On Wednesday, August 18, 1993, at 0530 hours Pacific daylight time, an Air Tractor AT-301, N2330V, collided with an irrigation ditch during an emergency landing near Dos Palos, California. The emergency landing was precipitated by a total loss of engine power. The pilot was conducting a local visual flight rules aerial application flight under 14 CFR Part 137. The airplane, operated by Spain Air, Inc., Dos Palos, sustained substantial damage. The certificated commercial pilot, the sole occupant, was not injured. The flight originated from a farm airstrip about 5 miles north of the accident site at 0500 hours. The pilot reported the airplane's engine began to run rough while returning to the San Juan Airstrip to pick up another defoliating chemical load. The pilot heard a grinding noise followed by a complete loss of engine power. The pilot landed the airplane in a cultivated field where it collided with the irrigation ditch during the landing roll. The airplane's engine was examined at Aero Engines, Inc., Los Angeles, California. During the examination, it was determined that the master rod failed. The master rod was too badly damaged to determine the mode of failure.