On July 12, 1993, about 1000 central daylight time, a Piper, PA-25-235; N8906L, sustained substantial damage when it struck a tree and subsequently crashed into a field during an aerial application flight near Clarence, Missouri. The flight was operating under 14 CFR Part 137 in visual meteorological conditions. The private pilot received minor injuries. The pilot was spraying an herbicide during an aerial application flight in Clarence, Missouri. Having just completed two "clean- up" passes in an adjacent field, the pilot proceeded to his target field. After circling the field to look for obstacles, the pilot completed a west to east run. The airplane turned and set up for an east-west pass. Lining up with his flagman and the tallest tree in the tree line along the west end of the field, the pilot proceeded to make a standard 900 yard ag run. The pilot completed the pass at approximately 105 miles per hour, shut off the spray booms before the required change in attitude to exit the field was made. While looking over the right wing root to be sure the tree had been cleared, so he was able to start an ag-turn, the pilot heard " a sound similar to a saw blade in a saw mill." He looked forward to find "foreign matter' being slung from the propeller. This foreign matter was the branches and leaves of the tree he thought he had cleared by 10 feet or more. The pilot attempted to control a chemical leak from the boom, to no avail, and made a controlled forced landing in the field. The aircraft's landing gear, hopper, cowling, propeller, wings and bottom skin were all damaged as the aircraft slid to a stop in the pea field.