The forward came close again minutes later as he was afforded space and time outside the box, only to put his effort just over the bar.
Piatti caused problems for the visitors again and he nearly caught Busch out with a lobbed effort to his back post, but the keeper managed to backpedal and tip the ball over.
In the second half, Cato released the run of Chris Wondolowski down the left and the forward raided into the box. He had space to get his effort away, but he scuffed his shot well wide of the post.
San Jose were beginning to take a hold of the game and Harris almost nodded them in front as he headed wide after latching onto a Shea Salinas cross to the near post.
McInerney made Harris pay for his missed chance as he gave his side the lead in fortunate circumstances, tapping the ball in from close range with his knee following Busch's parried save from Marco di Vaio.
The Impact secured the win as Duka received the ball down the left and cut inside onto his right foot to bend a curling shot into the bottom corner.