At Least Half of Iowa Counties Report Southern Rust Infections

According to the latest Crop Protection Network (CPN) map, positive cases of southern rust have been reported in 49 Iowa counties this growing season.

Southern rust has been confirmed in neighboring Illinois, Minnesota, Missouri, Nebraska, South Dakota, and Wisconsin. Cases also have been reported in Alabama, Arkansas, Florida, Georgia, Indiana, Kansas, Kentucky, Louisiana, Michigan, Mississippi, Ohio, Oklahoma, Tennessee, and Texas.

Weather

Across Iowa, July’s unusually wet weather has continued into the first half of August. During the first full week of the month, localized flooding challenged parts of the state, especially eastern Iowa, state Secretary of Agriculture Mike Naig reported in a press release.

The USDA’s Crop Progress report published Aug. 11 rated Iowa topsoil moisture 0% very short, 2% short, 68% adequate, and 30% surplus. Subsoil moisture was rated 0% very short, 3% short, 71% adequate, and 26% surplus.

The drought monitor map published Aug. 14 indicated no drought or abnormal dryness in the state.

In light of the extreme wetness, Iowa State University experts Alison Robertson and Rebecca Vittetoe said they were not surprised by the continued development of southern rust across the state. “Southern rust thrives in warm, wet conditions. At least 6 hours of leaf wetness is required for infection to occur,” Robertson and Vittetoe wrote in a recent Integrated Crop Management article. “Consequently,” they added, “southern rust epidemics can develop very quickly.” When fall’s cooler days arrive, southern rust development should slow, they said.

Positive 2025 Cases

Positive cases of southern rust have been documented in at least 49 of Iowa’s 99 counties. That’s a sharp increase from 39 counties at the end of July.

A regional crop update from Iowa State Extension agronomists published Aug. 12 noted southern rust in corn was one of the “biggest concerns coming in from fields across the state…”

Agronomist Josh Michel serves Allamakee, Buchanan, Clayton, Delaware, Dubuque, Fayette, Jones, Linn, and Winneshiek counties. Michel reported, “Minor reports of tar spot and southern rust are starting to show up in some isolated areas.”

Meaghan Anderson is the Extension agronomist for Boone, Dallas, Jasper, Madison, Marshall, Polk, Story, Tama, and Warren counties. In her update, Anderson advised farmers to keep an eye on disease when headed to the field to do yield checks. “During that time, I recommend evaluating the disease pressure in the canopy to compare hybrids, spray programs, and other factors that may be causing differences,” she wrote.

Vittetoe confirmed buzz around southern rust in her territory: Jefferson, Keokuk, Poweshiek, Washington, and Johnson counties. “In general, it is more prevalent in the lower canopy, especially in fields that have been sprayed with a fungicide,” she wrote, adding: “However, depending upon if a fungicide was sprayed, when it was sprayed, and hybrid susceptibility, I am seeing it move up in the canopy.”

Agronomist Logan McCrea has fielded questions about later fungicide applications as southern rust spreads across his territory, which includes Appanoose, Davis, Lucas, Mahaska, Marion, Monroe, Van Buren, Wapello, and Wayne counties. He points farmers to the CPN for research-based recommendations.

What Farmers Should Do

An overview of the disease published by the CPN explained, “Fungicides are effective at protecting uninfected leaf tissue from southern rust; however, there currently is no economic threshold for fungicide applications.”

The table below outlines possible benefits, by growth stage, from applying fungicides to protect corn against southern rust.

Iowa Corn Progress and Condition

The USDA published the latest corn progress and condition numbers Aug. 11. According to the report, 96% of Iowa’s corn had reached the silking stage as of Aug. 10. This is slightly behind the five-year average of 97%. This time last year, 95% of Iowa corn had reached the silking stage.

As of Aug. 10, 68% of the state’s corn crop had reached the dough stage. This is ahead of the five-year average by 1 percentage point and last year’s pace by 2 percentage points.

The USDA noted 15% of Iowa corn had reached the dent growth stage by Aug. 10. This is equal to the five-year average and 1 percentage point behind this time last year.

Iowa corn was rated 1% very poor, 2% poor, 11% fair, 57% good, and 29% excellent.

This is better than the national average corn condition rating of 2% very poor, 5% poor, 21% fair, 52% good, and 20% excellent.