If you have cornfields infested with southern rust, prepare for a down-and-dirty harvest. Literally.
“I can see it now in some of the leaves,” says Alison Robertson, Iowa State University Extension plant pathologist. “The pathogen will stop producing orange spores (a telltale sign of southern rust) and start producing black spores. The pustules can look very similar to tar spot. You’re going to drive into your field, and instead of coming out orange looking like Cheetos [such as during summer infestations], your combine will come out a bit black from all the spores.”
It’s important, though, to harvest infested fields first.
“If you have a field with severe southern rust, you absolutely have to schedule an earlier harvest,” Robertson says. “Standability will be horrible.”
Harvested corn also may be wetter. “You may have to think about incurring some more drying costs,” Robertson says.
Fungicides and hybrids
Left unchecked, southern rust can clip yields by 10% to 30%, Robertson says.
Preliminary results in 2025 ISU tests suggest the best fungicide treatment to reduce southern rust is a double shot of Quilt Xcel, or a generic mix of its active ingredients, azoxystrobin and propiconazole. In trials, ISU researchers applied the first treatment at R1 (silking), followed by a second application three to four weeks later around R3 (milk stage). Fields with this treatment have maintained 95% to 100% green canopy through R6 (black layer) stage, Robertson says.
It’s important to remember, however, that this treatment only concerns southern rust.
“It won’t necessarily be effective against tar spot or northern corn leaf blight or other diseases,” Robertson says. “But in this year, that double shot reduced [southern rust] disease considerably.”
Significant variation in southern rust susceptibility has been observed between corn hybrids. In one ISU trial at Muscatine, one hybrid showed 90% fewer southern rust pustules than adjacent hybrids, Robertson says.
Ear rots
Farmers also should be on the lookout for ear rots, particularly in areas such as northwestern Iowa, Robertson says. Besides yield loss, test weights will be down in affected fields. Thus, Robertson also advises targeting those fields for early harvest. Further, she advises marketing this corn early, as it will not store well.
“Those fields have ears kind of like a 6-year-old’s teeth, where the kernels are kind of wiggly when you run your hands up and down the cob,” Robertson says. “Those kernels just didn't fill as well as they could have.”
Some good news
The good news regarding southern rust is that it’s a clean slate for 2026. That’s because southern rust inoculum does not overwinter in Iowa. Instead, it blows in on southern winds.
If you graze your cattle on cornstalks in the fall, don’t worry about any toxins resulting from southern rust. Whether you graze them or bale stalks, no southern rust toxins are produced that will impact animal health, Robertson says.
However, if stalk rots are present, mycotoxins may result. Thus, it may be worth testing before feeding, she adds.