Bayer Takes the Hit After Monsanto Loses Roundup Cancer Trial

Two months after clinching its $66 billion purchase of Monsanto Co., Bayer AG faces a protracted legal battle over the U.S. company’s Roundup weed killer—a prospect that wiped more than $11 billion off the German conglomerate’s market value.

Bayer shares plunged the most in almost seven years after Monsanto was socked with $289 million in damages in the first trial over claims that the herbicide causes cancer. Now a deal Bayer pursued to keep pace with DowDuPont Inc. and China National Chemical Corp. is turning into a potentially expensive quagmire.

The verdict in favor of a California ...