The pilot stated that the tailwheel spring cross tube broke when the tailwheel landing gear touched down. After landing, the airplane made an immediate turn and spun around. The left main landing gear collapsed, and the left wing struck the ground. The airplane sustained substantial damage to the wing. Postaccident examination of the tailwheel spring cross tube revealed that the outside diameter of the tube was about 0.002 inch smaller than the specified nominal outside diameter of 0.875 inch, and the wall thickness was 0.0009 inch thinner than the specified nominal wall thickness of 0.049 inch for the original tube design. Examination of the tube showed signatures consistent with overstress deformation. The part dimensions indicated the part could have been from original manufacture and likely had not been replaced since at least 1985, when the drawing for replacement parts was changed to have a thicker wall, 0.120 inch. An uncoated steel part of that age used in an airplane used in aerial application service would be expected to have a more corroded surface than what was observed on the accident part. The surface condition suggests it may have been recently sanded, possibly removing about 0.001 inch of material around the outside diameter. The recent change in outside diameter likely did not significantly affect the strength of the tube. The bending strength of the tube was reduced by about 2 percent relative to the nominal tube based on a comparison of the bending moments required to reach given stress at the exterior surface. Similarly, the cross-sectional area was also reduced by just 2 percent. While some increase in impact loading on the tube could be expected due to the additional clearance between the tube and the fitting, the failure is more likely attributed to loads that exceeded the maximum design load for the tailwheel. An additional margin of safety could have been achieved if the tube had been replaced with the thicker-walled replacement part rather than apparently having been cleaned up and reused. The metallurgical examination of the tailwheel spring cross tube revealed that it failed in overload, which indicates that it was not the initiating event for the loss of control. It is likely that the pilot failed to maintain airplane control during the landing sequence, which resulted in a ground loop and the substantial damage to the left wing and the fractured tailwheel spring cross tube.