The Finn's Yaris rebounded out of a heavy compression and clipped a rock on the rear right, bouncing the car into a rock on the opposite side of the road which ripped off the rear left wheel and suspension components.
It was Lappi's second start of the season and first rally on gravel in the hybrid-powered Yaris.
His crash leaves Hyundai Motorsport's Ott Tänak leading the rally by 21.5sec over MSport Ford's Craig Breen.
Finland
Italy
Hungary