(Reuters) — A California appeals court on Tuesday heard arguments in the first case that went to trial over allegations that Bayer AG's glyphosate-based weed killer Roundup causes cancer, resulting in a $289 million judgment against the company.

The August 2018 jury verdict in favor of groundskeeper Dewayne Johnson, later reduced by a judge to $78 million, launched massive litigation over the weed killer that has led Bayer's stock price to tumble more than 30%.

A favorable appeals court decision in the Johnson case could help Bayer pressure plaintiffs' lawyers in settlement negotiations currently underway. Under California law, appeals courts have 90 days to issue a ruling following arguments.

During Tuesday's one-hour hearing, a three-judge panel peppered lawyers for Bayer and Mr. Johnson with technical legal questions that did not reveal which way they might be leaning.

Bayer has asked the court to overturn the verdict or order a new trial.

Bayer's lawyer, David Axelrad, said regulators around the world have found glyphosate to be non-carcinogenic, upending claims that Bayer has failed to warn about the product's cancer risks.

Mr. Johnson's lawyer, Michael Miller, argued the company has engaged in decades of deceit to mislead the scientific community about glyphosate's safety.

Bayer denies those claims and insists Roundup and glyphosate are safe for human use.

Bayer was found liable in two other U.S. Roundup jury trials and ordered to pay billions of dollars. Those verdicts were also reduced and are being appealed, but the massive awards sparked a slew of lawsuits.

Bayer in April said it was facing 52,500 U.S. lawsuits over Roundup, which it acquired with its $63 billion purchase of Monsanto in 2018.

Bayer is pursuing an out-of-court settlement of the litigation, which analysts estimate could result in a $10 billion agreement. The company in February said it would not have to write down the value of the Monsanto acquisition in case of such a deal with plaintiffs.