AUGUSTA, Ga. (Reuters) - Dustin Johnson finally clinched an elusive second major title with a five-stroke victory at the Masters on Sunday, staving off self-doubt as he overcame a shaky start to his final round to end with a tournament-record low score at Augusta National.American Johnson led throughout, though only by one stroke early, and did not drop a shot in the final 13 holes on his way to a four-under-par 68 and an unprecedented 20-under-par 268 total. Australian Cameron Smith and South Korean Im Sung-jae kept Johnson honest after starting four shots back. They both shot 69 to tie for second on 15-under, but in the end they had no answer to the champion, who played the final 13 holes bogey-free. “It was a very difficult day,” an emotional Johnson said after having a famous Green Jacket draped over his shoulders by last year’s champion Tiger Woods.