...a group of...tomato pickers...led a four-year boycott against [Taco Bell (part of YUM!)] until it agreed in March to pay a penny more per pound for Florida tomatoes and adopt a code of conduct that would allow Taco Bell to sever ties to suppliers who commit abuses against farmworkers...The coalition has sent letters to executives at McDonald's Corp., Subway and privately-owned Burger King asking them to follow Taco Bell's lead...McDonald's said it already has a code of conduct for suppliers that prohibits forced labor and child labor, and demands that workers receive fair compensation. A Burger King spokeswoman said the company's chairman hasn't yet read the coalition's letter, but she noted the chain also has a code of conduct for suppliers. Subway's spokeswoman said Wednesday the company couldn't immediately comment...