HOUSTON — The short porch in left field of Minute Maid Park got the best of the Seattle Mariners again Sunday.
After entering Houston on Thursday as winners of five of their previous seven, the sweep knocked the Mariners (10-15) five games below .500 for the first time since last April.
Trailing 6-2, the Mariners mounted a four-run seventh inning to tie the game before Gattis' homer.
"It was a tough game where they could have laid down, but they fought back and really grinded it out," manager Lloyd McClendon said. "They showed a lot of fortitude, which leads me to believe this team is going to be just fine."
Gattis hit a go-ahead solo shot in the eighth inning off Carson Smith (0-1). It was his fifth homer in six games.
Smith made his big league debut last year and this was his 21st appearance in the majors — he had not allowed a run in 19 1/3 innings until Gattis homered.
"It cleared by a few feet," Smith said. "It was a tough break. I made a bad pitch, and he took advantage of it... We definitely learned this series that they've got a short porch out there."
Gattis hit a three-run shot in the first, the third straight game he homered.
The Astros extended their top run since a 12-game winning streak in 2004.
At 18-7, Houston set a team record for its best start through 25 games. The Astros are coming off a 70-92 season, which followed three straight years of at least 106 losses.
Chris Carter also homered for Houston. The Astros hit five home runs Saturday night in roughing up the Mariners 11-4.
Houston has won 14 of 15 and will try to add to its winning streak when its hosts Texas in a three-game series starting Monday night.
After allowing the fewest homers in the AL in April (10), the Mariners gave up 11 homers to Houston in the first three games of May.
"They're a hot team right now with a lot of confidence," Seattle catcher Mike Zunino said. "They've had some momentum, and when things are going like that, things seem to go your way."
Gattis sent a changeup from J.A. Happ into the Crawford Boxes in left field for an early lead. Carter hit another changeup for a two-run homer just below the train tracks in left field for a 5-1 lead in the third.
"Two changeups I wish I could take back," Happ said. "That's really the story of the game, right there. I tried to battle as long as I could, and I needed to get us some innings as long as I could to limit the damage."
Brad Miller hit an RBI triple in the third for Seattle's first run. He later tried to score on a fly by Seth Smith to right fielder George Springer, and was originally ruled safe. The Astros challenged, and the replay showed he was tagged out before touching the plate.
"We're going to check on it, but we were told that when a foot is hovering over the plate, it's the same as touching it," Seattle manager Lloyd McClendon said. "We'll see what they say about the rule, but it is what it is."
McClendon said after the game the team had optioned relievers RHP Yoervis Medina and LHP Tyler Olson to Triple-A Tacoma. The corresponding moves will be made on Monday, and McClendon didn't give hints as to whether they would be bullpen pitchers or not.
"In the case of both of them, location and execution is important, and it's hard to get it straightened out in the big leagues," McClendon said. "Both of those kids need to go down, get a few things straightened out and they'll be fine."
Mariners: RHP Tom Wilhelmsen threw a bullpen session Sunday morning and is scheduled to throw a simulated game on Wednesday. He was placed on the disabled list on April 13 with a hyperextended elbow. ... Leadoff hitter Austin Jackson sprained his right ankle in the third when he caught the corner of first base running out a grounder. He left the game and will be re-evaluated on Monday.
Mariners: RHP Felix Hernandez (4-0, 1.82 ERA) goes for his sixth straight win against the Angels in Anaheim, where he has pitched below his high standards. He is 3-5 with a 3.11 ERA in Anaheim and has just one decision in his last seven starts at Angel Stadium.
Astros: LHP Dallas Keuchel (3-0, 0.73 ERA) has helped the Astros win in 10 straight starts, and he'll look to keep that streak going as Houston hosts the Texas Rangers. Keuchel allowed two runs and six hits in seven innings in an April 12 start against Texas.
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