BERKELEY, Calif. - With every punishing run, defensive stop or other big play in Southern California’s latest dominating victory, the Trojans sent a message back home to Stafon Johnson on his hospital bed.

“We were talking about him, before and after. He was a big motivator for us, and we might go see him later tonight, I’m not sure,” freshman quarterback Matt Barkley said. “Stafon knew we were on a mission tonight. I’m proud of our guys and how we played.”

Joe McKnight ran for 119 yards and two scores, Taylor Mays and the defense shut down Jahvid Best and No. 7 USC re-established itself as a national contender with a 30-3 victory over No. 24 California on Saturday night.

The win capped an emotional week for the Trojans (4-1, 2-1 Pac-10) that started with a frightening weight-room accident that sent Johnson to the emergency room with a crushed larynx and throat after a weight bar fell on his neck. With Johnson watching from his hospital room, his teammates delivered against Cal.

“Stafon is a leader and a big brother in the (locker) room, so I wanted to come out and win the game for him,” McKnight said. “We were thinking about him the whole time tonight, and we know he was liking what he saw back home.”

Damian Williams caught eight passes for 101 yards and also returned a punt 66 yards for a score, and Jordan Congdon kicked three field goals to give USC its second straight win following a 16-13 loss at Washington two weeks ago.

Mays set the tone with an interception on the first drive of the game for Cal (3-2, 0-2), and the defense kept it up all game, not allowing a point until the fourth quarter for the second straight week.

“That’s how we play,” Mays said. “We know what we have in the locker room, and we’ve been playing close to our potential for the last few weeks.”

With the defense playing this well, the Trojans are making it easy on freshman quarterback Matt Barkley. Barkley was inconsistent in this game, completing 20 of 35 passes for 283 yards and an interception. He moved the ball down the field with ease but struggled to punch the ball into the end zone.

“If our defense is playing like that, then we don’t need a whole lot of offense,” Barkley said. “I thought we executed well when we had to.”

The Trojans had the answer for whatever trick Cal coach Jeff Tedford tried. Best had five runs off direct snaps in Cal’s version of the wildcat and the Bears went for it twice on fourth down. But with a passing game that can’t click, there is no room to run for Best and no production for a Cal team that was held out of the end zone at home for the first time since 1998.

“We had a chance to make plays in the passing game and didn’t do a good job of it,” Tedford said. “We can’t be one-dimensional. We’re going to have to be able to throw the football to be successful. We were zero-dimensional today because we couldn’t run it and we couldn’t throw it.”

Even trick plays didn’t work. In the third quarter, Cal came out of a timeout with Shane Vereen standing next to its sideline, just on the field of play. With no defender near him, Vereen went out for a pass, but instead of a big play, Kevin Riley threw high for an incompletion. The next play was a screen pass that left tackle Mike Tepper caught for an illegal touch penalty.

That was the kind of day it was for Riley, who was 15 for 40 for 199 yards and an interception in his second straight sub-par performance. He was just 12 for 31 for 123 yards in last week’s 42-3 loss at Oregon.

Amy Sancetta / AP A frightening weight-room accident sent USC running back Stafon Johnson to the emergency room with a crushed larynx and throat after a weight bar fell on his neck.

Riley’s interception in the end zone on the opening drive was his first of the year. Six plays later, McKnight dived into the end zone at the end of his long run. Williams’ 66-yard punt return made it 17-0 in the second quarter and the rout was on. Williams caught eight passes for 101 yards.

After scoring 146 points in its first three games, Cal has just six the past two weeks. Cal has failed to score a touchdown just three times in eight years under Tedford, with two of them coming the past two weeks. The other was against USC in 2007, the last time Cal lost at home before Saturday.

“We have athletes all over the field. That’s the hardest part is how we’re not getting the ball in the end zone with everyone we have,” Riley said.

Even the speedy Best couldn’t get going against the Trojans. On one play in the first half, he was chased down by Mays for no gain on a third-and-2 run.

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