Plaintiff Dewayne Johnson reacts after hearing the verdict in his case against Monsanto at the Superior Court of California in San Francisco on Aug. 10. (Josh Edelson/Pool/AP)

Bayer’s stock slumped more than 10 percent in 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 such household brands 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. For years, the company has drawn sharp criticism and allegations about the health hazards caused by Roundup, and Monsanto faces thousands of lawsuits that assert its product is linked to cancer diagnoses.

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.”

Bayer had previously announced that the Monsanto brand name would be nixed as soon as this month. In a call with reporters in June, president of Bayer’s Crop Science Division Liam Condon said the move to lose Monsanto’s name was part of a wider strategy to win back consumer trust. Separately, Bayer chief executive Werner Baumann said the company would “aim to deepen our dialogue with society” and “listen to our critics.”

At the time, Bayer executives said they couldn’t predict what form this reimagined consumer engagement would take. Bayer did not respond to a request for comment.

"The more important point now, once we change the company name, is that we talk about what the new company will stand for,” Condon said on the June call. “Just changing the name doesn’t do so much — we’ve got to explain to farmers and ultimately to consumers why this new company is important for farming, for agriculture and for food, and how that impacts consumers and the environment.”

Monsanto has long maintained that its products don’t cause cancer, and the company doubled down on its stance after Friday’s verdict. Monsanto’s vice president, Scott Partridge, said in a statement that the verdict “does not change the fact that more than 800 scientific studies and reviews ... support the fact that glyphosate does not cause cancer, and did not cause Mr. Johnson’s cancer.”

Partridge said Monsanto would appeal the decision “and continue to vigorously defend this product.”

In an interview with The Post, Partridge asserted that Johnson’s attorneys made a series of false statements during the trial, including those that tied glyphosate to cancer and compared Monsanto’s behavior to that of tobacco companies.

“I have tremendous faith in our justice system in America at the end of the day to get the right result,” he said. “We started off with the wrong result.”

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.

Litzenburg said his firm is representing 2,000 people in their claims against Monsanto, and that there are probably 5,000 cases across the country.

In 2015, the World Health Organization said that glyphosate -- the herbicide in Roundup widely marketed by Monsanto -- was likely carcinogenic and could cause cancer in humans. For years, environmental groups and consumer advocates have raised alarms about the health risks associated with workers applying the weedkiller on farms. But Monsanto and other agricultural companies say long-standing research upholds glyphosate’s safety.

The herbicide is one of the most commonly used in the country, according to the Environmental Protection Agency. In his statement, Partridge said the EPA, the U.S. National Institutes of Health “and regulatory authorities around the world” have concluded that glyphosate does not cause cancer.

Johndrow said that when evaluating its merger with Monsanto, Bayer probably factored in the costs of future litigation over thousands of Roundup lawsuits. But it remains to be seen, Johndrow said, whether Bayer also calculated the cost of reputational risks, including from whopping jury verdicts.

However it chooses to protect its reputation, Bayer’s response must send a signal that it is concerned for consumer safety and “that they want to, and are trying to do right.”

“What they do will determine whether this will have a really, really bad effect," Johndrow said, “or just a moderately bad effect.”

As for the precedent set by Friday’s verdict, Johnson’s attorneys credited his bravery for helping bring the first trial against Monsanto. The ruling will help take care of Johnson’s wife and young sons after he’s gone. But it won’t keep him alive.

Another one of Johnson’s attorneys, Michael Miller, said that one of Johnson’s sons asked for a chemistry set for Christmas. When Johnson asked why, the young boy said he wanted to invent the chemotherapy that could save his dad.