THE PILOT WAS PRACTICING FOR AN UPCOMING AGRICULTURAL APPLICATION CERTIFICATION, AND WAS SPRAYING WATER. A WITNESS REPORTED THAT ON THE PILOT'S FIFTH PASS OF A FIELD, THE AIRPLANE COLLIDED WITH A TREE. THE AIRPLANE THEN CLIMBED ABOUT 300 FEET, AND 'NOSE DIVED' INTO THE GROUND. AN EXAMINATION OF THE WRECKAGE REVEALED NO EVIDENCE OF A MECHANICAL MALFUNCTION OR FAILURE.