Bayer’s stock slumped about 10 percent in midday trading Monday, three days after a California jury awarded $289 million to a former groundskeeper who said the popular weedkiller Roundup gave him terminal cancer.

The stock drop sent a cautionary signal to the company that acquired Monsanto, the maker of the weedkiller, in June for $63 billion. The merger created the world’s largest seed and agrochemical company, marrying Monsanto’s dominance in genetically modified crops with Bayer’s pesticide business. Bayer’s portfolio also includes pharmaceuticals with household brands such as Aleve to Alka-Seltzer.

The verdict poses a new challenge for Bayer in its quest to combat contempt swirling around Monsanto by consumer, health and environmental advocates.

Monsanto’s reputational problems are now Bayer’s problems, said Anthony Johndrow, a corporate reputation adviser. Lawsuits against Monsanto are nothing new, Johndrow said, adding that Bayer risks souring sales of its other products because of the public perceptions of Monsanto.

“Any stakeholder is going to be asking right now, ‘why would they buy Monsanto with this stuff hanging over its head?'” Johndrow said. “I think that’s a question they have to answer, and they have to answer it sooner than they planned to.”

The groundskeeper in Friday’s verdict, Dewayne Johnson, was the first to have his case go to trial after doctors pushed for it because he was close to death. Johnson’s attorney said that his client used Roundup 20 to 30 times a year between mid-2012 and early 2016 while working for a school district outside San Francisco.

He was diagnosed with non-Hodgkin’s lymphoma in 2014. Timothy Litzenburg, one of Johnson’s attorneys, told The Post that Johnson contacted Monsanto after the diagnosis to say that he had been exposed to Roundup and wanted to know if there was any correlation between the weedkiller and cancer. He was told there were no ties to Roundup and his cancer, and he continued to use the herbicide. Now, lesions cover up to 80 percent of his body.