Andrew McCutchen clubbed his 15th homer of the season and the Pittsburgh Pirates completed a doubleheader sweep of the St. Louis Cardinals with 6-0 victory on Tuesday night.Brandon Cumpton (1-1) allowed three hits over seven innings to pick up his first major league win. The sweep propelled the Pirates into first place in the NL Central. Pittsburgh is a season-high 22 games over .500 (64-42).Tyler Lyons (2-4) gave up four runs, three earned, in six innings. The rookie struck out five and walked one but received no help from a reeling offense. The Cardinals have dropped six straight and have scored five runs in their last 56 innings.St. Louis catcher Yadier Molina left in the fourth inning with a right knee strain.The Pirates took the opener 2-1 in 11 innings when Alex Presley's sharp grounder deflected off the glove of pitcher Kevin Siegrist and rolled into shallow left field, allowing Pittsburgh's Russell Martin to score from second.The fortuitous bounces kept coming a couple hours later. The Pirates took a 1-0 lead in the second when a rare passed ball by Molina let Jose Tabata sprint across the plate. Molina was done for the night in the top of the fourth when the right knee pain he's been battling for the better part of a month flared up again. He was replaced by Rob Johnson.The absence of their leader further weakened the Cardinals, and things quickly fell apart.While Cumpton kept the NL's top offense in check, Pittsburgh poured it on with a little help from St. Louis left fielder Matt Holliday. The Pirates took a 2-0 lead on Jordy Mercer's RBI single before McCutchen stepped in. He drilled Lyons' pitch deep to left but Holliday appeared ready to track it down at the wall. Instead, the ball caromed into the stands.Lyons placed both hands on his head in shock while the packed bleachers let Holliday have it. Holliday's night didn't get any better an inning later when he overran Josh Harrison's flyball to the wall, allowing Harrison to make it all the way to third.Harrison never made it home, but by then Cumpton had all the backing he would require.The rookie, who made a couple of spot starts earlier this season, needed just 87 pitches to get 21 outs. He received a loud ovation as he headed to the dugout, the latest in a series of unheralded Pirates pitchers to rise from obscurity and help propel the franchise into the midst of a pennant race.The staff takes its cue from ace A.J. Burnett, who worked seven emotional innings in the opener. Burnett struck out nine and walked three while allowing one run on three hits. He was long gone, however, by the time Presley stepped in with Martin on in the 11th."It's a battle, it's fun," Burnett said. "Let's roll from here. Let's keep going."NOTES: The series continues on Thursday. Adam Wainright (13-6, 2.51 ERA) starts for the Cardinals against Jeff Locke (9-3, 2.15) ... St. Louis sent struggling reliever Marc Rzepczynski to Cleveland on Tuesday in exchange for minor league infielder Juan Herrera. Rzepczynski had a 7.84 ERA in 10 1-3 innings this season with the Cardinals ... Pirates 2009 first-round pick Tony Sanchez went 0 for 3 with an RBI in his first major league start at catcher.