The pilot was tasked with spraying crop with fungicide in a paddock situated in an undulating timbered area. The wind was from the east at 10 - 15 kt, and the pilot elected to commenced the first spray run in a downwind direction. This required a steep descent from over the tree tops to pass beneath a swer power line located across his flight path. The pilot reported that he encountered severe turbulence and windshear in the lee of the trees, and was unable to prevent the aircraft, which had a full hopper, from descending into the top of a tree. This caused an immediate deterioration in airspeed, damage to the wing leading edges and removal of the spray pump. The aircraft flew out of the tree in a semi-stalled condition, and being unable to maintain height to fly over the swer power line the pilot dived the aircraft to pass beneath it. The aircraft wheels became entangled in the crop, slowing the aircraft further, and caused it to impact the ground heavily. The impact damaged the landing gear, and removed the hopper door allowing the fungicide load to dump. Due to the impact force, and the load being dumped, the aircraft became airborne again and was yawed to the right by the crop dragging on the landing gear. It then collided with a fence which turned it through 180 degrees as it came to a stop.