The pilot reported that he was taxiing for departure on runway 07. As he turned the aircraft to loine up, the left main gear leg fractured. The left wingtip and the left horizontal stabiliser contacted the ground. Subsequent examination of the fracture showed a fatigue failure emanating from a small impression or dent on the leading edge of the leg. The leg appeared to have been cracked for some considerable time, and eventually failed longitudinally in a straight line. The aircraft had been operating for some years on rough runways, and had been engaged in parachuting operations continually. This type of operation generates an unusually high number of cycles (takeoffs and landings) and would contribute to a fatigue failure of this nature.