DURING AN AERIAL APPLICATION FLIGHT, THE LEFT WING SEPARATED AND THE AIRCRAFT HIT THE TOP OF A HOUSE. EXAMINATION OF THE WING REVEALED A FATIGUE FRACTURE IN THE WING'S LOWER SPAR CAP. METALLURGICAL EXAMINATION OF THE FRACTURE REVEALED THAT IT ORIGINATED ON OPPOSITE SIDES OF A RIVET HOLE. INTERGRANULAR CORROSION WAS FOUND AT ONE OF THE HIGH CYCLE FATIGUE FRACTURE ORIGINS. THE OPPOSITE FRACTURE ORIGIN HAD BEEN MECHANICALLY MUTILATED. IN AUG 1977, ALL-METAL WINGS WERE INSTALLED ON THE AIRPLANE PER A SUPPLEMENTAL TYPE CERTIFICATE.