The roof of Minute Maid Park is growing like a Chia Pet. In the three years since the stadium opened, an unidentified organism has overtaken its white plastic roof like an out-of-control biology experiment, turning the top of the $250 million ballpark into what may be the world's largest petri dish. The growth -- which could be anything from mold to bacteria to algae, experts say -- has prompted the Houston Astros to file a warranty claim with the manufacturer, GenFlex Roofing Systems of Maumee, Ohio. "We noticed the discoloration last year," said Rob Matwick, senior vice president of operations...