America's largest agriculture companies are warning that spiking fuel and fertilizer costs due to the war in Iran are hampering South American farmers' crop-planting plans.

Executives at grain giant Bunge and seed-and-pesticide seller Corteva this week said rising costs and limited access to credit is expected to flatline production of Brazil's second-corn crop. The crop, known as safrinha, represents about 80% of Brazil's total corn production, which historically has grown at roughly 6% annually, according to analysts.

Corteva executives say a flat second-corn crop has slowed orders for its products compared with last year, pressuring margins. However, it said its orders are outpacing the rest of the market.

The company's stock was down about 10% on Friday.

Bunge CEO Greg Heckman said rising costs also could hurt crop production in Argentina, the world's fourth-largest corn producer.

"You'd want to watch yields closely there," he said this week on a call.