While returning to the airstrip to reload for another spray run, the aircraft experienced severe engine surge with associated rough running. The pilot selected the fuel boost pump on, but with little improvement. He then elected to make a forced landing. While stretching the glide, the aircraft stalled at about five feet AGL and landed heavily in a soft earth paddock. This resulted in the left main landing gear failing, followed by the nose impacting with sufficient force to tear the engine from its mounts. Inspection by the company found the main fuel filter not lock wired allowing the filter assembly to come loose and starve the engine of fuel.
