Southern rust in corn was confirmed recently in Turner and Lincoln counties in South Dakota.
"Southern rust is favored by warm (80-90 degree F) temperatures 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," says Emmanuel Byamukama, South Dakota State University Extension plant pathology specialist,
Southern rust may not significantly corn that has dented. However, for corn that was planted late and has not passed dent, the disease may be a threat, Byamukama says. He encourages growers to scout and prepare to apply fungicide if favorable weather conditions continue for southern rust continue.
The general recommended threshold for rust severity is 15% 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 says. "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.
Fungicide label should be consulted to crosscheck pre-harvest restrictions.
Common rust versus southern rust
Common rust and southern rust can be easily differentiated by the color and distribution on the leaf surface.
Common rust has circular to elongate golden brown to cinnamon brown pustules that are usually randomly distributed on the leaves. These pustules can also be seen on lower side of the leaf."
Southern rust pustules are circular to oval light brown to orange and occur in clusters on the upper side of the leaf. Southern rust pustules have a yellow halo surrounding the pustules when the leaf is held against light.
Neither common rust nor southern rust overwinters in South Dakota. Rust spores are blown from southern states. Corn residue or crop rotation does not affect the rust disease development.