A drive in college towns aims to get the fast-food chain to follow McDonald’s and Walmart in buying tomatoes grown under strict labor standards.

A program created by a group that organizes farmworkers has persuaded companies like Walmart and McDonald’s to buy their tomatoes from growers who follow strict labor standards. But high-profile holdouts have threatened to halt the effort’s progress.

Now the group, a nonprofit called the Coalition of Immokalee Workers, is raising pressure on one of the most prominent holdouts — Wendy’s — which it sees as an obstacle to expansion.

“They’ve already been successful in a measurable way at effectively eliminating modern-day slavery and sexual assault, and greatly reducing harassment,” said Susan L. Marquis, dean of the Pardee RAND Graduate School in Santa Monica, Calif., who has written a book on the program. “Pay is substantially higher for these people.”

The Immokalee workers’ initiative, called the Fair Food Program, currently benefits about 35,000 laborers, primarily in Florida. Over the last decade, it has helped transform the state’s tomato industry from one in which wage theft and violence were rampant to an industry with the some of the highest labor standards in American agriculture.

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But only 20 to 25 percent of tomatoes in the United States are purchased from growers that take part in the program, the organizers estimate.

By late 2014, a few years after the program was up and running, Wendy’s had ceased buying winter tomatoes from Florida and was importing most of that supply from Mexican farms, where forced labor and physical abuse are common. Wendy’s said the change was unrelated to the Fair Food Program.

“The smaller brands look at Wendy’s refusal to join the program and make a decision,” said Gerardo Reyes Chavez, a former farmworker who is a leader of the Coalition of Immokalee Workers. “If this is how big corporations are behaving, then it is O.K.”

Under the Fair Food Program, buyers like Walmart and McDonald’s agree to pay 1 to 4 cents more per pound of tomatoes. The growers, in turn, agree to pay farmworkers at least the local minimum wage, to which the premium adds a bonus, and to meet a set of labor standards like providing shade and water for workers and ensuring freedom from physical and sexual abuse. Some of the practices are required by law but flouted on many farms.

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Heidi Schauer, a Wendy’s spokeswoman, said in an email that the company required its tomato suppliers to submit to third-party reviews of their human rights and labor practices. She said the chain had recently committed to buying all of its tomatoes from indoor greenhouse farms, most of them in the United States and Canada, which “strengthens our commitment to treat people with respect.”

Image Protesters outside a Wendy’s restaurant in Carrboro, N.C., who are part of an effort by a nonprofit, the Coalition of Immokalee Workers, to pressure the fast-food chain to buy its tomatoes from growers who have agreed to follow strict labor standards. Credit... Travis Dove for The New York Times

Several experts questioned the value of such reviews, which are often superficial, as well as the suggestion that greenhouse farms provide more humane work environments.

“Indoor greenhouse farms are not inherently better in terms of labor conditions,” said Margaret Gray, an associate professor of political science at Adelphi University who has studied farm labor conditions.

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In recent weeks, activists affiliated with the Immokalee workers have stepped up pressure at several universities for Wendy’s to sign on to the Fair Food Program.

The effort borrows its strategy from a campaign that student activists connected to the Immokalee workers waged against Taco Bell. Over nearly four years, supporters on several campuses persuaded officials to either remove the chain from campus or block it from doing business there in the future. In 2005, Taco Bell’s parent company agreed to buy its tomatoes through the program, becoming the first major company to sign on.

Like Taco Bell, Wendy’s is potentially susceptible to protests on college campuses. A significant number of Wendy’s customers are in their teens and 20s, according to Mark Kalinowski, an industry equity analyst.

One recent focus of the campaign against Wendy’s is the University of Michigan, where, according to a statement from the company in late January, its campus franchisee has chosen not to seek to renew its lease.

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Wendy’s later said the franchisee had made the decision a few years ago. But the announcement came just before the local City Council and the university’s student government passed resolutions advocating boycotts of Wendy’s.

Last week, the Board of Aldermen of Carrboro, N.C., a town near the University of North Carolina at Chapel Hill, passed a resolution urging Wendy’s to join the Fair Food Program. And the mayor of Gainesville, home to the University of Florida, has placed a similar resolution on the agenda of a City Commission meeting this week.

In an interview, the mayor, Lauren Poe, said that he intended the resolution as a way of supporting farmworkers, not opposing the university, but that he hoped the school would ban the Wendy’s that operated on campus until the company joined the Fair Food Program.

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The actions coincided with a series of demonstrations organized by the Coalition of Immokalee Workers, whose members are descending on the universities of North Carolina, Michigan and Florida and Ohio State University this week and next.

Image Juan Antiaon was among demonstrators who marched through the campus of the University of North Carolina at Chapel Hill. There is a Wendy’s restaurant on campus. Credit... Travis Dove for The New York Times

On Tuesday, scores of students and other supporters gathered in front of an administration building at the University of North Carolina, where several speakers, including the human rights activist Kerry Kennedy, implored the school to oust Wendy’s from campus.

Officials at the University of North Carolina and the University of Florida say the Wendy’s on their campuses obtain their tomatoes through another company, Aramark, which takes part in the Fair Food Program.

While Wendy’s is the immediate target, there are other big companies refusing the Immokalee group’s demands, including Costco and the grocery chains Publix and Kroger. None of those companies responded to a request for comment, but a statement on the Publix website says that the chain regards the campaign as a labor dispute between its suppliers and their employees and that it is “not our place” to get involved.

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Unless more companies commit to buying tomatoes through the Fair Food Program, growers who haven’t joined will continue to have a market for their products and may not feel pressure to raise their labor standards.

“I’m sure there are people that look at all of us who did join the program and viewed it as an opportunity to go do business with brands that didn’t want to sign up,” said Jon Esformes, chief executive of Sunripe Certified Brands, which grows tomatoes in Florida, Georgia and Virginia.

Sunripe agreed to join the program in 2010. Like other growers in the program, it must show new workers a video about their labor rights and let the Immokalee group provide education sessions for workers at least once per season. Workers are urged to report abuses to a 24-hour hotline, which is monitored by an independent council that investigates complaints.

The council also regularly audits farms for compliance. Fair Food auditors interview at least half the workers on a farm — often hundreds of them — which is far more than conventional auditors typically interview. Growers found to have violated the program’s code of conduct can lose access to buyers.

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The stakes go far beyond tomatoes. In the coming years, the Immokalee workers hope to bring their model to a variety of crops in many states, where tens of thousands of workers are still vulnerable to abuses.

But to do so effectively, said James Brudney, a law professor at Fordham University who has studied farm labor, it is important to show buyers and growers that they can’t avoid taking part in the long run.

“They want to be able to make clear that this is a serious market penetration,” Mr. Brudney said.

Follow Noam Scheiber on Twitter: @noamscheiber.

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