On June 16, 2013, about 0720 central daylight time, an Air Tractor 802A, N8522S, registered to AGviation Inc., of Minot, North Dakota, sustained substantial damage after a collision with trees and the ground, during an aerial application flight. The pilot sustained serious injuries. Visual meteorological conditions prevailed in the vicinity and no flight plan was filed. The aerial application flight was being conducted under the provisions of 14 Code of Federal Regulations Part 137. The flight originated from the Minot International Airport (MOT) about 0700.The pilot reported that he took off about 0700 to apply the first load of the day. He commenced spraying the field utilizing east to west passes along the south end of the field. On the accident pass, the pilot pulled up over trees and felt a small bump, surmising that the airplane had struck branches. The airplane flew straight for a second or two and then started rolling to the left. The pilot applied full right aileron and full right rudder, but the airplane continued to roll to the left. The airplane then struck the ground nose low, about 100 degrees angle of bank to the left. The airplane sustained structural damage to the fuselage and both wings. An FAA inspector who examined the accident site and airplane wreckage reported that no mechanical anomalies were discovered that could have contributed to the accident. The pilot reported that he was well rested and the flying shift was during his normal working hours.