Southern rust is a fungal disease that affects corn crops in the United States. Previous research has estimated that for every 10 percent of rust infection there is an accompanying 6 percent reduction in corn yield This fungus is dispersed by spores that are carried on the wind from tropical climates in the South, where this disease overwinters. The severity and occurrence of the disease depends largely upon the weather conditions. Six hours of leaf wetness or relative humidity of at least 95°F is enough for infection and development.
Southern rust disease is a parasite that diverts nutrients away from the plant and towards the fungus. Severe infections can cause leaf defoliation and senescence, causing the plant to cannibalize the stalk for nutrients during grain fill. This weakening of the corn stalk can lead to stalk rot disease and lodging, which may also affect yield.
Identification of Southern Rust
Southern rust is identified by circular, cinnamon to orange colored pustules (raised masses of fungal spores) located primarily on the upper surface of the leaf. A yellow, chlorotic halo around the pustules is often present, and only these chlorotic spots are found on the underside of the leaf. If the pustules are rubbed off the leaf, they will leave a reddish orange streak on the hands or clothing. These characteristics are important to distinguish this disease from common rust, which has more elongated pustules located on the top and bottom of the leaf surface.
Rust pustules interfere with the stomata’s water regulation, resulting in plants exhibiting symptoms of drought stress. Pustules are commonly located on the upper leaves of the plant due to infection occurring while the leaves are in the whorl. As infection continues, pustules appear in dense clusters on the leaves. While the pattern of infection on the plants are similar, the pattern of infection in the field is often random.
Management of Southern Rust
There is susceptibility to southern rust in most corn hybrids, while some moderately susceptible hybrids may use multiple genes to slow down southern rust development. Planting corn earlier or use of short season hybrids can help prevent the severity of southern rust because the leaves will be more mature by the time rust spores arrive late in the growing season, around August and September.
Fungicide applications are most effective when applied soon after the initial infection occurs. Research indicates that fungicide applications between silking (R1) and milk stage (R3) are most beneficial at reducing yield loss. However, since there is no research on the economic threshold of southern rust, fungicide applications may only be economically beneficial when applied to high value crops such as seed corn or popcorn. Another factor in fungicide applications is weather conditions--if the forecast is high humidity andtemperatures around 77-82°F, conditions are favorable to continued disease development.