Southern Rust in Corn: A Growing Concern for 2025

Southern rust is a lesser-known but potentially devastating fungal disease in corn, caused by Puccinia polysora. Unlike the more common rust diseases in North America, southern rust cannot overwinter in the Corn Belt. Its life cycle begins in southern states, where urediospores develop and spread northward on weather systems. While it doesn’t occur as often in the Corn Belt, when conditions are right, it can cause significant damage.

This past summer presented the perfect storm for southern rust, with high temperatures exceeding 77°F and high relative humidity, creating ideal conditions for its spread. Here’s what every grower should know about this disease, its impacts, and how to manage it effectively.

What Makes Southern Rust So Dangerous?

Southern rust can be more destructive than common rust due to its rapid spread and impact on plant health. Here are some of the ways it affects corn production:

Reduced Photosynthesis: The disease decreases the functional leaf area, limiting the plant's ability to produce energy. This can lead to poor kernel fill and reduced yields.
Weakened Stalks: Less photosynthesis means reduced carbohydrate reserves for stalks, which can compromise integrity and increase the risk of lodging.
Timing Matters: Later-planted corn is particularly vulnerable. If the disease takes hold before the dent stage, the yield impact is much higher.

The Life Cycle of Southern RusT

Southern rust thrives on windborne urediospores, which can travel hundreds of miles. Here’s a breakdown of how it develops:

Initial Infection: Spores from infected corn residue spread to growing plants early in the season.
Secondary Inoculum: Once infected, these plants produce spores that further disseminate the disease.
Continued Spread: Favorable conditions enable secondary infections to occur multiple times within a field.

Southern rust typically emerges later in the growing season, especially in southern states. However, it can move deeper into the Corn Belt in years with above-average temperatures, as observed in late summer 2024.

Infections were first reported in the Corn Belt on July 26, 2024, and peaked again on September 10, 2024. Look for:

Small, circular, orange pustules on the upper surface of leaves.
Symptoms are concentrated initially on lower leaves, spreading upward as the disease progresses.

Managing and Preventing Southern Rust

While prevention is challenging due to the disease's reliance on weather conditions, growers can take several steps to minimize its impact:

Stay Informed: Regularly monitor disease reports and weather forecasts during the growing season.
Scouting: Inspect fields regularly for signs of southern rust, especially during late summer.
Fungicides: Timely application of fungicides can protect leaf tissue and reduce the spread of the disease. Consult with your agronomist for specific product recommendations and timing.

Looking Ahead

While southern rust remains unpredictable, understanding its life cycle and impact can help growers mitigate losses. As we approach the 2025 growing season, proactive management and vigilance will be key to protecting your corn yields.

For more agronomic insights and to connect with experts. Together, we can tackle challenges like southern rust and ensure successful harvests for years to come.