Southwest (17-8, 10-2 Imperial Valley League) also didn't do itself any favors at the plate as Sommer gave up only one hit, a single to Jessica Conway in the fourth. Lerno was the only other Southwest player to reach base, walking twice.
Sommer struck out 17, facing three batters over the minimum of 21.
"She was a great pitcher. She pitched a great game, but I think we may have helped her a little," said Southwest coach Stephanie Niebla, who won her first league title this year. "But this game was very winnable. She was good, but we could've hit her. I don't know if it was nerves or what, but this team was beatable."
Tied 0-0 after one, Serra — which played in Division III for the first time after playing in Division II in past years — got on the board in the second as Sara Rohman scored on a passed ball to give the Conquistadors the lead.
Errors struck again in the third as an overthrow to first base allowed Maryssa Garcia to score the second run of the game.
After the errors, the Southwest defense played solidly, not committing any errors as Lerno held Serra to one hit the rest of the way. Lerno finished with four hits allowed, two strikeouts, a walk and no earned runs.
Still, the Eagles couldn't mount any offense and couldn't get back in the game.
For the most part, the game was a learning experience for the underclassmen-laden Southwest team, which will return eight starters next season.
But for one Eagle, senior Melissa Frausto, the game was a heartbreaking end to an otherwise stellar season.
"I kind of have mixed emotions about this whole thing. I think it's great that we made it this far and that we had the chance to play in the championships, but it's sad because this was my last game," said Frausto. "It's a little disappointing for me knowing that we have such a good team and that I'm the only one that's leaving and I won't be able to be a part of it next year. But this season was great. I couldn't have asked for anything else."
The championship game was the furthest any Imperial Valley team has gotten since Holtville lost in the Division IV championship game in 1999. In 1998, Holtville won that title.
Despite coming up short, Niebla said the loss does not diminish what the team accomplished this year, winning league after finishing below .500 last season and not making the playoffs.
"I was very proud of everything this team's accomplished this year. This is just one of those learning experiences for us and now we know what needs to be done," said Niebla. "You know, every coach's goal is to win it all at the beginning of the year. And with me, I like to take one goal at a time and not really look too far ahead. But with the team we have, I think it's very possible that we can be contenders for the next few years."
Added Lerno: "I think we could've and we should've won this game. I think this is going to stick with me until next year. I'm only a freshman and I still have a few more years, so I just want to win this whole thing before it's all said and done."