On August 2, 2018, about 1035 central daylight time, a Cessna 188 airplane, N5523S, was substantially damaged during a forced landing following a partial loss of engine power near Watkins, Minnesota. The pilot was not injured. The airplane was registered to a private individual and operated by Classic Arrow Inc. as a Title 14 Code of Federal Regulations Part 137 agricultural application flight. Day visual meteorological conditions prevailed. The flight was not operated on a flight plan. The local flight originated from the Litchfield Municipal Airport (LJF), Litchfield, Minnesota, about 0945.The pilot reported that the airplane was fully fueled before the flight. After takeoff, he flew about 20 minutes to the field to be sprayed. He proceeded to survey for hazards and completed a few spray passes. He subsequently noted a loss of engine speed (rpm) after completing an application pass. He pushed the propeller control forward in an attempt to increase the engine/propeller speed which was successful for a short period of time. However, the engine speed began to decrease again, and he observed a decrease in oil pressure combined with an increase in the oil temperature. He moved all engine controls – throttle, propeller, and mixture – full forward. As he returned for a forced landing to the corn field being sprayed, the oil pressure was about 7 psi. As the airplane came over the field, the engine was "coughing." After impacting the corn, the airplane "angled up on the nose" before coming to rest. He recalled the weather conditions as "very good," with a light wind, clear sky, and more than 10 miles visibility. The pilot informed a Federal Aviation Administration (FAA) inspector that the loss of engine power was not catastrophic in nature. Rather, the pilot observed the oil pressure dropping with the oil temperature rising and decided to pull the mixture to shut the engine off. He subsequently landed in the corn field just below the airplane. The FAA inspector reported that the airplane sustained substantial damage to the right wing. Postaccident airframe and engine examinations conducted by the inspector revealed no anomalies that could be attributed to a preimpact failure or malfunction.