Dodgers star closer Eric Gagne will undergo season-ending back surgery on Saturday, the team announced prior to Friday's game against the Giants. The 30-year-old righthander has a herniated disc in his lower back, a condition that is pressing against a nerve and causing pain down his legs. Gagne, whose contract contains a $12 million club option for 2007 or a $1 million club buyout, is expected to be ready for spring training next year.