The right-hander allowed another single to Alvarez in the eighth, issued only the one walk and struck out four in the dominating performance, tossing 65 of his 106 pitches for strikes.
Chase Headley drove in three runs with a solo homer, an RBI groundout and a bases-loaded walk while Will Venable also went deep as San Diego clinched its three-game series at PNC Park.
Pittsburgh starter A.J. Burnett (14-4) fanned 10 but allowed five runs (four earned) on nine hits through 5 2/3 innings.
The second-place Pirates fell 4 1/2 games behind National League Central-leading Cincinnati with the loss.