On September 11, 1993, at 0915 central daylight time, an Airtractor AT301, N36613, collided with a tree while attempting an emergency landing near Deatsville, Alabama. The aerial application flight operated under 14 CFR Part 137 with no flight plan filed. Visual weather conditions prevailed at the time of the collision. The airplane sustained substantial damage and the pilot was not injured. The flight departed Deatsville at 0900 hours. The pilot dispensed the first load of chemicals and was enroute to the staging area for another load. While at cruise flight, the pilot reported that the engine backfired three times, followed by a loss in engine power. He immediately increased the ower lever, but the engine did not respond. The pilot selected an open field and attempted an emergency landing. During the landing roll the airplane struck a tree. During the wreckage examination at the accident site, the engine was started on the airplane with all existing systems; however, the engine would not develop full power. A subsequent examination of the engine at an overhaul facility, disclosed a crack in the number three cylinder head. The crack extended the distance between the two spark plugs, and according to the overhaul facility, cylinder head cracking is common on the old radial type engines.