A 17 inch portion of one of the prop blades fractured and separated from the 76 inch diameter prop while the aircraft was at cruise. During the forced landing, the aircraft struck a fence post. Examination of the prop revealed the presence of filiform corrosion across the backs of both blades. Metallurgical analysis of the fracture surface revealed that the fracture originated at a corrosion pit on the blade back. Total time of the prop was 2225 hours. The prop was reconditioned by the manufacturer 1241 hours prior to the accident. The manufacturer recommends that this model prop be reconditioned every 1000 hours. The original reconditioning procedure specified anodizing the prop and painting with lacquer after repair. Examination of the prop indicates that this was done. The manufacturer has since revised the reconditioning procedure to use a chemical conversion coating instead. The manufacturer specifies that the prop be frequently wiped with an oily rag between reconditionings to assist in corrosion prevention.