On August 16, 2008, at 1000 central daylight time, an Air Tractor AT-400A, N7307W, operated by Wilbur-Ellis Air LLC, collided with a tree during an off airport forced landing in Luverne, Minnesota. The commercial pilot was not injured. The airplane sustained substantial damage to the left wing. The aerial application flight was operating under Title 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed and no flight plan was filed. The local flight originated from the Luverne Municipal Airport (LYV), Luverne, Minnesota, at 0950. The pilot reported she had sprayed approximately 40 acres when the engine began making a "popping" sound with a higher pitched whistle. She reported she pulled back on the elevator control to gain as much altitude as possible and began looking for a place to land. She stated that while looking out the right side of the airplane, she noticed flames coming from the right side exhaust stack. The pilot determined she was not able to land on the country road, so she decided to try and fly between a couple of trees to land in an open field. She stated the airplane touched down prior to the trees and it spun to the right, impacting one of the trees with the left wing. Inspection of the engine revealed that all of the compressor turbine blades were damaged with most of them having broken off. Damage was also present to the outlet side of the compressor turbine guide vanes.