Adam Armstrong scored his eighth goal of the season as Southampton eased to a 3-1 Sky Bet Championship victory and condemned Wayne Rooney to a third straight defeat as Birmingham manager.

Forward Armstrong now has eight career goals against Blues, more than he has scored against any other side, as he settled the game with a fine finish.

He had set up Taylor Harwood-Bellis’ opener before Carlos Alcaraz tapped in – both players’ first goals of the season. Jay Stansfield pulled one back for the visitors but it could not help end Rooney’s winless run.

Southampton had monopolised the opening stages without creating anything clear-cut until Harwood-Bellis nodded in the ninth-minute opener.

Armstrongs Stuart and Adam worked a short-corner routine before the latter lifted for the Manchester City loanee to power in his third professional goal, and first since last September.

Blues rallied but Oliver Burke’s lashed effort into the side netting – their only shot of the first half – poked the hosts back into life.

Kamaldeen Sulemana and Stuart Armstrong linked up smartly on the left flank before the Ghanaian slid across the face of the goal for Alcaraz to push in.

Rooney had been booed after Wednesday’s 2-0 home defeat by Hull, and Saints supporters rubbed their advantage in with a round of “sacked in the morning” aimed at the Manchester United great.

But rather than rub further salt into the wound of Rooney’s poor start, Birmingham fans supported their boss with cries of “Rooney, Rooney” and “Wayne Rooney’s Blue Army”.

Their support should have been rewarded with a spot-kick but goalkeeper Gavin Bazunu got away with flattening Burke in the box.

Saints should have gone into the break with more than a two-goal advantage as Harwood-Bellis’ free header from a corner skipped wide and Adam Armstrong clipped a one-on-one over John Ruddy but wide of the goal.

After the break, Stuart Armstrong tamely ended a well-worked move and Adam Armstrong’s diving header flashed wide.

But the hosts floundered and Blues capitalised. Stansfield jumped off the bench, met Lukas Jutkiewicz’s knockdown, bullied his way past Kyle Walker-Peters and rifled into the top corner – all within 52 seconds of his 57th-minute introduction.

It was Stansfield’s fourth goal of the season and extended Southampton’s wait for a home clean sheet to 28 matches.

But Saints held onto the ball well and made sure of the result in the 86th minute when top-scorer Adam Armstrong pounced and swivelled onto Sam Edozie’s nod down.

Scott Hogan curled one onto the roof of the goal in additional time but it could not stop Saints moving to a sixth game unbeaten to cement their place in the play-off spots.