A C172 EXPERIENCED AN NMAC WITH A CROP DUSTER AT VTI.

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ON A TRAINING FLT TO VTI FOR TOUCH-AND-GOES AT VTI ON RWY 16. PVT PLT RATED STUDENT WAS FLYING WHILE I WAS PROVIDING INSTRUCTION AND WATCHING FOR TFC. THE STUDENT ANNOUNCED OUR POS ON THE CTAF FOR EACH LEG OF THE PATTERN EXCEPT UPWIND LEG. WE DID OUR FIRST 2 TOUCH-AND-GOES WITH NO EVENT. AFTER WE TOOK OFF FROM THE SECOND LNDG; I NOTICED A CROP DUSTER LAND ON RWY 16 AS WE TURNED XWIND. THERE WAS NO ANNOUNCEMENT ON THE CTAF. AS WE PROCEEDED ON DOWNWIND; I NOTICED ANOTHER CROP DUSTER DEPARTING RWY 27. AGAIN; THERE WERE NO ANNOUNCEMENTS ON THE CTAF BESIDES US. I WATCHED THE CROP DUSTER TURN OUT TOWARDS THE S. I TURNED BACK TO WATCH FOR MORE TFC ON BASE AND FINAL. AS WE TURNED FINAL AT APPROX 500 FT AGL; I NOTICED THE CROP DUSTER THAT HAD DEPARTED RWY 27 WAS NOW FLYING LOW DOWN RWY 9. IT LOOKED AS IF HE WAS OVER THE S SIDE OF THE RWY SPRAYING THE GRASS. I'M NOT SURE IF IT WAS TRAINING OR IF THERE WAS ACTUAL AERIAL APPLICATION BEING DONE AT THE ARPT OR IF IT WAS SHOWBOATING FOR THE OTHER AERIAL APPLICATORS TEMPORARILY LOCATED ON THE FIELD (I SAW AT LEAST 4 OTHER CROP DUSTERS ON THE RAMP). WE MADE OUR CALL FOR FINAL AS THE OTHER ACFT PASSED IN FRONT OF US FROM OUR L TO R. I MADE ANOTHER CALL THAT WE WERE ON FINAL. THE CROP DUSTER PULLED UP SHARPLY BUT DIDN'T INITIATE AN IMMEDIATE TURN. WE MISTAKENLY THOUGHT THE CROP DUSTER NOTICED US AND WAS GOING TO LET US COMPLETE OUR LNDG BEFORE COMING BACK IN. HOWEVER; THE CROP DUSTER DID MAKE A TURN AT AN ALT OF ABOUT 500 FT AGL. IT BEGAN A DIVE BACK DOWN TOWARD THE RWY. WE WERE AT APPROX 100 FT AGL AT THE TIME. I TOOK CTL OF THE AIRPLANE FROM THE STUDENT AND IMMEDIATELY INITIATED A GAR. I FELT IT PRUDENT TO VEER TO THE L TOWARD THE CROP DUSTER IN ORDER TO PASS BEHIND AND ABOVE IT. A R TURN WOULD'VE KEPT US ON A COLLISION COURSE AND COULD'VE POSSIBLY CAUSED US TO LOSE SIGHT OF IT. WE PASSED WITHIN 200 FT OF THE OTHER AIRPLANE. I RETURNED CTL OF THE AIRPLANE TO THE STUDENT AND WE CONTINUED ON AROUND IN THE PATTERN FOR ANOTHER ATTEMPT AT MAKING A LNDG. I INSTRUCTED THE STUDENT TO MAKE A WIDE PATTERN IN CASE THE CROP DUSTER MADE ANOTHER PASS SO THAT WE WOULD BE OUTSIDE OF THE CROP DUSTER'S TURN. SURE ENOUGH; THE CROP DUSTER CONTINUED TO MAKE AERIAL APPLICATION PASSES ALONG THE RWY IN BOTH DIRECTIONS. THERE WAS NEVER ANY ACKNOWLEDGEMENT OF OTHER TFC OR ANNOUNCEMENTS ON THE RADIO. WE CONTINUED WITH OUR APCH AS FAR AS ABOUT 400 FT AGL ON FINAL AND THEN DECIDED TO BREAK OFF THE LNDG AND DEPARTED THE ARPT TO THE W.