On August 5, 2005, at 1700 central daylight time (cdt), an American Champion 7GCBC (Citabria), N515KG, was substantially damaged when it nosed over while landing on runway 23 (4,430 feet by 75 feet, asphalt) at the Watertown Municipal Airport (RYV), Watertown, Wisconsin. Visual meteorological conditions prevailed at the time of the accident. The instructional flight was operating under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91 without a flight plan. The pilot and his flight instructor were not injured. The local area flight departed RYV around 1630. The pilot reported that he was obtaining instruction toward a tailwheel endorsement. The pilot stated that he had no experience in the accident airplane make/model prior to the accident flight. The pilot reported that after touchdown the airplane veered to the left and he used right rudder input in an attempt to realign with the runway centerline. The pilot stated that "next we were heading extreme right, I had a feeling the tailwheel was off the ground." The pilot reported that the airplane departed the runway and entered a muddy ditch where it nosed over. The flight instructor reported that after departure they practiced slow flight, power on/off stalls, and steep turns before returning to the airport to practice landings. The flight instructor stated that she "briefed" the student on "how the approach to landing would go." The flight instructor reported that because the reported winds were "calm" they decided to use runway 23 because of its longer length. The flight instructor stated that she was "talking [the pilot] through the landing, and also helping out with [control] stick movement." The flight instructor reported that the pilot landed the airplane on the runway centerline. The flight instructor stated that after touchdown she told the pilot to "put the [control] stick in your lap and pin the tail." The flight instructor reported that as she began to bring the control stick aft, the pilot pushed the control stick forward which elevated the tailwheel off the runway. The flight instructor stated that the pilot also "pushed right rudder" which turned the airplane "sideways." The flight instructor reported that she "immediately brought the [control] stick back and tried to correct the rudder." The flight instructor stated that the tailwheel came back down to the runway, but she was unable to "correct the rudder" because of "resistance." The flight instructor reported that she felt the pilot had "frozen on the controls" and she told the pilot "my plane, my plane." The flight instructor stated that the only thing she could do was "slam on the brakes" in an attempt to stop the airplane before it entered a ditch alongside the runway. The flight instructor reported that the "mud was so deep that the brakes were not effective and [airplane] kept sliding" into the ditch where it nosed over. The operator of the aircraft reported that the flight instructor had approximately 20 hours of experience in conventional gear (tailwheel) airplanes. The flight instructor reported having 16.6 hours in the accident airplane make/model. Subsequent to the accident, the operator raised their minimum experience requirements for flight instructors in tailwheel aircraft. Additionally, the operator requires that tailwheel flight instructors demonstrate proficiency in tailwheel instruction via a checkride with the chief pilot or manager.