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...