China didn’t follow proper procedures when it imposed trade restrictions on agricultural imports, the World Trade Organization said on April 18 in a ruling that bolsters President Donald Trump’s dispute against Beijing.

China used a flawed and opaque approach when it administered tariff-rate quotas for rice, wheat, and corn, the Geneva-based arbiter of global trade disputes said on its website. It’s tariff-rate quota “administration contains legal flaws from the beginning through to the completion of the process,” the text of the ruling said.

The decision could increase U.S. farmers’ access to China at a time when they are struggling...