A 2011 study led by the Parkinson’s Institute and the National Institutes of Health drew on a federal survey of farmers and their spouses in Iowa and North Carolina, as well as others who handled pesticides. Those studied were two and a half times more likely to develop Parkinson’s if they used paraquat or rotenone. A 2012 study found that those who used paraquat, and who also had a certain genetic variation, were 11 times more likely to develop Parkinson’s, suggesting that certain people could be more at risk from exposure than others.

“It’s a poison, and we really shouldn’t be using this as an herbicide in the way we do,” said Dr. Beate Ritz, a professor of epidemiology at the Fielding School of Public Health at the University of California, Los Angeles, who has studied health risks to people living near where pesticides are used.

Syngenta has been known for aggressively defending its products, including engaging in a bitter feud with a scientist whose research it once underwrote. The company argues that studies implicating paraquat do not adequately consider other environmental factors or chemicals that are present. Syngenta also studied the work force at an old British manufacturing site that produced paraquat and found a lower-than-expected rate of Parkinson’s, though its study relied on death certificates, which often underreport the disease.

“I do believe paraquat is safe when it’s used appropriately,” said Dr. Botham of Syngenta. “I will, though, always as a scientist be open to any new findings.”

In a filing with the E.P.A. this year, Syngenta said it had hired Dr. Colin Berry to help lead a scientific review of the Parkinson’s issue. The review “concluded that the evidence available from epidemiological studies was fragmentary and insufficient to establish whether herbicides and paraquat in particular” increase the risk of Parkinson’s, according to Syngenta’s E.P.A. filing.

Dr. Berry, an emeritus professor at Queen Mary University of London, was at the center of high-profile civil cases in 1999 and 2000 that led to more than $1 million in payments to two of his patients who were given misdiagnoses of breast cancer and had needless double mastectomies. His hospital’s internal review, which was read in court by the judge handling one of the cases, cited “a serious and indefensible diagnosis” and raised questions of basic competency, according to accounts of the case.