Tomatoes are one of the most widely grown crops in the world. They’re also up against a persistent, expensive problem: bacterial spot. The disease is caused by Xanthomonas bacteria, and it doesn’t stop at ruining the fruit. Leaves develop dead spots and fall off. Photosynthesis slows way down. The fruit gets blemished and loses market value. In bad years, growers can lose half their crop. Back in 2010, the USDA estimated that bacterial spot cost tomato farmers in the midwestern US somewhere between 7 and 8 million. For a long time, farmers had two main options: copper sprays, and antibiotics like streptomycin and oxytetracycline. Those are becoming less useful by…