On June 14, 1995, at 1000 central daylight time, an Ayres S2R-T34, N627LL, was substantially damaged during landing near Iowa, Louisiana. The commercial pilot was not injured. Visual meteorological conditions prevailed for the local aerial application flight operated by Deep South Flying Service, Inc., under Title 14 CFR Part 137. The flight originated at 0950 and a flight plan was not filed. During a telephone interview, conducted by the investigator-in- charge, the pilot reported that the aerial application flight originated from a private grass airstrip. A drainage ditch approximately 18 feet wide and 4 feet deep was located along the west side of the airstrip. While landing to the north, the right brake malfunctioned during the landing roll. Subsequently, the airplane departed the left side of the airstrip, rolled through the drainage ditch and came to rest in a field. The pilot examined the brakes and stated that brake fluid leaked from the brake piston "O" ring. The Federal Aviation Administration (FAA) inspector examined the airplane and reported damage to the left wing spar and stringers. He further reported damage to the empennage tubing, engine, and propeller.