At Washington, D.C. United extended its lead to four points over second-place Sporting Kansas City in the Eastern Conference with a 2-0 win over the New York Red Bulls.
Robbie Keane beat Jhon Kennedy Hurtado at the top of the box and lobbed the ball over Kennedy, who was caught in no-man's land, for a 2-0 lead at the 68th minute and Zardes made it 3-0 in the 71st after a set of deft passes and misdirection between Donovan and Keane.
The Galaxy are two points off the lead in the Western Conference behind Seattle.
Luis Silva's goal in the 57th minute proved the difference and Eddie Johnson sealed the win in stoppage time for D.C. United's victory over New York.
United 'keeper Bill Hamid sent a long ball across midfield and Fabian Espindola flicked a header forward to Silva. Silva then beat Red Bulls goalkeeper Luis Robles to the ball and punched it forward to set up an open-net finish.
Johnson looped the ball over Robles in the first minute of added time after he got behind the defense on a ball sent in from near midfield from Perry Kitchen.
New York remains two points behind in the race for the final Eastern Conference playoff spot.