(Reuters) - Bayer won a big legal victory in its fight to limit liability from claims its Roundup weed killer causes cancer, as a U.S. appeals court on Thursday said federal law shields the German company from a lawsuit by a Pennsylvania landscaper.

Shares of Bayer rose as much as 13.2% in Friday trading in Germany, on optimism the decision could eliminate much of its remaining legal liabilities for Roundup, made by its Monsanto unit.

The 3rd U.S. Circuit Court of Appeals in Philadelphia rejected plaintiff David Schaffner's claim that Monsanto violated state law by failing to put a cancer warning on the label for Roundup.

Schaffner was diagnosed in 2006 with a kind of cancer called non-Hodgkin's lymphoma, a common claim for Roundup plaintiffs.

He and his wife Theresa sued Bayer in 2019, partly over how his illness affected their relationship.

Chief Judge Michael Chagares wrote for a unanimous three-judge panel that the Federal Insecticide, Fungicide, and Rodenticide Act requires nationwide uniformity in pesticide labels, and prevented Pennsylvania from adding a cancer warning.

COURTS ARE DIVIDED

The decision conflicts with rulings from federal appeals courts in San Francisco and Atlanta in similar cases.

That raises the prospect the U.S. Supreme Court could resolve the split if the Schaffners appealed, perhaps as soon as next year. The court is not required to hear an appeal, but often accepts cases when lower courts disagree on the law.

Chip Becker, a lawyer for the Schaffners, said his clients were disappointed with the decision and reviewing their legal options, and that federal law should not preempt their failure-to-warn claim.

In a research note, Barclays analysts said "this ruling was not expected," and Bayer believed a positive Supreme Court resolution "could bring glyphosate litigation to an end."

Bayer said it was pleased with the decision, and the Supreme Court should "settle this important issue of law."

The German company has long maintained that Roundup and its active ingredient glyphosate are safe.

SHARE SLUMP

Bayer has faced extensive litigation over Roundup, and through Thursday had seen its share price fall more than 73% since buying Monsanto for $63 billion in June 2018.

It settled much of the litigation for $10.9 billion in 2020, but still faces about 58,000 claims. Another 114,000 claims have been settled or rejected.

Though Bayer won 14 of 23 Roundup trials through July 23, one victory was overturned on appeal, and the losses saddled it with billions of dollars of damages awards.

The Schaffners settled with Bayer in September 2022, conditioned on Bayer being unable to convince courts that federal law preempted Pennsylvania's cancer warning.

Chagares said it did, and that this approach "best achieves Congress's stated aim of uniformity in pesticide labeling."

Roundup is among the most widely used weed killers in the United States. Bayer phased out sales for home use last year.

The case is Schaffner et al v Monsanto Corp, 3rd U.S. Circuit Court of Appeals, No. 22-3075.