As Monsanto challenged a $78.5 million damage award to a Bay Area groundskeeper who was stricken with cancer after spraying the company’s herbicide, California Attorney General Xavier Becerra stepped into the case Wednesday, telling a state appeals court that the verdict was validly based on state laws requiring warning labels for cancer-causing chemicals.

The 2018 verdict was the first of three, all in the tens of millions of dollars, in Bay Area trials of suits by users of the glyphosate herbicide, widely sold as Roundup, who were later diagnosed with cancer. A central issue is whether the suits could rely on California law, which classifies glyphosate as a carcinogen, or should have been dismissed under federal law because the U.S. Environmental Protection Agency considers it a safe chemical.

In a filing with the First District Court of Appeal in San Francisco supporting the plaintiff, Dewayne “Lee” Johnson, Becerra’s office argued that the EPA’s statements about glyphosate, and its approval of a product label without cancer warnings, “do not carry the force of law.”

The EPA’s opinion, stated most recently in an announcement last August that did not involve any legal proceedings or regulations, does not prohibit a state from finding that the agency-approved label “is inadequate to protect public health and therefore constitutes misbranding,” state lawyers told the court.

“California protects its residents from dangerous pesticides,” Becerra said in a statement. “We shouldn’t be forced to put our heads in the sand simply because the EPA won’t do its job.”

Monsanto first appealed the verdict in April 2019, arguing that there was no credible evidence its herbicide was dangerous. The court then asked opposing parties about the legal implications of the EPA’s findings, and Monsanto lawyers replied Wednesday that they should require dismissal of Johnson’s suit, or at least a new trial.

Federal law “limits the role of states in pesticide regulation” and prohibits states from requiring labels that differ from federally mandated labels, Monsanto told the appeals court.

Since 1991, under five presidential administrations, the EPA “has repeatedly and consistently determined that no cancer warning is warranted or would be approved on glyphosate-based herbicides,” the company’s lawyers said.

Johnson’s lawyers have contended that Monsanto, now a subsidiary of German pharmaceutical company Bayer AG, unduly influenced EPA decisions and had “ghost-written” reports for the government agency. The lawyers also cited the 2015 assessment by the International Agency for Research on Cancer, an arm of the World Health Organization, that glyphosate was a probable cause of cancer in humans, a conclusion disputed by both Monsanto and the EPA.

Over Monsanto’s objections, the judge in Johnson’s case, Suzanne Bolanos of the San Francisco Superior Court, and judges in the other two cases allowed lawyers for the cancer patients to argue to their juries that the company knew its product was dangerous and should have informed the public.

Johnson, of Vallejo, sprayed a high-concentration glyphosate product called Ranger Pro on school grounds in Benicia, where he worked as a groundskeeper, from 2012 to 2016, even after he was diagnosed with non-Hodgkin’s lymphoma in 2014. The jury found unanimously that Monsanto was responsible for his illness and should have known of the product’s dangers.

The court has indicated it would hear the company’s appeal sometime in the next two months.

The two other verdicts, both last year, were $25.2 million in a federal court suit by a Sonoma County man, Edwin Hardeman, who sprayed Roundup on his property for more than 26 years before being diagnosed with non-Hodgkin’s lymphoma, and $86.7 million in a Superior Court suit by a Livermore couple, Alva and Alberta Pilliod, who used the weed-killer on their property for more than 30 years before their cancer diagnoses. Monsanto is appealing both verdicts.

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter: @BobEgelko