Southern Rust Cases Rising in Minnesota, Yield Loss Possible

According to the latest Crop Protection Network map, positive cases of southern rust have been reported in 5 Minnesota counties so far this growing season. 

Reports started coming in on July 29, 2025 — just over two weeks ago.

Complicating things is the fact that states as far north as Minnesota have less experience with the disease compared to other Corn Belt states, according to Syngenta agronomist Tim Dahl, because “southern rust does not overwinter on residue.”

Weather

Minnesota weather has been just right for the disease to thrive. 

“The weather, again, was warm, with some areas getting an abundance of rain across much of Minnesota,” Dahl said, referring to the week ending Aug. 10.

“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,” according to a disease overview published by the Crop Protection Network.

Positive 2025 Cases

“Ample moisture and warm weather are moving the corn crop right along,” Dahl said. But the same conditions that are pushing the crop along are favoring disease. 

“The warm humid weather has been favorable for southern rust and a few other leaf diseases,” Dahl shared. “These pathogens could take the top off of yields if crops have not been protected with a fungicide,” he said. 

The following Minnesota counties have reported positive cases this year:

• Lyon County (first reported July 29)
• Murray County (first reported July 29)
• Olmsted County (first reported Aug. 4)
• Wabasha County (first reported Aug. 9)
• Fillmore County (first reported Aug. 11)

There is reason to be concerned about the disease spreading further, but Dahl is more concerned about something else. 

“I am not as concerned about southern rust spreading as I am about the yield impact in fields that are already affected by this disease,” he said. “As we get later in the season, there is a shorter period of time before maturity, so the impact will be less significant. But, if fields are already affected and fungicide treatments have not been made, yield impacts could be very significant.”

What Should Farmers Do

Dahl said that even though we’re past the point for typical fungicide applications in corn, he thinks the crop can still benefit from an application, especially in fields affected by multiple pathogens including northern corn leaf blight and tar spot. “Those fields could really benefit from an application like Miravis Neo,” he said.

Timeliness is critical, according to Dahl. “We need to make those treatments as soon as possible,” he said.

Minnesota Corn Progress and Condition

USDA published the latest Crop Progress report Aug. 11.

According to the report, 95% of Minnesota’s corn had reached the silking stage as of Aug. 10. This is just shy of the five-year average but ahead of last year at this time by 5 percentage points.

As of Aug. 10, 42% of the state’s corn crop had reached the dough stage. This is behind the five-year average by 11 percentage points, and it’s four points behind last year’s progress.

Five percent of Minnesota’s corn had reached the dent stage by Aug. 10, on track with the five-year average and ahead of progress a year ago.

“Corn is progressing at a rapid pace,” Dahl said. “Most corn in Minnesota is at either the R3 (milk stage) or R4 (dough stage),” he shared.

USDA rated Minnesota corn 1% very poor, 5% poor, 19% fair, 51% good, and 24% excellent.

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