Cars and steel may be grabbing all the headlines as trade tensions mount, but countries that really want to needle the U.S. are springing retaliatory tariffs on the cranberry.

A quintessentially American export, cranberries are nearly all grown in the U.S. and Canada. It’s a trade duopoly, and most of the cranberry bogs in the U.S. happen to be in House Speaker Paul Ryan’s home state of Wisconsin.

“If...