In the 80th minute, Le Toux took a pass from Roger Torres and shot just wide on a low roller past the far post from outside the box.
Union midfielder Amobi Okugo got a weak shot off from the top of the box during second-half stoppage time.
Over the final 25 minutes, the Union applied the same type of relentless pressure that they had for most of the first half.
Union forward Danny Mwanga didn't dress for the game. Afterward he said his right knee was bothering him.
"I wasn't 100 percent, and the training and coaching staff decided not to risk and wait until I was 100 percent," said Mwanga. He added that he wasn't sure he would be able to play in Saturday's home game against Chivas USA.
Even without Mwanga, the Union went with more of an attacking lineup.
Midfielder Veljko Paunovic made his second straight start. Jack McInerney got a start in midfield, the third of his career.
Ruiz insisted that recent reports out of Mexico that he was headed to that country to play once the transfer period begins July 15 have not been a distraction.
"Not at all," he said. "Like I said Tuesday, nobody has contacted me, and it's just rumor, and I am very happy to be here."
When asked if there was any truth to the rumors, team manager Peter Nowak said: "He made it very clear [Tuesday] that it is not true."
The Union certainly didn't lack scoring chances in the first half. In fact, they dominated a Kansas City team that was sloppy with its passing.
Guided by former Delran star Peter Vermes, Kansas City appeared content to sit back, playing for the tie.
Kansas City keeper Jimmy Nielsen kept his team in the game in the first half, with his best moment coming on a kick save on a shot by McInerney just inside the box.
Contact staff writer Marc Narducci at 856-779-3225, email@example.com, or @sjnard on Twitter.