On March 26, 2002, at 1122 eastern standard time, an Air Tractor AT-400A, N3159G, collided with the ground following an aerial application maneuver in Hastings, Florida. The airplane was registered to and operated by the commercial pilot under the provisions of Title 14 CFR Part 137, and visual flight rules. Visual meteorological conditions prevailed and no flight plan was filed. The pilot received serious injuries and the airplane sustained substantial damage. The flight originated from a private strip in Hastings, Florida, on March 26, 2002, at 1050. According to the pilot, while spraying fungicide to a potato field and with no indication of anything wrong, he heard a loud explosion in the vicinity of the wing center section at about 60 feet above the ground. After the explosion, he had no control over the airplane and it impacted the ground separating the right wing. Examination of the wreckage found the right wing tip near the initial impact point. The path of the airplane's debris field was to the north with a turn towards the east. The airplane then impacted into an irrigation ditch 261 feet from the initial impact point. Further examination of the wreckage found portion of the right wing spar imbedded into the ground sticking up in a near vertical attitude. This section of the inboard right wing spar was recovered and forwarded to the NTSB Materials Laboratory in Washington, DC, for further examination. Examination of the section of the wing spar by the NTSB Materials Laboratory found that the wing separated from the airplane due to undetected fatigue cracking in the most-outboard splice bolt hole in the lower spar cap of the right wing. Air Tractor reported that this was the first known instance of fatigue cracking or fracture on a modified lower spar cap. As a result of this investigation the FAA issued an airworthiness directive (AD) that applies to all Air Tractor, Inc., Models AT-300, AT-301, AT-302and AT-400A airplanes that have aluminum spar caps; certain Air Tractor Models AT-400 airplane that have aluminum spar caps; and all Models AT-300 and AT-301 airplanes that have aluminum spar caps and are or have been converted to turbine power. In part the AD requires operators to inspect the wing centerline splice joint for cracks and, if any crack is found, replace the affected wing spar lower cap.