On June 5, 2024, about 0630 eastern daylight time, a Piper PA-25-235 airplane, N8715L, was substantially damaged when it was involved in an accident near Colquitt, Georgia. The commercial pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The flight originated from a private airstrip. The pilot reported that preflight operations were normal. He reported the engine contained 10 quarts of oil and there was no water found in the fuel samples. The engine run-up and takeoff were normal; however, the engine started “missing” as the airplane climbed to about 200-300 ft mean sea level. The engine speed dropped about 50 rpm. The pilot confirmed the engine controls were fully forward and the engine then lost all power. He then made a forced landing into a wooded swamp area. The airplane came to rest upright in shallow water. The pilot exited the airplane and was met by first responders. Both wings and the fuselage were substantially damaged during the accident sequence. The wreckage was recovered to a salvage facility where an examination of the wreckage was performed. The engine remained partially attached to the firewall through the tubular engine mount. The firewall was cut away and the engine was placed on a hoist. The engine crankshaft was rotated by turning the propeller and continuity of the crankshaft to the rear gears and valvetrain was confirmed. Compression and suction were observed on all six cylinders. The interiors of the cylinders were examined with a lighted borescope and no anomalies were noted. The carburetor was partially disassembled and about two ounces of liquid was found in the bowl. The liquid tested positive for the presence of water with water-finding paste. The carburetor floats and other internal components were undamaged. The magnetos were removed from the engine; both produced spark on all towers when rotated with an electric drill. The airplane’s fuel system was then examined. The airplane was fitted with a single, fuselage-mounted fuel tank. The fuel filler cap was secure and in place. No signs of fuel leakage were observed. The tank vent was unobstructed. The gascolator bowl and screen were removed. Residue on the screen and in the bowl resembled sand, grit, unknown organic matter, and water. Of note, the lower fuselage in the area of the gascolator was found submerged in swamp water at the accident site. The fuel shutoff valve from the fuel tank to the gascolator was removed. The position of the valve before the accident could not be determined due to impact damage and deformation to the forward fuselage and engine mount. When the valve was manipulated manually, the through-port was not totally obstructed when placed to the “closed” position. The valve arm moved back and forth with minimal resistance. The valve was then disassembled and the connection to the interior plastic gate was broken and the gate did not move when the valve stem was actuated. The valve was sent to the NTSB Materials Laboratory for further examination. The examination revealed that the valve failed from the two fractured rivets that joined the stem flange to the adjacent internal component of the valve. The lower rivet failed from intergranular corrosion attack. The upper rivet showed signs of overstress in bending and shear. Once the failure occurred, the valve stem could move without manual input, obstructing the port and compromising fuel flow to the engine.