THE PILOT WAS CONDUCTING AERIAL SPRAYING OF AN ORCHARD. HE HAD COMPLETED 12 PASSES AND WAS MANEUVERING FOR THE LAST PASS. ACCORDING TO THE PILOT, HE WAS HEADED IN 'AN EASTERLY DIRECTION, WENT INTO AN AG TURN, TO HEAD WEST, WHEN AIRCRAFT WAS IN WESTERLY DIRECTION, STILL IN TURN, WE HIT TREES AND GROUND. ACCORDING TO THE FAA, THE PILOT DID NOT MAINTAIN AN AIRSPEED OF 80 MPH IN THE TURN AND THE AIRPLANE STALLED.