About 80% of the diesel Brazil imported in September came from the United States, according to the daily Folha de S.Paulo. On Wednesday, the news site Politico reported that the White House is finalizing a plan to bar US exports of the fuel for 90 days, and the combination set off alarms in the Brazilian market. Diesel moves the trucks that carry food and supplies in Brazil, so a squeeze on supply tends to show up first in freight costs and then in grocery bills.

In the US, the fuel cost the equivalent of US$ 1.72 a liter on Wednesday ($6.52 a gallon of 3.79 liters), 76% more than a year ago and 91 cents higher than a week earlier, according to figures from the automobile association AAA cited by InfoMoney. It is as if a fill-up that cost R$ 300 (about $58) a year ago came to more than R$ 500 today. The White House wants energy prices down before the midterm elections in November, the vote for Congress in which polls point to a strong win for the Democratic Party.

The plan has critics in the oil industry, in Congress and inside the administration itself. According to Politico, producers warn that short-term relief would be offset by higher prices later, including jet fuel, because refineries would cut output once they lose foreign customers; one source described an "apocalyptic concern" about pump prices inside the White House. Donald Trump is inclined to announce the measure this week, and if it goes through it will be the first restriction on US energy exports since 2015, when then-president Barack Obama ended a decades-old ban on crude oil sales abroad.

Why Brazil is exposed

Brazil refines most of the diesel it burns, but the shortfall comes from abroad, and mostly from the Americans. Last October the country imported 1.6 billion liters of the fuel, according to the ANP, the national oil and fuels regulator; for this October, Petrobras, the state-controlled oil company, has already bought 650 million liters abroad, less than half the volume of a year ago. October is the peak month for consumption, when farms plant the summer crop and the newly harvested corn reaches consumers by truck.

Understand: an export ban does not shut down US refineries; it only changes where the fuel goes. The diesel that would have been shipped to buyers like Brazil stays in the US market, prices there fall and prices elsewhere rise, because importers end up competing for a smaller pool of supply. That is why analysts quoted by the American press say the measure relieves US consumers in the short run and makes fuel more expensive everywhere else, Brazil included.

Companies say they are ready

Petrobras logistics director Angélica Laureano said purchases for October "are already done" and that no large imports are expected in November, when the Repar refinery in the state of Paraná returns from scheduled maintenance. Ernesto Pousada, president of Vibra, Brazil's largest fuel distributor, said the company can raise imports even paying 2 to 3 reais more per liter abroad, a gap that would add about R$ 180 (roughly $35) to a 60-liter tank. In his view, the main effect would be on prices, not on availability: suppliers that traditionally sold to Brazil have already left the market, "and at no point was there a shortage of diesel for our clients".