Southern Rust- The Yield Robber You Can't Ignore

By: Craig, Lead Agronomist at Cornelius Seed

As I travel across Iowa and Illinois, I’m seeing Southern Rust rear its ugly head earlier and further north than usual this season. Having dealt with this disease in southern Illinois, I can tell you it’s a yield robber that can strip 60-70 bushels per acre when it teams up with other diseases like Tar Spot, as we saw in Western Illinois a few years back. Southwestern Iowa growers got a harsh reminder of this last year with heavy Southern Rust pressure. Let me be clear: we cannot let this disease get out of hand.

Why Southern Rust is a Threat

Southern Rust, caused by Puccinia polysora, thrives in the warm, humid conditions we’re experiencing right now. Those orange pustules on the leaves can spread rapidly, covering the plant and reducing photosynthesis, which directly hits your grain fill and yield. The current weather is like a buffet for this disease, allowing it to cycle and spread quickly. If you’re not scouting, you’re playing with fire.

How to Fight Back

• Fungicide Timing is Critical: Apply a long-residual fungicide at the R1 (silking) stage. I’ve reviewed the data, and long-residual fungicides provide very acceptable control for Southern Rust. If you’ve already applied one, you’re likely in good shape, but don’t skip scouting to confirm.
• Choose Resistant Hybrids: For top performers, look at C5527, C7026, and C7235 (rated Excellent) or C5704, C5806, C6523, and C7399 (rated Very Good). These hybrids can help mitigate losses, but they’re not a silver bullet—fungicides are still key.
• Scout and Report: If you spot Southern Rust in your fields, let our team know immediately. Some hybrids in the guide are marked “N/A” (Not Available) for Southern Rust ratings, and your reports can help us update those to better serve you.

A Word of Caution

Southern Rust doesn’t play nice when combined with other diseases like Tar Spot or Gray Leaf Spot, which is already showing up in central Iowa. The yield losses can be catastrophic if you don’t act early. August will bring cooler temps, which could slow Southern Rust but ramp up other diseases, so stay vigilant.

Take Action Now

Don’t wait for Southern Rust to take hold. Scout your fields and reach out to our team for fungicide recommendations tailored to your operation. If you would like more detailed ratings on Southern Rust as it relates to our hybrids contact us at Cornelius Seed. Let’s protect your yields together!