THE PILOT STATED THAT HE WAS APPLYING AGRICULTURAL CHEMICALS. HE SAID THAT DURING CLIMBOUT AT THE COMPLETION OR A SWATH RUN, HE CROSSED OVER A RIDGE. AFTER CROSSING THE RIDGE LINE, HE EXPERIENCED A SEVERE DOWNDRAFT. HE WAS UNABLE TO ARREST THE AIRCRAFT'S DESCENT AND COLLIDED WITH THE TERRAIN. NO MECHANICAL PROBLEMS WITH THE AIRPLANE WERE REPORTED BY THE PILOT.