Nick Kurtz set the bar high before making his Major League debut for the A’s earlier this season.

The 6-foot-5 left-handed slugger hit seven home runs in his first 20 games with Triple-A Las Vegas — a 53-home run pace for a full 150-game slate in the Pacific Coast League — but the power didn’t immediately translate with the A’s after getting the promotion.

The prized prospect didn’t hit his first homer until his 17th big league game on May 13. But he still showed signs of plate discipline the organization recognized as a strength to complement his immense power.

“He’s really advanced,” A’s manager Mark Kotsay said Monday. “He’s got an advanced approach for a young kid, and I think that showed up tonight.”

That approach showed up Monday in the form of a 447-foot walk-off home run in the ninth inning to give the A’s a 3-1 win over the first-place Houston Astros, who were winners of five straight games coming in. It marked the second straight game Kurtz hit a decisive ninth-inning home run. He gave the A’s a sweep over the Kansas City Royals on Sunday with a ninth-inning bomb off right-hander Carlos Estévez.

Monday’s blast came on a 2-1 pitch after he badly missed on a breaking pitch in the dirt from Astros reliever Bryan Abreu, who entered the game with a 1.45 ERA while allowing 19 hits with 41 strikeouts in 31 appearances.

After Kurtz swung and missed a breaking ball, Abreu left another one up in the zone, and Kurtz hit it over the trees in the right field grass area at Sutter Health Park. It was his first career walk-off home run.

“I knew I got it pretty much right away,” Kurtz said. “That’s the scenario you dream about as a kid.”

Four straight

It was Kurtz’ seventh home run of the season. He was in the No. 5 spot in Kotsay’s lineup for the first time.“I think he’s getting a lot more comfortable,” Kotsay said. “The at-bats are looking better, and the results are obviously showing up. And tonight was a big, big swing. Feel great for him. You can there’s more confidence. You can him, in some form, starting to be a leader in the clubhouse.”

The A’s (30-44) are hoping their winning ways can continue after going 7-21 May that included losing 18 of 19, their worst stretch since they played in Philadelphia in 1943. They’ve won four straight and are 7-8 in June.

“I think you’re seeing some momentum right now with this team,” Kotsay said. “Which is great to see. They’re starting to have some fun playing the game.”

For Houston, Jose Altuve hit a 439-foot home run to left field to get the Astros on the board in the first inning. After the pitch of his next at-bat went high and inside, home plate umpire Doug Eddings issued a warning to A’s starter Mitch Spence and both dugouts. The A’s tied the game at 1 on a JJ Bleday solo homer in the bottom of the fifth.

Spence shines

Spence lasted five innings and allowed one run, marking the third straight start he threw five innings and allowed one run or fewer since he was put back into the rotation June 5.

“We had a little bit of a rough stretch there (earlier in the season),” Spence said. “And we just needed something to give us a confidence boost and I think this is 100% what it is.”

The A’s will look to increase their winning streak to five on Tuesday against Houston when they send left-handed starter JP Sears (5-5, 5.08 ERA) to the mound. The Astros will counter with Jason Alexander (0-0, 18.00), a native of Santa Rosa who attended Cardinal Newman High School.