Ryan Dempster tossed six shutout innings Saturday and the Chicago Cubs handled the Arizona Diamondbacks 4-1. Dempster (5-3) lowered his major league baseball-best ERA to 1.86 with the six scoreless frames, during which he scattered four hits, issued three walks and struck out five. The Chicago right-hander, who is the subject of trade rumors, stretched his scoreless streak to 33 innings in winning his fifth straight start. Shawn Camp hurled a scoreless eighth inning and Carlos Marmol picked up his ninth save for the Cubs, who clinched their three-game series at Wrigley Field. Starlin Castro, Darwin Barney and Luis Valbuena supplied RBI in the victory. Joe Saunders (4-6) absorbed the loss for Arizona, allowing four runs (three earned) on eight hits over six frames. Chris Young socked a solo homer for the Diamondbacks.