On April 7, 1993, at about 1006 hours Pacific daylight time, a Rockwell Aero Commander ST-R, N1783S, crashed in an open field about 13 miles southeast of San Jose, California. The pilot was conducting a local visual flight rules aerial application flight. The airplane, operated by Ag-Rotors, Inc., dba Cantrell Ag Air, was destroyed. The certificated commercial pilot, the sole occupant, sustained serious injuries. Visual meteorological conditions prevailed. The flight originated at Hollister Airport, Hollister, California, at about 0930 hours. Mr. Jim Friel, principal operations inspector, Federal Aviation Administration (FAA), San Jose [California] Flight Standards District Office, conducted the on scene investigation. Inspector Friel reported that the ground loader stated that the pilot was spraying the field with Bamville mixed with MCTA/AMINE herbicide. At the completion of a spray run the aircraft pulled up sharply and nosed over during the procedure turn-around. The operator reported that the pilot executed 4 to 5 aerial application passes over the wheat field. During the last spray pass, the airplane entered into a "stall and spun into the ground."