SAN FRANCISCO (AFP) – A California groundskeeper dying of cancer prepared to testify Monday before jurors hearing evidence in his lawsuit blaming Monsanto weed killer Roundup for his terminal illness.

The trial pitting 46-year-old Dewayne Johnson against the agrochemical colossus is expected to last into August, with the potential for a major impact on the company recently acquired by German-based Bayer.

The case is the first to reach trial alleging a cancer link from Roundup, one of the world’s most widely used herbicides.

Johnson arrived in court Monday with his lawyer and wife, and watched as his dermatologist Ope Ofodile testified in the California Superior Court trial.

The legal clash involves dueling studies, along with allegations Monsanto connived behind the scenes to thwart potentially damning research.

Diagnosed in 2014 with non-Hodgkin’s lymphoma, a cancer that affects white blood cells, Johnson used a version of Roundup called “Ranger Pro” repeatedly in his job at a school in Benicia, California, after being promoted to groundskeeper in 2012.

In this file photo taken on July 09, 2018, plaintiff Dewayne Johnson listens as attorney Brent Wisner (out of frame) speaks about his condition during the Monsanto trial in San Francisco, California. Photo: AFP/Josh Edelson

Ofodile testified that Johnson consulted her when he noticed a rash on his body starting in 2014.

“He was frightened by the state of his skin,” the physician told the trial.

After seeing the rash, Odofile said she sent a letter to the school district board saying “that he shouldn’t be exposed to any airborne chemicals that could worsen his condition.”

Asked whether she was referring to Ranger Pro, she said, “Yes.”

But the physician said she did not investigate what caused the rash, and that she was focusing on treating the patient rather than establishing a link to Roundup.

Johnson had little warning about the risks of Roundup, his lawyer said in opening statements earlier this month.

He was told you could drink it, it was completely non-toxic, lawyer Brent Wisner claimed in his opening remarks.

The lawyer said Johnson, who is between rounds of chemotherapy, “is actually on borrowed time — he is not supposed to be alive today.”

A key to Johnson’s case will be convincing jurors that Monsanto’s pesticide — whose main ingredient is glyphosate — is responsible for the illness. Wisner contended that glyphosate combined with an ingredient intended to help it spread over leaves in a cancer-causing “synergy.”

Bottles of Monsanto’s Roundup are seen for sale June 19, 2018 at a retail store in Glendale, California. Photo: AFP/ Robyn Beck

POTENTIAL IMPACT OF THE CASE

Whether glyphosate causes cancer has been the source of long debate among government regulators, health experts and lawyers.

If Monsanto loses, the case could open the door to hundreds of additional lawsuits against the company.

Monsanto has denied any link with the disease and says studies have concluded the product is safe.

“Mr Johnson’s cancer is a terrible disease. We all do and we all should have great sympathy for what he is going through,” Monsanto defense attorney George Lombardi said during his opening remarks.

But the lawyer maintained that “the scientific evidence is overwhelming that glyphosate-based products do not cause cancer, and did not cause Mr Johnson’s cancer.”

Monsanto’s flagship herbicide Roundup was launched in 1976.

Roundup has been approved by the US Environmental Protection Agency, according to Lombardi.

In 2015, the International Agency for Research on Cancer — a World Health Organization body — classified glyphosate as “probably carcinogenic,” and as a result, the state of California listed it as carcinogenic.

Argentinian former farmworker Fabian Tomasi looks through the window during an interview with AFP at his home in Basavilbaso, Entre Rios province, Argentina on February 7, 2018. Tomasi used to work supplying herbicides for plane spraying and did not use protection. Soybean fields in Argentina are often fumigated with glyphosate, a herbicide which is probably carcinogenic according to the World Health Organization (WHO), but which is needed to maintain GMO crops. The first trial for the possible effects of Round Up -Monsanto’s polemic herbicide containing glyphosate- starts on July 9 in the US. Photo: AFP/Pablo Aharonian

Founded in 1901 in St Louis, Missouri, Monsanto began producing agrochemicals in the 1940s and became a pioneer of genetically-engineered crops in the 1980s.

Monsanto’s roles in agricultural changes and biotechnology products has resulted in controversies, such as its practice of seed patenting.

The company once manufactured controversial products such as the insecticide DDT and the herbicide Agent Orange, which was used by the US military as part of its herbicidal warfare programme during the Vietnam war.

Monsanto was acquired by Bayer for more than $62 billion in June.