NEAR COLLISION ON GND AT 6DB WHEN CROP DUSTER ACFT TOOK OFF DOWNWIND FROM RWY 31 OVER RPTR'S ACFT IN POS FOR TKOF AT DEP END OF RWY 13. CROP DUSTER PLT DID NOT UTILIZE CTAF TO ANNOUNCE HIS MOVEMENTS OR INFORM OTHER ACFT.

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WIND 170 DEGS AT 15-20 KTS (SOCK ERECT). I CALLED ON CTAF FOR BACK-TAXI ON RWY 13. UPON OUR ARR AT THE TURNAROUND ON THE RWY 13 END; WE SAW THE CROP DUSTER BACK-TAXIING BEHIND US. WE TAXIED ONTO THE FAR EDGE OF THE RWY TO GIVE THE CROP DUSTER ENOUGH ROOM TO TAXI INTO THE TURNAROUND BEHIND US. WE WERE SURPRISED TO SEE HIS WHEELS LEAVE THE RWY. INSTEAD OF A BACK-TAXI; HE WAS ON A DOWNWIND TKOF RUN WITH A 20 KT TAILWIND. I DID NOT EXPECT ANYONE TO TAKE OFF FROM SUCH A STRONG TAILWIND. HE CLRED OUR ACFT BY MAYBE 50 FT AT THE MOST. IN THE FUTURE; I WILL ASSUME ALL ACFT ON A RWY AT AN UNCTLED ARPT ARE ON A TKOF RUN UNTIL I CAN CONFIRM OTHERWISE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISES THAT ALTHOUGH HE WAS MONITORING THE CTAF AND UTILIZING IT TO BROADCAST HIS INTENTIONS; AT NO TIME DID HE HEAR A XMISSION FROM THE CROP DUSTER ACFT. HE STATES THAT; ALTHOUGH HE ISN'T FAMILIAR WITH SPECIFIC CROP DUSTER ACFT TYPES; THIS WAS A TYPICAL LOW WING; HIGH PWRED ACFT WITH A VERY NOSE HIGH ATTITUDE WHICH MAKES FORWARD VISIBILITY DIFFICULT UNTIL THE TAIL IS RAISED ON THE TKOF ROLL.