The USDA projected in August a record yield of 188.8 bushels per acre for the 2025 corn crop. Yet, analysts now expect a downward revision, estimating closer to 186.2 bushels per acre. Even if realized, this would still be a record, but the rise of fungal outbreaks could make harvesting far more complicated.

The primary culprit is southern rust, a disease that typically arrives too late in the season to cause major damage. This year, however, it reached Iowa by mid-July, fueled by the state's second-wettest July on record, creating an ideal breeding ground for fungal spread.

"I've never seen disease as severe as I've seen this year," said Alison Robertson, plant pathologist at Iowa State University.

In addition to southern rust, tar spot has also been widely detected across the Midwest. Both diseases attack corn leaves, reducing photosynthesis and resulting in smaller kernels. Yield losses can reach up to 45% under severe conditions, according to the Crop Protection Network.

"We kind of had the perfect storm for rust," Robertson added.

Farmers applied fungicides to slow the spread, but the measures are costly. With fertilizer, seed, and energy expenses already climbing, growers worry they may not break even.

In southwest Iowa, farmer Roger Cerven reported heavy damage despite spraying:

"The fungicide was a Band-Aid, and we needed a tourniquet," he said, estimating losses of 30 bushels per acre.

Some growers have begun early harvesting, as infected plants stopped growing and dried prematurely. Others face additional challenges: diseased stalks hollow out and collapse, making mechanical harvests resemble "driving through a pile of Pick-Up Sticks," Robertson noted.

The final impact on yields will only become clear as the harvest progresses. But with southern rust and tar spot hitting earlier and harder than usual, the U.S. Corn Belt may deliver smaller returns than expected, complicating balance sheets for farmers and influencing commodity markets worldwide.