A year ago, the active ingredient in Roundup, the nation’s most widely used weed-killing herbicide, was added to California’s official list of chemicals known to cause cancer. The state’s warning about glyphosate followed a similar caution issued by the World Health Organization and coincided with hundreds of lawsuits across the country focused on the herbicide. The first very jury trial to involve Roundup recently started in San Francisco —the plaintiff is a groundskeeper who believes he developed non-Hodgkin’s lymphoma by using the weedkiller on the job.