To Jody Herr, it was a telltale sign that one of his tomato fields had been poisoned by 2,4-D, the powerful herbicide that was an ingredient in Agent Orange, the Vietnam War defoliant. “The leaves had curled and the plants were kind of twisting rather than growing straight,” Herr said of the 2009 incident on his vegetable farm in Lowell, Ind. He is convinced the chemical, as well as another herbicide called dicamba, had wafted through the air from farms nearly 2 miles away. Herr recalled the incident because he is concerned that the Dow Chemical company is on the verge of winning regulatory approval for corn that is genetically engineered to be immune to 2,4-D, allowing farmers to spray the chemical to kill weeds without harming the corn. That would be a welcome development for corn farmers like Brooks Hurst of Tarkio, Mo., who are coping with runaway weeds that can no longer be controlled by Roundup, the herbicide of choice. But some consumer and environmental groups oppose approval of Dow’s corn, saying it will lead to a huge increase in the use of 2,4-D, which they say might cause cancer, hormone disruption and other health problems. They are being joined by a coalition of fruit and vegetable farmers like Herr and canners like Red Gold and Seneca Foods, which filed petitions with the government last week seeking a delay in the corn’s approval. The Save Our Crops Coalition, as it calls itself, says it is not opposed to biotechnology. But it fears that fruits and vegetables, which will not be immune to 2,4-D, will become unintended casualties of herbicide drift as the chemical is sprayed on tens of millions of acres of corn. The dispute is the latest iteration in the intense and often bitter battle over genetically modified crops, made even more emotional in this case because of the connection between 2,4-D and Agent Orange, the notorious defoliant that has been linked to birth defects, cancer and other health problems in Vietnamese civilians and U.S. veterans. Dow hopes the approval will come in time for planting next year. For farmers like Hurst, the approval couldn’t come too soon. “I think it’s a crisis, and we need something to have a solution to get rid of resistant weeds,” Hurst said. He said without new chemical approaches, farmers would have to plow more, increasing soil erosion.