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 school groundskeeper who said exposure to Roundup caused his non-Hodgkin’s lymphoma — which Bayer denies — is just one of thousands of cases related to the weed killer that are working their way through the courts. If more go against the German company, the costs could be “ruinous,” Sanford C. Bernstein analysts led by Jonas Oxgaard said in a note.