Bayer AG shares dropped more than 13% in trading on Aug. 13 after its newly acquired unit Monsanto Co. was ordered to pay $289 million in damages over allegations that its glyphosate-based weedkiller Roundup caused cancer in a school groundskeeper.

This was the largest fall in the German agrochemical company's stock prices in almost seven years, Bloomberg News noted.

Bayer shares were down 13.36% as of 1:42 p.m. CEST on Aug. 13, 2018.

Lee Johnson, a former school groundskeeper, alleged that Monsanto, the world's top seed manufacturer, knew about the cancer risk associated with its top-selling herbicide Roundup but failed to disclose it to the public.

The jury at San Francisco's Superior Court of California awarded Johnson $39 million for his losses and $250 million as the penalty for Roundup's design defect and failure to warn the public.

The California trial will serve as a benchmark for thousands of other Roundup-related lawsuits filed against Monsanto and a series of large verdicts could have a material impact on Bayer, Bloomberg added, citing analysts.

Monsanto, which argued that its product is safe, is planning to appeal the verdict, according to the report.

The verdict also led to a drop in the shares of Australia's Nufarm Ltd., which manufactures a product similar to Roundup that also has glyphosate as the main ingredient.