The pilot of a tailwheel equipped airplane reported that during the landing roll the tailwheel shimmied, and the airplane veered right of the runway centerline. He was able to maintain directional control and taxied to the loading area to prepare the airplane for the next agricultural application flight. He reported that this was the fourth landing of the day and there were no anomalies with the previous landings. While taxiing the airplane to the runway for takeoff the pilot had to use left rudder and brake to keep the airplane going straight on the taxiway. During the takeoff roll the tailwheel began to shimmy again and the airplane veered to the right. The pilot reduced the engine power and applied brakes to abort the takeoff but was unable to maintain directional control of the airplane. The airplane exited the runway and ground looped in a grassy area adjacent to the runway. Substantial damage was noted to the tailwheel’s empennage supporting structure and the tailwheel exhibited deformation to the right. Examination of the airplane revealed the tailwheel assembly was partially separated from the empennage supporting structure and all observed fractures were consistent with overload separation. It is likely that the tailwheel was damaged during a previous landing and that damage resulted in the shimmy during the previous landing and the pilot’s difficulty in maintaining control during the taxi and takeoff. An examination of the of the tailwheel assembly, tailwheel control, and rudder revealed no mechanical anomalies that would have precluded normal operations before the damage occurred. Given the difficulties taxiing for takeoff, the pilot should not have continued with the takeoff with a known anomaly.