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Southern corn rust can blow into corn country this time of year like a bundle of bad news. In 2025, the fungus caught many farmers unaware -- stripping the corn of yield potential and hastening harvest.

The good news is farmers are on high alert this year. Outbreaks in southern regions give corn growers to the north a healthy heads-up.

States indicating positive detections as of July 28 include Alabama, Arkansas, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Missouri, Mississippi, Nebraska and Texas. Maps showing the spread can be found here: https://corn.ipmpipe.org/… and https://cropprotectionnetwork.org/….

So far, there have been fewer detections of rust in the south this year, which hopefully means less inoculum to travel north, noted Alison Robertson, Iowa State University plant pathologist, in a recent news release.

However, the need to scout continues to be important. Southern rust is particularly concerning when it arrives early enough to influence grain fill. The pathogen reduces the plant's ability to photosynthesize and can cause premature leaf death when disease pressure is high.

"Wet leaves for 10-plus hours are more important than hot and humid," Robertson told DTN via email. "I expect this past weekend may have favored it somewhat. I guess we just need to wait and see what the next three to four weeks bring. It's important to keep scouting and monitor the situation."

Find the Iowa State University release here: https://crops.extension.iastate.edu/….

Purdue University plant pathologist Darcy Telenko said delayed reports and limited lesions exhibited where the disease has been found bodes well for farmers.

"Our southern counterparts have found it, but most of the corn there is being harvested," Telenko reported. "We have very little tar spot as well, but gray leaf spot is active if they planted a susceptible hybrid. Find a recent disease update from Purdue here: https://extension.entm.purdue.edu/….

In the presence of disease, timely fungicides with multiple modes of action applied between VT and R3 growth stages are recommended to reduce disease severity and protect yield, according to a University of Illinois release: https://farmdoc.illinois.edu/….

Fungicide treatments won't help protect against southern rust beyond the R5 (dent stage), Robertson added. She also recommended targeting hybrids known to be susceptible based on 2025 information.

"Hybrids tolerant of southern rust or that exhibited low incidence in 2025 likely will not need a fungicide," she added.

For information on the efficacy of specific fungicides against southern and other foliar diseases, farmers can use a table available through the Crop Protection Network: https://cropprotectionnetwork.org/….

It is important to distinguish between southern rust and common rust when making treatment decisions. Common rust is typically mild and rarely causes economic damage, according to university sources. It also favors cooler temperatures and shows up earlier in the season. Common rust pustules are brick red and more elongated in appearance.

Southern rust pustules are more round and orange brown in color. The disease favors warmer temperatures and arrives later in the season. It can be aggressive and fast spreading.

A video from Beck's Hybrids can help distinguish southern rust and common rust: https://youtu.be/….

Pamela Smith can be reached at pamela.smith@dtn.com

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