Southern rust, a destructive fungal disease in corn, has now been confirmed in 39 Iowa counties, according to the latest data from the Crop Protection Network. The outbreak coincides with exceptionally wet and humid July weather, which has created near-perfect conditions for the disease to thrive.

Southern rust cases have also been reported in South Dakota, Nebraska, Missouri, Illinois, and across much of the southern U.S., including Kansas, Texas, Georgia, and Florida, signaling a widespread threat to corn producers nationwide.

Warm, Wet Weather Driving Disease Risk

"Warm temperatures in the 80s and sustained moisture from rainfall or irrigation are prime conditions for southern rust," wrote Dr. Alison Robertson, plant pathology professor at Iowa State University. The fungus can infect plants after just six hours of leaf wetness, with dew alone sufficient to trigger infection. Continuous rainfall only worsens disease development.

Iowa Secretary of Agriculture Mike Naig noted that July 2025 could go down as one of the ten wettest on record, due to persistent storms and a heat dome that brought "sweltering heat and humidity" to the region. More storms are expected before month's end.

Corn Outlook Remains Strong, But Concerns Mount

Despite the disease risk, Iowa's corn condition remains robust. The USDA's July 28 Crop Progress report rated 28% of Iowa corn as excellent, with another 59% rated good-outperforming the national average. However, agronomists warn that continued rainfall may exacerbate nitrogen deficiency and increase disease pressure during key grain-fill stages.

Agronomic Insight: Localized Impact, Continued Scouting

Eric Wilson, an agronomist with Wyffels Hybrids, reported sporadic southern rust activity in southeast Iowa, with concerns about rain stress on both corn and soybean crops. "We are hopeful that cooler temperatures next week may help slow disease progress," he said.

Extension agronomist Angie Rieck-Hinz emphasized the importance of scouting. "Most fields I visited last week had low disease pressure, with symptoms below the ear leaf. But southern rust was detected weeks ago, and in susceptible hybrids, damage could escalate quickly," she warned.

What Farmers Should Do: Fungicide Guidance

Experts recommend considering fungicide applications through the R3 (milk) stage, and potentially into R4 (dough) under severe disease pressure. Decisions should be based on hybrid susceptibility, rainfall patterns, and disease scouting.

Iowa Corn Development Ahead of Schedule

According to USDA, as of July 27, 84% of Iowa's corn had reached the silking stage, with 34% in the dough stage-ahead of both last year and the five-year average. This progress, coupled with strong early ratings, makes protecting yield potential more critical than ever.

Farmers like Kelly Garrett in Crawford County remain optimistic: "The corn crop is the best I've seen." Still, others point to signs of nitrogen loss in waterlogged soils as a warning.

Counties Reporting Positive Southern Rust Cases: Dickenson, Kossuth, Clay, Hancock, Cerro Gordo, Plymouth, Cherokee, Buena Vista, Wright, Franklin, Woodbury, Sac, Webster, Hamilton, Boone, Story, Marshall, Tama, Harrison, Guthrie, Polk, Jasper, Poweshiek, Johnson, Cedar, Muscatine, Louisa, Keokuk, Mahaska, Madison, Adair, Cass, Pottawattamie, Montgomery, Fremont, Page, Decatur, Wayne, Appanoose.

With disease conditions aligning perfectly, agronomists stress the importance of scouting, hybrid assessment, and timely fungicide action to safeguard yield. Southern rust isn't widespread in severity yet-but with nearly half the state affected, vigilance will be essential into August.