The pilot reported that during approach, as the airplane crossed the runway 9 threshold, he reduced engine power and the airplane experienced a severe sink. Upon touchdown the airplane veered left and the he applied right rudder pedal; however, the airplane continued off the left side of the runway into a ditch and came to rest upright. During the accident sequence, the airplane's left main landing gear collapsed, which resulted in substantial damage to the lower fuselage and left wing. The recorded wind at an airport located approximately 15 miles south of the accident site, about the time of the accident, was from 110 degrees at 11 knots, gusting to 20 knots. Examination of the wreckage by a Federal Aviation Administration inspector did not reveal any preimpact mechanical malfunctions.