A jury in the first federal court trial of thousands of lawsuits by cancer victims against the manufacturer of the world’s most widely used herbicide found Tuesday that Monsanto’s Roundup was a likely cause of a Sonoma County man’s non-Hodgkin’s lymphoma.

The six jurors deliberated for nearly four days after a two-week trial in San Francisco before agreeing unanimously that Edwin Hardeman, 70, had proven the herbicide was probably a “substantial factor” in causing the cancer with which he was diagnosed in 2015. Hardemen said he’d sprayed Roundup on his property for decades.

The jury returns Wednesday to hear evidence on whether Monsanto is legally responsible for Hardeman’s cancer and, if so, determine the amount of damages he should receive. The case is one of three “bellwether” trials scheduled before U.S. District Judge Vince Chhabria and could establish guidelines for settlements of about 760 cases against Monsanto from around the nation that have been transferred to his court, and more than 10,000 pending elsewhere.

The first trial against the company took place in state court last summer in San Francisco, where a Superior Court jury awarded $289 million in damages to Dewayne “Lee” Johnson, 46, who was diagnosed with non-Hodgkin’s lymphoma after years of spraying school grounds in Benicia with a concentrated brand of the same herbicide. A judge later reduced Johnson’s damages to $78.5 million. Monsanto is appealing.

Unlike Johnson, whose doctors have described his illness as terminal, Hardeman’s cancer is in remission. He testified that he sprayed Roundup for nearly 30 years to kill poison oak on his 56-acre tract in Forestville, often feeling the liquid on his hands or inhaling it.

The herbicide’s active ingredient, glyphosate, was classified in 2015 as a probable cause of cancer in humans by the International Agency for Research on Cancer, an arm of the World Health Organization. California’s Office of Environmental Health Hazard Assessment lists it as a chemical known to cause cancer.

But the U.S. Environmental Protection Agency and regulatory agencies in other nations have found glyphosate to be a safe product, and it remains legal in both the United States and Europe. Monsanto, now a subsidiary of the German pharmaceutical giant Bayer AG, held the initial patent and remains its leading distributor.

The verdict brought an audible sigh of relief from Hardeman, who hugged his lawyers and declined to speak to reporters.

In pretrial rulings, Chhabria described evidence linking Roundup to cancer as “shaky” but decided there was enough to bring the case to a jury. He rejected Monsanto’s motions for dismissal. Federal law, he said, “allows states to regulate or ban pesticides that have been federally approved.”

Chhabria granted the company’s request, however, to divide the trial into phases, requiring jurors to consider first whether Roundup was a likely cause of Hardeman’s illness without hearing evidence of Monsanto’s conduct in marketing the product, a central factor in determining the company’s liability for damages. That will come in the next phase.

Lawyers for Hardeman and other plaintiffs contend the company hid evidence of the herbicide’s danger from its users and “ghost-wrote” some of the purported favorable research findings.

“Now we can focus on the evidence that Monsanto has not taken a responsible, objective approach to the safety of Roundup,” Hardeman’s lawyers, Aimee Wagstaff and Jennifer Moore, said in a statement after the verdict.

“Instead, it is clear from Monsanto’s actions that it does not particularly care whether its product is in fact giving people cancer, focusing instead on manipulating public opinion and undermining anyone who raises genuine and legitimate concerns about the issue.”

Environmental groups chimed in.

“It is also a resounding verdict against our pro-industry EPA regulators, who have failed miserably in their duty to protect us all from the known health risks of dangerous pesticides,” said Nathan Donley, a scientist at the Center for Biological Diversity.

Bayer, Monsanto’s parent company, said it was disappointed by the verdict but still maintains that “the science confirms glyphosate-based herbicides do not cause cancer” and that it was not responsible for Hardeman’s illness.

Roundup and glyphosate “have been used safely and successfully for over four decades worldwide and are a valuable tool to help farmers deliver crops to markets and practice sustainable farming by reducing soil tillage, soil erosion and carbon emissions,” Bayer said.

During the trial, Monsanto lawyers argued that there was no provable cause of Hardeman’s illness, but that the most likely factor was hepatitis C, from which he suffered for nearly three decades.

His lawyers countered that Hardeman had been free of hepatitis since 2004, but an oncologist testifying for Monsanto said the illness had lasting effects on the genetic system — a “hit-and-run” impact, as the witness put it — that left its victims vulnerable to cancer.

A pathologist testifying for Hardeman told the jury that Roundup was the most likely cause of his illness.

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