CHICAGO -- Right-hander Jason Bergmann had one bad inning Sunday and it proved to be costly, as the Nationals lost to the Cubs, 6-1, at Wrigley Field.

The Cubs ended up taking two out of three games in the series.

Early on, it looked like Bergmann would be in for a rough afternoon, as the Cubs scored three runs in the top of the second inning. Mark DeRosa led off the inning by hitting a home run over the right-field wall. It was the third home run of the series DeRosa, who has hit a home run in four consecutive games.

"He's very aggressive," Nationals manager Manny Acta said of DeRosa. "We didn't help ourselves being behind in the count in most of the series. He is a good fastball hitter. Whenever he got his fastball, he hit it. In that particular at-bat, when the wind was blowing in, it was blowing to right field and it kind of helped him out."

After Jim Edmonds walked, Geovany Soto hit a two-run home run to make it a 3-0 game. Bergmann would settle down after the second inning, allowing one run in the next 4 1/3 innings.

"I thought he had just one rough inning where the long ball hurt him, especially that walk mixed in there," Acta said of Bergmann's outing. "He threw the ball very well other than that inning. He really gave us a chance."

Bergmann believes he had too much confidence in his fastball. After giving up the three runs, he relied heavily on his breaking stuff.

"After that, my curveball took over as my fastball. I kept throwing it because it was effective," Bergmann said.

Steve Shell replaced Bergmann in the seventh inning and wasn't nearly as effective. He gave up a two-run homer to pinch-hitter Kosuke Fukudome, who was in a 30-for-151 (.199) slump prior to the at-bat. Shell was trying to pitch Fukudome on the outside part of the plate, but the fastball ended up on the inner half.

"I fell behind in the count," Shell said. "When you fall behind, you have to throw a strike in there. The ball came back over the middle and he bails out a lot. He's a good hitter. I just made a mistake."

The Nationals have struggled all season offensively, and Sunday was no different. Right-hander Rich Harden was dominating for Chicago. He pitched seven innings, gave up a run on two hits and struck out 11. The Nationals' lone run was scored in the third inning, when Austin Kearns hit a home run.

"Harden threw strike one a lot on the outer half of the plate," Acta said. "Then, he has that tough slider. Also ... he changes speeds so well with his fastball. He'll go from 89-95 miles per hour on one hitter."

Ryan Zimmerman had one of his toughest games of his career, striking out four times. Three of his strikeouts were against Harden.

"[His pitches] were tough to pick up," Zimmerman said. "He does everything from the same arm angle. He does a really good job of not changing anything with his offspeed stuff and his fastball."

After Harden left the game, the Nationals had a chance to score off reliever Carlos Marmol in the eighth. They had the bases loaded, two outs and Cristian Guzman at the plate. He saw seven pitches and worked the count to 3-2. On the eighth and final pitch, Guzman struck out looking on strikes. The replay showed that the ball was inside.

"It was way inside," Acta said. "But instant replay is only going to be used for home runs. You can't change balls and strikes. I can't see that from the side during the game. I saw the video after the game and it wasn't even close."

Home-plate umpire Angel Hernandez was not available for comment after the game.