Southern Rust Spreads to 39 Iowa Counties Amid Wet July Weather

According to the latest Crop Protection Network map, positive cases of southern rust have been reported in 39 counties so far this growing season.

This season, southern rust has also been confirmed in neighboring South Dakota, Nebraska, Missouri, and Illinois. Kansas, Indiana, Kentucky, Tennessee, Oklahoma, Arkansas, Texas, Louisiana, Alabama, Georgia, and Florida have reported positive cases in 2025 as well.

Weather

Unusually wet July weather has created the right conditions for disease. “Warm (80s [degrees Fahrenheit]), wet (high humidity and/or frequent precipitation/irrigation) conditions favor southern rust development,” wrote Alison Robertson, Iowa State University professor of plant pathology in a mid-July blog post.

“The fungus that causes southern rust can infect a plant after approximately six hours of leaf wetness. Dew usually provides enough moisture to cause infection, but frequent rainfall can promote severe disease development,” explained a disease overview published by the Crop Protection Network.

“Last week was hot and muggy as a heat dome settled over the Midwest, producing some sweltering heat and humidity,” said Iowa Secretary of Agriculture Mike Naig in a press statement. “Forecasts show chances for a few more thunderstorms this week, followed by a welcome break from the hot weather. With a few days to go in the month, preliminary estimates indicate it could rank among the top 10 wettest Julys on record.”

Positive 2025 Cases

Wyffels Agronomist Eric Wilson said “heavy rains are starting to take a toll on corn and beans both” across southeast Iowa. “Southern rust is showing up very sporadically across my southern areas,” Wilson said. “We are continuing to monitor this, but are hopeful that as temperatures cool in the coming week disease progress will slow.”

The following Iowa counties have reported positive cases this year:

• Dickenson County
• Kossuth County
• Clay County
• Hancock County
• Cerro Gordo County
• Plymouth County
• Cherokee County
• Buena Vista County
• Wright County
• Franklin County
• Woodbury County
• Sac County
• Webster County
• Hamilton County
• Boone County
• Story County
• Marshall County
• Tama County
• Harrison County
• Guthrie County
• Polk County
• Jasper County
• Poweshick County
• Johnson County
• Cedar County
• Muscatine County
• Louisa County
• Keokuk County
• Mahaska County
• Madison County
• Adair County
• Cass County
• Pottawattamie County
• Montgomery County
• Fremont County
• Page County
• Decatur County
• Wayne County
• Appanoose County

What Should Farmers Do

Angie Rieck-Hinz is an Extension field agronomist for Iowa State covering north central Iowa. “Rain concerns and fungicide decisions continue to dominate the discussion,” in her area, she wrote.

“Across all the fields I was in last week, the disease pressure is still low and everything was below the ear leaf. I think we all have concerns about southern rust of corn, but it was detected several weeks ago, and with the exception of susceptible hybrids, does not appear to have done much,” Reick-Hinz explained.

“I realize your situation may be different and that is why I am continuing to encourage you to scout,” she added. Her peers from across the state echoed the recommendation.

According to the Crop Protection Network, there is likely a benefit from spraying up until R3, or the milk growth stage. There could be benefit to spraying at R4, or the dough growth stage, in cases of severe disease pressure.

Iowa Corn Progress and Condition

USDA published the latest Crop Progress report July 28.

According to the report, 84% of Iowa’s corn had reached the silking stage as of July 27. This is ahead of the five-year average and last year at this time by 1 percentage point.

As of July 27, 34% of the state’s corn crop had reached the dough stage. This is ahead of the five-year average by 8 percentage points and two points ahead of last year’s progress.

Wilson noted some signs of nitrogen deficiency in areas that have received heavy rainfall, “which is concerning as we move into grain fill stages,” he said.

On the western side of the state, Crawford County farmer Kelly Garrett said, “The corn crop is the best I’ve seen.”

USDA rated Iowa corn 1% very poor, 2% poor, 10% fair, 59% good, and 28% excellent.

That is better than the national average rating of 2% very poor, 5% poor, 20% fair, 53% good, and 20% excellent.