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Agronomist says southern rust created unprecedented challenges for 2025 corn crop

An agronomy expert says disease pressure was the biggest growing season issue for corn farmers in 2025.

“Southern rust was a beast this year. Something we have never seen,” said BASF’s Mark Storr on Wednesday.

Storr says the severity of southern rust was unexpected.

“Southern rust decimated the plants so that tar spot couldn’t continue to grow. It actually kind of shut it down,” he said. “It was incredible the amount of response we saw to fungicides in multiple trials. Fifty bushels per acre was not unheard of.”

According to ISU Extension, southern rust thrives in warm (77-82°F), wet conditions. At least six hours of leaf wetness is required for infection to occur. Under these conditions, the time between a spore infecting a leaf to the production of a pustule filled with new spores is 7 to 10 days.

Storr tells Brownfield he expects it to be hit or miss in 2026.

“It’s a visitor, and that one blew up from the South,” he said. “We are at the mercy of what happens in the Gulf of Mexico. South Texas, it overwinters on crops down there and of course they have an early corn crop down there.”

He spoke to Brownfield at the 2025 Integrated Crop Management Conference hosted by Iowa State University.