During an aerial application maneuver, the acft collided with the ground. An exam of the wreckage indicated that it impacted in a steep nose down attitude while rotating to the left. The flaps were found extended 25 deg & the airspeed gage was found indicating 55 kts. The T-handle for dumping the chemical load was found in the stowed position & had not been activated. The fuel selector was positioned to the right fuel tank & both fuel tanks were found with fuel remaining. Flt control continuity was established & the throttle was found in the full forward position. Neither the plt nor the owner possessed a certificate to conduct aerial applications. Also, there was no record that the plt had obtained knowledge & skill test, required by 14 CFR 137. The plt had been issued a 2nd class medical certificate on 10/23/79 with a limitation to wear corrective lenses. However, no glasses or contact lenses were found. The density alt was approx 7200 ft. Reportedly, the winds were strong & gusty.