The pilot reported that after successfully completing three practice touch-and-go landings, he extended the landing gear for a fourth landing and noted that the left main gear indicator light was not illuminated. The pilot cycled the landing gear and all three lights then illuminated. A touch-and-go was accomplished and the landing gear was retracted. During the final landing, the landing gear was extended and the left main landing gear light did not illuminate. The pilot cycled the landing gear, however this time the light did not illuminate and he could not confirm that the left main gear was down and locked. The landing approach was continued and shortly after touchdown, the left main landing gear collapsed. The left wing contacted the runway surface and the airplane departed the left side of the runway, coming to rest in a ditch. The left wing and stabilator were substantially damaged. Postaccident examination of the left main landing gear wheel well revealed excessive corrosion, dirt, and wear on the landing gear control arm, strut, torque link, gear spring, and associated connections. During a manual extension of the left main landing gear, the gear locked into the down position and the cockpit gear indicator light illuminated. A review of the airplane's maintenance records revealed that a 100-hour inspection was completed about 2 months before the accident.