N9148C

While back-taxiing the tail wheel-equipped agricultural application airplane for takeoff, the pilot reported that he encountered a gust of left quartering tailwind and was "unable to maintain directional control" of the airplane. The airplane veered left and departed the left side of the runway. The tail wheel block attaching bolts subsequently failed and the tail wheel spring then impacted the ground, stopping the airplane's lateral rotation, and shearing all four of the longerons just forward of the vertical stabilizer. The pilot reported no preaccident mechanical malfunctions or failures that would have precluded normal operation.