Talks to overhaul the North American Free Trade Agreement resume this weekend in Mexico, the second round after U.S. President Donald Trump’s renewed threats to withdraw from one of the world’s biggest trade blocs.

In one letter seen by Reuters, sent to U.S. Commerce Secretary Wilbur Ross and Trade Representative Robert Lighthizer on Thursday, retailers argue that the U.S. proposal to allow more complaints about the dumping of perishable produce would have “dangerous implications for U.S. businesses and consumers.”

The retailers and food industry groups argue that American producers could be left open to retaliatory measures if more complaints were to be filed, for instance, against avocados, tomatoes and other produce imported from Mexico.

The letter was signed by large trade groups including the National Council of Chain Restaurants, National Restaurant Association, National Retail Federation, Retail Industry Leaders Association and the Fresh Produce Association of the Americas.

A separate letter was sent on Wednesday by 26 U.S. agriculture groups – addressing Ross, Lighthizer, Agriculture Secretary Sonny Perdue and Gary Cohn, the top White House economic adviser. It too urged American negotiators to abandon the fresh produce proposal because it risks damaging U.S. producers.

“Once seasonal tariffs were put in place for tomatoes, for example, Mexico or Canada may initiate trade cases of their own on any of a wide range of U.S. agricultural products, beginning a tit-for-tat cycle that could broadly limit agricultural trade,” the letter states. “At a time of low commodity prices in much of the United States, US agriculture can hardly afford to see a primary market disrupted.”

The letter was signed by several large agriculture groups, including the U.S. Grains Council, National Association of Wheat Growers, National Cattleman’s Beef Association, National Corn Growers Association, National Cotton Council of America and Ocean Spray Cranberries.