Helicopter was being used to spray fungicide on fields of corn. The pilot approached and overflew one field checking for hazards and wind conditions, and then began a spray swath across the field. Midway through the first swath, the pilot heard the low rotor horn sound. Manually rolling the throttle up did not prevent the rotor RPM from continuing to drop. The pilot then attempted a quick stop and a controlled autorotation into the corn. The helicopter was substantially damaged by the impact but the pilot who was wearing a four point shoulder harness and a helmet was not injured.