The Pittsburgh Pirates lost their seventh straight game Friday and kept fading in the NL wild-card race as Starlin Castro hit a three-run homer that sent the Cubs to a 7-4 win.
The Pirates have dropped 11 of 13 overall. They began the day three games behind St. Louis for the final NL playoff spot.
The Cubs have won four straight against the Pirates, including a three-game sweep last weekend in Pittsburgh.
Chris Rusin (1-2) notched his first major league victory, allowing two runs in five-plus innings in his fourth career start.
The Cubs led 4-3 in the sixth before Castro connected after Luis Valbuena walked and Alfonso Soriano was hit by a pitch.
Andrew McCutchen homered in the Pittsburgh seventh, but Cubs reliever Sean Camp tossed a perfect eighth and Carlos Marmol earned his 20th save in 22 chances.
Pirates starter James McDonald (12-8) struggled once again in the second half. He lasted just 3 2-3 innings, allowing four runs on five hits and four walks. He is 2-5 in his last 10 starts.
McDonald, who also hit a batter, hurt himself with wildness. He walked three batters in the fourth before Anthony Rizzo delivered a two-run double down the right-field line and Alfonso Soriano hit an RBI single that made it 4-0.
The Pirates got an RBI single from Pedro Alvarez in the fifth and closed within a run in the sixth.
Gaby Sanchez singled home a run and Alvarez had an RBI grounder, but Pittsburgh left the bases loaded when James Russell struck out pinch-hitter Neil Walker.
On Aug. 8, the Pirates were 63-47, but have gone 9-24 since. They are now just a game over .500 for the first time since June 15 when they were 32-31. Pittsburgh hasn't finished with a winning record since 1992.
NOTES: Cubs' first-round draft pick Albert Almora took batting practice with the team at Wrigley Field prior to the game. He will report to the instructional league on Monday. ... Alvarez (sore wrist) returned to the lineup after missing Wednesday's game. Walker (lower back tightness) did not start for the 16th consecutive game. ... Jason Berkan makes his Cubs debut Saturday against Wandy Rodriguez (10-13, 3.72).