Victory for the home side sees them extend their unbeaten run to four games, while the Sounders are now without a win in four and sit two points off the pace.
The only goal of the evening came on the stroke of half time and it was an all-around defensive calamity.
That left him out of position as the ball fell to Kalif Alhassan on the edge of the area, and the midfielder drilled a low effort into the net, with Yedlin turning his back on the shot and Ianni sticking out a leg half-heartedly.
The match became ill-tempered in the second half, which resulted in a flurry of yellow cards and a red for Osvaldo Alonso for fighting.
Despite having a man disadvantage, the Sounders could still have got something from the game. However, Steve Zakuani's shot came crashing off crossbar in the last minute of the game as they threw caution to the wind.