On August 9, 1995, about 1930 Pacific daylight time, N9574J, a Cessna T188C, operated by B & R Aerial Crop Care, Inc., collided with terrain during an emergency landing in Mattawa, Washington, and was substantially damaged. The emergency landing was precipitated by a partial loss of engine power while maneuvering for an aerial application. The commercial pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The local flight was conducted under 14 CFR 137. In a written statement, the pilot reported that he took off with 170 gallons of spray mix and flew five miles to the spray site. The pilot stated that as he pulled up from the first spray pass of the second load, he made a right turn, then rolled into a left turn to re-enter another spray pass. As the airplane entered the left turn, the engine lost partial power. The pilot extended the flaps to 20 degrees and applied full power. The engine did not react, and the pilot applied the remaining flaps and initiated a forced landing to the adjacent field. During the landing, the airplane collided with the terrain. The airplane spun around and slid sideways before coming to rest. The engine was removed to a secured facility for teardown and inspection. The inspection was witnessed by Federal Aviation Administration inspectors from the Spokane, Washington, Flight Standards Field Office. No evidence was found to indicate a mechanical failure or malfunction during the inspection. The inspectors noted a small amount of a "fuzzy" type substance on the back side of the screen in the manifold valve. One inspector reported that the substance partially covered the screen in an area where fuel would naturally concentrate. The inspector also stated that the substance appeared to have been on the screen for an extended period of time. The inspector also stated that the airplane had been frequently operated under full power settings without difficulty.