The weather conditions of 2025 were perfect for a southern rust outbreak across the Midwest, resulting in many growers having to make late-season decisions on disease management.
Southern rust is a fungal foliar disease that flourishes in hot and humid conditions and requires a living host to reproduce. The spores are typically blown from the south by wind and rain. In a recent Pioneer webinar 86% of attendees experienced southern rust in 2025 and almost 50% dealt with southern rust at least once between 2020 and 2024.
“June was warm, but July was warmer and one of the wettest on record in the North-Central Corn Belt. Those are the conditions you need for southern rust to thrive,” said Carl Joern, Pioneer Field Agronomist in Indiana.
Southern rust can be identified by the small, circular, light-orange pustules that form in dense clusters almost exclusively on the upper leaf surface and leaf sheath and leave a powdery residue when touched.
“An easy way to determine if you have southern rust is to take your fingers and run it through the leaves and see if you have an accumulation of spores on your hands,” said Krystel Navarro, North America Pathology Leader, Corteva Agriscience.
Visible symptoms develop in 7-14 days after infection. Pustules eventually rupture and the disease spreads. As the disease progresses, leaf tissue around the pustules turns yellow (chlorosis) and then brown (necrosis), leading to rapid dieback of infected leaves. Severely infected plants look “fired” or as if they are drying down prematurely from the mid-canopy up, which can be mistaken for early maturity or drought stress.
Common vs Southern
Southern rust and common rust are different species of the same genus. While they are both rusts, they appear different and thrive in different conditions. Southern rust pustules are found almost exclusively on the upper leaf surface, while common rust appears on both the upper and lower surfaces, and may also infect the husk.
Additionally, southern rust has small, circular and light-orange pustules, whereas common rust has larger, elongated, brick-red pustules.
Common rust thrives on cool-to-warm, moist conditions, usually between 60°F and 77°F. Southern rust, on the other hand, prefers warm-to-hot, moist conditions, usually higher than 77°F.
Why 2025 Was Unique
A large and persistent heat dome covered much of the U.S. in late June, resulting in several consecutive days of temperatures in excess of 90°F. Additionally, overnight low temperatures remained greater than 72°F, providing no break from the heat.
Leaf wetness — between 6 and 8 hours continuously — also plays a critical role in spore germination for southern rust. During this period of high temperatures, dew point values were greater than 70°F, resulting in high humidity. Many areas exceeded normal precipitation for June, adding to the moisture for the area.
Krystel Navarro and her team monitored conditions throughout the year to help better predict the spread of southern rust.
“If you think about it, every 14 days, you’re producing thousands and thousands of spores that continue to infect, and that availability of corn throughout that period is really what’s enhancing [the spread],” she said. “Plus, with the warmer temperatures, the fungi just had the perfect conditions to really develop.
“If you were in fields, you could feel the heat and humidity, even late in the afternoon,” Navarro added.
The last environmental component for southern rust propagation is spore transportation via wind. Southern rust spores mostly come from the south, usually in areas with continuous corn in tropical/subtropical regions, and are blown north by wind. Southern rust does not overwinter in cold conditions or survive in crop residue – it requires a living plant to survive.
While wind speeds in June were not abnormally high, much of the country had a stronger southwesterly component than normal.
“When you follow the wind patterns, you can see [southern rust] progress north,” Navarro said.
Predicting the Disease
“The biggest challenge for scouting for this disease is that it’s so sporadic. You can find it in many different areas of the field because it comes from up high and settles down,” Joern said.
Because of this, Pioneer has leveraged predictive insights to help with southern rust risk projections. Farmers are able to partner with their Pioneer sales reps to receive weekly disease forecasts on a county level through Granular Insights.
“We look at the specific amount of environmental conditions that are needed to ‘cross that line,’” Navarro said. “Using mathematical and statistical models, we were able to create tools for predictability.”
The tool helps forecast and guide field scouting by identifying when and where fields are at elevated risk.
Southern Rust Management
Timely foliar fungicide applications can help reduce leaf damage from southern rust, but its fast-moving nature requires vigilant scouting and fast-acting decisions.
“When you get something that shuts the plant down hard, it can really impact stalk integrity, and kernel and grain quality. You need to be a little proactive,” Joern said. “This year, we had about a week to make that decision. That was a pressure cooker for many growers.
“This is a very aggressive disease. Because it can take a week for spores to land, pustules to form and then more spores to spread, you are going to want something with multiple modes of action and a long residual,” Joern added. “You also have to be prepared to make that fungicide decision if you have severe disease pressure as late as the R4 dough stage.”
Because it damages leaves and forces plants to consume stalks for energy, the risk of stalk rot and lodging is increased. A timely harvest can help mitigate yield loss from southern rust. However, premature plant death can halt grain fill, leading to lower test weights and smaller kernels. Growers should anticipate lower grain quality in this situation.
2026 and Beyond
Building a successful management plan for southern rust requires a mix of genetics, crop protection and data.
Corteva and Pioneer continue to push the limits of genetics using CRISPR gene-editing technology, including multi-disease resistance (MDR) corn, which is expected to launch late this decade pending regulatory reviews and the completion of field testing.
“What gene editing has allowed us to do is to stack disease-resistance genes and make them really easy to work with,” said Brandon Wardyn, North America Corn Evaluation Zone Lead for Corteva Agriscience. “We’re even able to stack resistant genes for the same disease, giving even more protection.”