The pilot had completed two thirds of the task of spraying a field of cotton which was bounded by a road and powerlines at one end. The pilot had been flying under the wires and had kept a watch for traffic along the road. On what turned out to be his last pass, he again lined up to fly under the wires. As he approached the wires he saw a truck travelling along the road, on a collision course with his aircraft. Trees had obstructed the truck from view. As the pilot took avoiding action, the right mainwheel of the aircraft struck the road surface. The right main gear was bent backwards. The pilot flew his damaged aircraft to the airstrip and landed. At a slow speed it ground looped and came to rest without any further damage.