THE COMMERCIAL AG PLT WAS OPERATING A SINGLE PLACE CESSNA 188 FROM A ROAD NEAR RAPELJE, MT. THE PLT ESTIMATED THE WINDS FROM THE WEST AT 5 KTS, GUSTING TO 12 KTS. DURING A LANDING TO THE NORTH TO RELOAD WITH FERTILIZER, A GUST OF WIND STRUCK THE ACFT AND THE PLT SAID HE LOST DIRECTIONAL CONTROL. THE ACFT DEPARTED THE ROAD TO THE LEFT AND STRUCK A DITCH, DAMAGING THE LEFT MAIN GEAR AND LEFT WING.