WASHINGTON — In the biggest game of the season, the Montreal Impact faltered badly in the second half.

Luciano Acosta had a goal and three assists, Wayne Rooney and Paul Arriola each scored two second-half goals, and D.C. United beat the Impact 5-0 on Saturday night.

United (10-11-8) closed within two points of the Impact (12-15-4) for the sixth and final playoff spot in the Eastern Conference. D.C. has two games in hand.

"In the first half, I wasn't under the impression that we would play such a horrible second. There are certain behaviours we need to correct," said Impact coach Remi Garde. "These are important games in which we need all 11 players on the field to be at their best physically, mentally, and in their aggressiveness. Today, we were outclassed by our opponent in these categories."

Acosta turned on Yamil Asad's lead pass with a right-footed shot to open the scoring in the 17th minute.

Rooney swerved past Acosta's cross, created an opening with his first touch and slotted it back inside the left post to make it 2-0 in the 48th.

United made it 3-0 in the 61st when Rooney found Acosta open on the left side of the area and Acosta sent a pinpoint pass to Arriola for the tap-in. Arriola chipped home Acosta's pass in the 78th and Rooney caught the goalkeeper out of the net in the 82nd.

Bill Hamid had five saves for his second shutout of the season.

"When we look back on the season, this will be the moment that we decided if we stepped up or we crumbled," said Impact striker Quincy Amarikwa. "I'm confident of the quality we have in the locker room, and we're going to use this as motivation to move forward into the playoffs."