On August 9, 2005, at 1430 central daylight time, a Grumman G-164, N553Y, operated by Mueller Aviation, LLC., received substantial damage on impact with terrain following a reported loss of engine power near Georgetown, Minnesota. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 137 aerial application flight was not operating on a flight plan. The commercial pilot received minor injuries. The flight originated from a private airstrip near Georgetown, Minnesota, at 1420. The pilot stated that while completing a turn on the fifth pass over the field he was spraying, the engine speed decreased "rapidly." The engine began to fluctuate when he "fully" advanced the throttle. The engine continued to lose power as the airspeed decreased "quickly" while coming out of a turn. The airplane impacted trees along the field, impacted a powerline pole along the field, and then impacted a field. Inspection of the engine by the Federal Aviation Administration revealed no anomalies associated with the reported loss of engine power.