GIANTS 6, BREWERS 1

(07-07) 04:00 PDT Milwaukee -- For the record, underage Madison Bumgarner only wore a gallon of beer after dominating the Brewers for his first major-league win. He drank Gatorade.

What else were his teammates supposed to dump on the left-hander after Tuesday night's 6-1 victory, soda pop? This is Milwaukee.

Bumgarner, who turns 21 on Aug. 1, had an inkling of what was coming. The Giants are not very original in celebrating milestones.

"When they told me they wanted to talk to me, especially in the shower, I figured," Bumgarner said. "I went to peek in there and see, and I got caught."

That was his only misstep on a night when yet another catastrophic error by a Milwaukee shortstop on what should have been an inning-ending double play led to a huge Giants rally, this time a five-run sixth inning that included a Bumgarner single for his first big-league RBI.

Bumgarner allowed three singles in eight shutout innings, and Edgar Renteria should have turned one of those into an out. The lefty also struck out five in his 115-pitch start.

He passed his first test by pitching scoreless ball in the first two innings. Of the eight runs he surrendered in his first two games, both losses, six came before the third inning. Bumgarner did walk three in the first three innings at Miller Park but coasted from there, retiring 17 of his last 19 hitters.

"At 20 years old, he's got great composure and poise," manager Bruce Bochy said. "He's got a good look about him. He has a presence on the mound."

Catcher Buster Posey considered this Bumgarner's third quality start but said this time he had a little extra.

"He threw the ball in and out well," Posey said. "He had his slider working. He also threw his cutter when he needed that and, late in the game, he flipped some curveballs in there. I thought he did a great job."

Posey was a wreck. He had ice on his tender left wrist and more on his left knee, which was bruised pretty badly by a Randy Wolf cutter that found skin and bone in the first inning.

Posey stayed in the game, singled in the fourth and hit a sacrifice fly in the ninth to help the Giants win consecutive games for the first time since a four-game winning streak June 11-14 and even their record on the season's longest trip to 3-3.

The Giants again posted a crooked number after a big mess-up by a Brewers shortstop. On Monday it was 23-year-old Alcides Escobar. On Tuesday it was 39-year-old Craig Counsell, who gloved Pablo Sandoval's bases-loaded double-play ball in the sixth inning of a scoreless game and sailed his throw into right field. Andres Torres and Freddy Sanchez, who both had walked, scored.

With two outs and the bases again loaded, Travis Ishikawa, in only his fifth at-bat against a left-handed pitcher this year, lined a two-run single off the glove of leaping second baseman Rickie Weeks into right field. Bumgarner followed with his own line single to right for an RBI.

Ishikawa could not recall two games in a row in which errors at the same position led to two wins. The Giants have scored 12 runs in the series. Eleven should not have been. This is not how the season has gone for this team.

"We've definitely been hitting a lot of hard balls lately and had some bad luck as far as finding holes," Ishikawa said. "Hopefully this will be a little turning point and we can get on a roll here."

E-mail Henry Schulman at hschulman@sfchronicle.com.

This article appeared on page B - 1 of the San Francisco Chronicle