On a crisp morning in Citrusdal, trucks brimming with oranges idled under the blazing South African sun, farmers watched helplessly as port authorities halted shipments to the United States. A new 30 percent tariff, one of the highest imposed by the Trump administration, had shut off access to one of South Africa’s largest export markets. For families whose livelihoods depend on agriculture, the shock was immediate, putting an estimated 30,000 jobs at risk.