The pilot stated that after takeoff the flight proceeded to the destination airport and the automated weather observing station (AWOS) indicated the wind was favoring runway 29. The flight continued and while on final approach to runway 29, the AWOS indicated the wind was from 360 degrees at 7 knots; no gusts were reported. The airplane was landed first on the main landing gear, and within seconds of lowering the tailwheel, the airplane suddenly veered to the right. She applied right aileron, left rudder, and left brake to stop the turn but was unsuccessful. The airplane departed the runway and collided with a ditch. After coming to rest she and her son exited the airplane. The spars of the lower left wing were damaged. Inspection of the airplane following recovery by a FAA certificated airframe and powerplant mechanic with inspection authorization (A&P/IA) revealed no evidence of preimpact failure or malfunction of the tailwheel assembly. The tailwheel pin was not fractured or failed, and he did not detect any excessive play or looseness in the tailwheel assembly.