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Southern Rust Impact on Corn

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% 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.

The disease primarily targets the leaves of the corn plant, with pustules often densely clustered on the upper leaf surface. In extreme cases, the disease can also affect the husks and tassels, hampering the plant's ability to photosynthesize and produce kernels.

Southern Rust curtails the corn plant's photosynthetic capacity, leading to premature leaf death and reduced kernel development. This can result in poor grain fill, lighter test weights, and ultimately, a reduction in yield.

How to Identify 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.

Prompt identification of Southern Rust is key to mitigating its effects. It is marked by small, round to oval pustules on the upper surface of corn leaves. These pustules are packed with a bright orange to tan spore mass, lending the disease its characteristic rust-like appearance.

Unlike other corn diseases, Southern Rust pustules are predominantly found on the upper leaf surface and are densely clustered. As the disease advances, the pustules may also emerge on the husks and tassels.

(Image Credit: Travis Faske, University of Arkansas - Division of Agriculture, Bugwood.org)

How to Manage 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 and temperatures around 77-82°F, conditions are favorable to continued disease development.

When it comes to managing Southern Rust on corn, our agronomists recommend GCS Azoxy or GCS Azoxyprop.

Remember to always refer to the product labels for specific application rates and instructions.

Disclaimer

The information provided above was authored by Lanae Ringler and provided by Farmers Business Network, Inc. for informational purposes only. It does not constitute an endorsement or recommendation of a particular course of action or product. Please conduct your own due diligence prior to selecting a particular course of action or product.

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Southern Rust

Overview

Southern Rust Impact on Corn

How to Identify Southern Rust

Geographic Impact of Southern Rust

Timing of Southern Rust

How to Manage Southern Rust

Learn More About Southern Rust

Disclaimer

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