On August 2, 2007, about 1100 central daylight time, a Piper PA-25-235, N7263Z, operated by Dan's Crop Service, and piloted by a commercial pilot sustained substantial damage while maneuvering near Overly, North Dakota. The 14 CFR Part 137 aerial application flight was operating in visual meteorological conditions without a flight plan. No injuries were reported. The local flight originated about 10 minutes prior to the accident. The pilot reported that he was spraying his own wheat crop when the accident occurred. He stated that he had completed a pass and was executing a left turn when he decided to make a "clean-up" pass. During the turn, the left spray boom, which hung about 18 inches below the wing, contacted the crop resulting in the airplane continuing to turn and descend into the terrain. The pilot reported that there were no mechanical problems with the airplane prior to the accident.