Bayer won a legal victory in its fight to limit liability from claims that 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.

David Schaffner, Jr. and Theresa Sue Schaffner filed a lawsuit against Monsanto Corporation, alleging that Monsanto violated Pennsylvania law by failing to include a cancer warning on the label of its...