PILOT STARTED PULL UP FOR REVERSAL TURN IN AERIAL APPLICATION MANEUVER. THE RIGHT WING SEPARATED FROM THE AIRPLANE. AIRPLANE CRASHED IN FIELD AND WAS CONSUMED BY POST CRASH FIRE. METALLURGICAL EXAMINATION REVEALED PREEXISTING FATIGUE CRACKS IN LOWER SPAR CAP OF THE RIGHT WING ORIGINATING FROM TWO MISDRILLED HOLES. AIRWORTHINESS DIRECTIVE TO INSPECT FOR WING SPAR FATIGUE CRACKS HAD NOT BEEN COMPLIED WITH.