WHILE SPRAYING MELONS, THE PILOT STARTED A PULL UP FROM A SWATH RUN, BUT WAS NOT ABLE TO COMPLETE THE MANEUVER BECAUSE THE FLIGHT CONTROLS WERE JAMMED. DURING THE ATTEMPT TO FREE THE CONTROLS THE RIGHT MAIN LANDING GEAR STRUCK THE GROUND. ACCORDING TO THE PILOT, THE FLIGHT CONTROLS JAMMED WHEN A STRING USED AS A GUST LOCK WEDGED INTO THE STICK CONTROL. THE AIRPLANE COLLIDED WITH THE GROUND BEFORE THE PILOT COULD RELEASE THE GUST LOCK LINE. THE STRING WAS NOT PROPERLY SECURED PRIOR TO THE DEPARTURE.