The pilot reported that he was spraying a crop with the runs oriented in a racetrack pattern across the wind, which was south-easterly at 15 knots. This resulted in a turn into wind at one end and a turn downwind at the other end. He experienced difficulty in preventing the aircraft drifting away from the paddock during the downwind turns and was widening the turns in an attempt to correct the problem. During the third downwind turn the aircraft stalled but the pilot was unable to level the wings due to trees in the path of the aircraft. The right wing struck a fence and cartwheeled into a paddock.