscheck pre-harvest restrictions.
Common rust versus southern rust
The two rusts can be easily differentiated, said Byamukama by the color and distribution on the leaf surface.
"Just like the name suggests, common rust is very common, we have found this rust in every corn field we have scouted. Common rust severity of up to 50 percent on the lower leaves has been observed in a few corn fields," he said. "Common rust has circular to elongate golden brown to cinnamon brown pustules that are usually randomly distributed on the leaves (Fig.1). These pustules can also be seen on lower side of the leaf."
On the other hand, Byamukama explained that southern rust pustules are circular to oval light brown to orange and occur in clusters on the upper side of the leaf (Fig. 2). Southern rust pustules have a yellow halo surrounding the pustules when the leaf is held against light (Fig. 3).
Both the common rust and southern rust do not overwinter in South Dakota. Rust spores are blown from southern states into South Dakota. Therefore corn residue or crop rotation does not affect the rust disease development.