Diesel fuel prices reached a record national average of $6.05 a gallon on Friday, according to AAA data, just as many U.S. farmers started harvesting corn and soybeans. The surge comes amid ongoing supply disruptions tied to the six-month war with Iran.

The price marks a sharp increase from $5.85 a gallon last week and $3.70 a year ago. On Sept. 4, the average first topped $5.85 before climbing further this week.

Farmers in key Midwest states report the higher costs are cutting into already thin margins. Jason Kurtz, a corn and soybean grower near Forest City, Missouri, said his diesel expenses have doubled compared to last year. His combine uses about 200 gallons daily during a roughly 30-day harvest, and he also needs fuel for tractors and trucks to move grain.

"We have to harvest. We have to run the machines. We have to use the diesel, so it cuts into our bottom line," Kurtz said. He noted his budget was already tight after paying more for fertilizer and chemicals earlier in the year.

Iowa farmer Mark Mueller, president of the Iowa Corn Growers Association, faces similar pressure. He said filling his 5,000-gallon tank at over $5 a gallon would cost $25,000, nearly double last year's expense. Mueller indicated some farmers may buy smaller amounts to manage cash flow during harvest.

The spike affects more than field operations. Higher diesel prices raise costs for hauling grain from fields to storage and then to market, according to Paul Mitchell, a professor of agricultural and applied economics at the University of Wisconsin-Madison.

A congressional analysis found that elevated diesel prices increased planting costs for major crops by more than $1.4 billion nationwide in 2026 compared to the prior year, a 63 percent jump. Iowa ranked among the states with the largest increases.

Farmers have limited ability to pass along these costs, as they sell into commodity markets where prices are set globally. Some operations are considering adjustments such as fewer field passes or volume discounts on fuel purchases.

The elevated fuel expenses add to broader challenges this year, including higher prices for fertilizer, seeds, and equipment. Transportation costs for grain and other goods could also rise, with potential effects on supply chains.

AAA data shows diesel prices have risen about 60 percent since late February, when the national average stood near $3.76 a gallon before the conflict with Iran intensified supply pressures.