LaRoche hit a tiebreaking homer and the Washington Nationals handed Dickey his first loss in three months, beating the sinking New York Mets 5-2 on Tuesday night behind Gio Gonzalez's seven crisp innings.
"I've been using a heavier bat and obviously it hasn't been working against Dickey, so I figured it was time to try something new," said LaRoche, who was 2 for 14 against the knuckleballer before his long home run.
Gonzalez (13-5) gave up only two singles in a rematch of All-Star pitchers, five days after New York hit him hard to back Dickey during his 13th win.
This time, it went the other way.
"Just wanted to redeem myself from the last start," Gonzalez said. "You always want to bounce back as a pitcher. You don't want to feel down and out. A lot of these guys came up to me and gave me some positive energy, positive feedback. So the best I wanted to do was return the favor."
Dickey (13-2) allowed five consecutive hits in the sixth, ending his 11-game winning streak, as the Mets lost their fifth straight and 11th in 12 games. Their lone victory since July 7 was Dickey's 9-5 win over Gonzalez last Thursday in Washington.
In a mini shake-up before the game, New York demoted slumping right fielder Lucas Duda to the minors.
"When you're not playing good baseball against teams like this, they exploit you pretty quickly," Mets third baseman David Wright said. "This stretch is not going to define us over the course of the year."
Ryan Zimmerman kept up his tear at the plate with three hits and the Nationals won their fourth in a row to improve the NL's best record to 57-39. They are 8-3 against the Mets this season, winning all four series so far, and will go for a three-game sweep Wednesday at noon with ace Stephen Strasburg on the mound against rookie Jeremy Hefner.
"I think we're clicking pretty good," LaRoche said. "It can get better, but it's pretty good right now."
Sean Burnett got three outs in the eighth and Tyler Clippard fanned two in a perfect ninth, finishing the three-hitter for his 17th save. Gonzalez struck out four, walked two and did not allow an earned run in his longest outing since May 27.
"It was a masterpiece," Nationals manager Davey Johnson said. "He had something to prove."
Jordany Valdespin set a Mets record with his fifth pinch-hit homer of the season. The rookie connected off Ryan Mattheus leading off the eighth, breaking a mark he previously shared with Mark Carreon (1989) and Danny Heep (1983).
Dickey was 11-0 in his previous 16 starts since getting roughed up in a 14-6 loss April 18 on a rainy day in Atlanta. Trying to become the first 14-game winner in the majors, he was unable to stop New York's nasty slide. He yielded five runs -- four earned -- and eight hits in six innings.
The 37-year-old right-hander tossed an inning of relief Saturday against the Dodgers to help an overworked bullpen, only two days after throwing 111 pitches over 7 1/3 innings in Washington. He has a 5.36 ERA in his last seven outings.
"I had a good knuckleball," Dickey said. "I should have pitched into the eighth inning. I felt too good for that to happen."
Dickey whiffed star rookie Bryce Harper all three times on knuckleballs ranging from 66-81 mph and was in command until the sixth, when he gave up five straight hits with two outs.
Michael Morse singled and LaRoche drove a low knuckler into the upper deck in right, snapping a 1-all tie with his 17th homer. Danny Espinosa singled, Bernadina beat out a slow dribbler toward first and Jesus Flores hit an RBI single to left. Jason Bay's wide throw to the plate got away for an error that allowed Bernadina to score, making it 5-1.
Espinosa's throwing error at shortstop led to an unearned run in the second. Ronny Cedeno hit an RBI single to right with two outs, giving New York its first lead in 42 innings since last Thursday, but was thrown out by Harper trying to stretch it into a double.
Gonzalez retired his next 11 batters and did not permit another hit until Ike Davis' two-out single in the seventh.
A switch-hitter, Espinosa batted right-handed against Dickey to get a better look at the knuckleball. The first time up, Espinosa struck out swinging. But in the fourth, his two-out double tied it 1-all after Zimmerman opened the inning with a bloop double.
Flores (back) was back behind the plate after sitting out two games. ... Washington is 9-1-3 in series against NL East opponents. ... Harper is 0 for 10 against Dickey with six strikeouts.