For some Iowa farmers, corn disease cuts into what could have been record yields

After years of drought, spring this year brought relief to Iowa in the form of regular rainfall.

However, for some farmers, the precipitation — which fell heavy at times — combined with high temperatures, led to other difficulties including plant disease.

Background

Agricultural experts began noticing an increase in crop diseases this summer, after heavy rain in July.

Iowa recorded its second-wettest July in more than 150 years this year. The average rainfall across the state that month was 9.2 inches, more than double the average July total of 4.17 inches.

Southern rust, specifically, was first detected in Iowa in mid-July as the disease began popping up across the state at low levels.

In the following weeks, the disease continued to spread, with 49 of Iowa’s 99 counties reporting at least one positive case of the disease.

Alison Robertson, a professor of plant pathology and microbiology and an extension field crops pathologist with Iowa State University, previously told The Gazette Southern rust was likely in every county in Iowa, but she couldn’t confirm a positive case until someone in that county reports the crops with a photo showing its symptoms to ISU or the Network.

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, corn infected by the disease has pustules that look similar to 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 leaves. They are usually an orange-brown color but turn dark brown or black as the pustules age.

  Buy Photo

What’s happened since

Southern rust was identified in corn in 60 counties across the state, according to the Crop Protection Network, which is a multistate program comprised of extension specialists from various land grant universities. The network produces agricultural information for producers.

  Buy Photo

Aaron Lehman, president of the Iowa Farmers Union, said the disease affected some farmers’ yields this season.

Iowa farmers “were really hoping for some really outstanding yields at the end of July, and then Southern rust issues emerged, and so a lot of farmers are reporting yields significantly lower than what they were anticipating,” Lehman said.

According to a report issued Nov. 14 by the U.S. Department of Agriculture, corn yields this year are expected to increase by 2.4 percent statewide — from 211 bushels per acre in 2024, to about 216 acres per bushel this year.

Lehman said it’s difficult to quantify how much of Iowa’s corn crop was affected by Southern rust because it varied from field to field. He said reliable yield numbers also have been difficult to come by because of the 43-day federal government shutdown, which began in October and lasted into November.

Robertson said estimates from the Crop Protection Network put statewide yield losses due to Southern rust at about 7 percent of Iowa’s 2025 corn crop.

“This is the highest disease loss estimate we have ever had in all the years we have been estimate disease losses, to all diseases, not just Southern rust,” she said.

Megan Cady, who oversees District 6 — which includes Cedar Rapids — for the Iowa Corn Growers Association, said disease was one of the “main drivers” for some Iowa farmers seeing lower yields this year than in 2024.

When “the rain shut off, the disease really set in, and we saw a lot of Southern rust come up,” Cady said. “And that's not typically the case in Iowa.”

Cady said Southern rust was a more acute problem for farmers who did not apply fungicides to their fields this year.

  Buy Photo

“If farmers didn't apply fungicide, I think they're seeing some extra detrimental numbers to their yield, just because the fungicide’s job is to protect the corn from the disease,” Cady said. “If you applied at the right time, or just even applied a pass at all, you might have been protected a little bit more than those who didn't choose to do that this year.”

Cady said Southern rust was a bit more common in Iowa’s southern counties, like Iowa, Johnson, Cedar and Muscatine, but not as common for more central Eastern Iowa counties, like Linn, Johnson and Jones counties.

“Southern rust really just kind of surprised everyone in Iowa,” she said. “It's not typically something that creeps its way all the way up here, but it happened to this year.”

Cady said corn affected by Southern rust still can be harvested and sold.

“It really doesn't affect the fruit of the corn,” she said. “It mostly just affects the plant.”

She said infected corn could have a less firm stalk, or the disease may have caused the stalk to fall over.

Robertson added 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 it would be in corn that is not a hybrid.

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.

Sign up for our curated, weekly environment & outdoors newsletter.

Comments: olivia.cohen@thegazette.com