The pilot reported that during an aerial application flight he was spraying a mature wheat field along north facing hillsides. As the pilot finished his last pass he flew west along the steep hillsides. A strong gust of wind struck the airplane and the left wing dropped into the wheat. Subsequently, the airplane spun and cartwheeled in the wheat field before coming to a rest; the fuselage and empennage were substantially damaged. The pilot reported no mechanical failures or malfunctions with the airframe or engine prior to the accident.