The pilot reported that the tailwheel airplane began to "pitch left" during landing. The pilot made crosswind corrections and could not maintain wings level. The left wing touched the runway and the airplane continued straight. The airplane subsequently yawed to the right. The pilot was unable to correct the yaw with rudder application and engine power, and the airplane exited the runway. The airplane's left landing gear folded when it contacted the terrain off the runway. The airplane sustained substantial damage to its left wing spar. The pilot did not indicate that there were any airplane mechanical malfunctions prior to the airplane exiting the runway.