It’s a corn disease issue creeping north and infiltrating fields earlier than it has in the past and one that thrives in high heat and humidity. With increasing reports of southern rust now being found as far north as Iowa, agronomists are encouraging farmers to scout early and often.
Southern rust is a fungal disease that affects corn crops. It’s trademark is small, orange-colored spots that look like rust.
And as Farm Journal’s Rhonda Brooks reported last week, southern rust can be difficult to distinguish from common rust.
The spots erupt on the upper surface of corn leaves. The disease likes warm and humid conditions, and once it spreads, it can be a major hit to yield, especially if it develops during the critical period of kernel development.
According to Crop Protection Network, southern rust was first reported in Florida and Georgia in mid-June, but the disease has quickly made its way to the Midwest. While the disease has been reported in Alabama, Florida, Georgia, Mississippi, southern Texas, it’s now been reported in a number of counties across the South and Midwest, including:
- Arkansas: eight counties
- Illinois: eight counties
- Indiana: two counties confirmed, but three counties listed as probably cases
- Iowa: 30 counties
- Kansas: 16 counties
- Kentucky: eight counties
- Missouri: 12 counties
- Nebraska: 27 counties
- Oklahoma: three counties
“In addition to tar spot, there’s also been some reports of southern rust, depending on which area you’re in, as that stuff has blown up more from the south,” Missy Bauer told Farm Journal’s Rhonda Brooks during Corn College this week. “If the environment’s conducive for these things to continue, we could see some pretty big disease issues across the Corn Belt this year. We want to be prepared and make sure you’ve got plans in place to deal with this potential disease pressure.”
Dan Quinn, who is an assistant professor of agronomy and Extension corn specialist at Purdue University, says southern rust has been confirmed in southern Indiana, but he has also detected it in northern Indiana, which is early.
“We always track its movement from the south,” Quinn says. “That’s the key. Southern rust in a state like Indiana, we kind of get a clean slate every year. Tarspot is always here, it kind of hangs around, but southern rust we really have to track its movement as it moved up from the South.”
Quinn says Kentucky recently confirmed the disease, and then it appeared in Indiana.
“The thing with southern rust is, one, make sure it is southern rust. So we do have a lot of leaf rust out there. It’s always good to, you know, a lot of universities have diagnostic labs that can confirm that for you. So, first make that it is truly southern rust, but also be mindful of managing it. When it comes in this early, it can impact that crop pretty significantly. So a lot those areas are looking at a fungicide management for that disease.”
He says in his experience, southern rust can take 10% to 20% off yield.
“It’s like tar spot tar spot where I’ve seen 40% to 50% yield reductions. So any of those diseases that impact that green leaf area of that plant and really start to move into the ear, leaf and into the upper canopy, can have pretty severe impacts on yield.”
Quinn’s advice for farmers is to pay attention to where it’s moving. Farmers can track the disease here.
He says farmers need to pay attention to that map daily, which is tracked county by county, to see where the disease has popped up, but also be mindful of weather conditions.
Other advice for farmers includes:
1. Scout fields thoroughly and frequently
- Southern rust can spread rapidly under favorable conditions (high temperatures and humidity), so early detection is critical
- Inspect fields often, especially those that are late-planted or in low-lying areas, where humidity and moisture tend to linger
- Focus on the upper leaves, looking for small, circular, light orange to cinnamon-colored pustules, primarily on the upper surface
- Consult resources like the Corn ipmPIPE tool to monitor the disease’s progression in the region
2. Differentiate southern rust from common rust
- While symptoms can be similar, common rust is generally less aggressive and causes less yield loss
- Southern rust pustules are typically circular or oval, light cinnamon to orange, and primarily found on the upper leaf surfaces, while common rust pustules are more elongated, dark brown and can be found on both upper and lower leaf surfaces
- The spores of southern rust will easily rub off on your fingers, unlike tar spot, which is a different disease that can initially look similar to southern rust
3. Consider fungicide application
- If Southern rust is detected on multiple plants during the early dent stage or earlier, and hot, humid conditions are expected, a fungicide application might be warranted
- Fungicide applications are most effective when applied at the onset of the disease and can help prevent further spread
- The optimal timing for fungicide application is generally between the R1 (silking) and R3 (milk) growth stages of corn development
- If conditions remain favorable for disease development, a second fungicide application might be necessary, especially for late-planted fields
4. Monitor stalk quality and consider early harvest
- Southern rust can weaken stalks and increase susceptibility to stalk rot, potentially leading to lodging and harvest losses
- As corn maturity progresses, regularly check stalks for signs of deterioration
