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Fungal disease becoming more widespread in Iowa’s growing corn
Southern rust is spreading and growing in intensity due to several factors, but the wetter than average summer has helped infect more crops
Aug. 21, 2025 5:30 am, Updated: Aug. 25, 2025 12:16 pm
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A fungal disease is spreading and growing in severity in Iowa, affecting the state’s corn production.
Southern rust was first detected in Iowa in mid-July as the disease began popping up across the state at low levels.
Since then, Alison Robertson, professor in the Department of Plant Pathology, Entomology and Microbiology at Iowa State University, said the disease has continued to increase statewide.
One of the key reasons the fungus has spread, Robertson said, is wet conditions throughout Iowa this summer.
“The frequent precipitation we have had across the state has favored further development of the disease,” Robertson said. “Had we turned hot and dry like we did the past couple of years, we probably would not have seen the disease increase and spread like it has.”
Iowa saw about 9.2 inches of rainfall in July, five inches above normal. This past July was ranked as the second wettest July out of the state’s 153 years of records.
The most recent data shows 49 of Iowa’s 99 counties have confirmed cases of Southern run. Robertson said the fungus likely is in every Iowa county, but ISU can’t confirm a positive case unless it is reported by someone in that county with a photo showing the symptoms.
Southern Rust is caused by the fungus Puccinia polysora. It is generally considered a tropical disease, but it can impact corn production throughout the United States and Canada.
According to the Crop Protection Network, a multistate collaboration of university extension specialists, corn infected by the disease has pustules that look similar to typical rust on metal.
The pustules tend to be smaller and occur on the top of the leaves. The pustules are circular and densely scattered across the leaf. They are usually an orange-brown color but turn dark brown or black as the pustules age.
In addition to moist conditions, Robertson said there are other factors that may be causing more corn to become infected.
She said that some corn hybrids are more susceptible to Southern rust than others and that if infected, hybrids will develop the disease quicker and it will be more severe than corn crops that aren’t hybrids.
“Unfortunately, companies do not publish this information, so farmers are unable to determine if their hybrid is resistant or not,” Robertson said.
Mitigation
Farmers can use fungicides to protect against Southern rust, but Robertson said it may be too late in the season for those products to be effective.
Corn plants go through two phases of development: vegetative and reproductive. The reproductive phase has six stages.
- Silk stage: One or more silks — the long, hairlike strands that emerge from the developing corn ears — extend outside of the husk leaves
- Blister stage: The corn kernels resemble “blisters” with clear liquid
- Milk stage: The corn’s kernels fill with “milky” fluid
- Dough stage: The inside the kernels have a “doughy” consistency
- Dent stage: Dents form on the kernels and the milk line progresses toward the kernel tip
- Physiological maturity: Kernels are at maximum dry matter accumulation
Robertson said most of the corn that is currently growing in Iowa is between the dough and the dent stages.
Pointing to an interactive map from the Crop Protection Network that she contributes to, Robertson said the use of fungicides is not recommended for producers to use when the corn enters the later stages of development, starting at the dough phase.
According to the Network’s map, spraying fungicides on the crop during the dough phase could only be beneficial if the disease is severe. Fungicide sprays would have an unlikely benefit to the crop during the dent stage and would serve no benefit when the corn is in the final reproductive phase when the plant reaches full maturity.
Effect on yields
Rebecca Vittetoe, a field agronomist with ISU’s Extension and Outreach, said that although experts are seeing more Southern rust in the state, the impact the disease will have on this year’s yields won’t be known until harvest this fall.
The infected corn “can still be harvested and sold,” Vittetoe said. “The biggest concern with Southern rust is the potential impact it could have on the yield — which it’s too early to know exactly what that impact is — for farmers.
A crop production report published by the U.S. Department of Agriculture this month predicts corn production across the country this year will total 16.7 billion bushels, an increase of 13 percent over last year. It would be the largest U.S. annual grain production on record.
The projected increase is due to an increase in the number of acres planted in corn — up 7 percent over last year — and a projected increase in the number of bushels per acre — 188.8 this year, compared with 179.3 last year.
Vittetoe said that corn with Southern rust does not contain any mycotoxins — toxins produced by certain types of fungi or molds — and there are no other safety concerns regarding the grain itself.
Olivia Cohen covers energy and environment for The Gazette and is a corps member with Report for America, a national service program that places journalists in local newsrooms to report on under-covered issues. She is also a contributing writer for the Ag and Water Desk, an independent journalism collaborative focusing on the Mississippi River Basin.
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Comments: olivia.cohen@thegazette.com
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