BERLIN -- Shares in Germany's Bayer have plunged after a U.S. jury found the Roundup weed killer was a substantial factor in a California man's cancer -- the second case that has gone against manufacturer Monsanto, acquired by Bayer last year.

Bayer AG shares dropped 13 per cent to 60.66 euros (US$68.84) in Frankfurt Wednesday. On Tuesday, a federal court jury in San Francisco ruled unanimously in a lawsuit against Monsanto. Attorneys say the trial, which will determine in a second phase whether the company is liable and if so, for how much, could help determine the fate of hundreds of similar lawsuits.

Monsanto says studies have established that Roundup's active ingredient, glyphosate, is safe. It has appealed a separate U.S. court decision last year in favour of a man who used Roundup.