Source : Webster's Revised Unabridged Dictionary (1913)

Disparager \Dis*par"a*ger\, n.
   One who disparages or dishonors; one who vilifies or
   disgraces.

Source : WordNet®

disparager
     n : one who disparages or belittles the worth of something [syn:
          {detractor}, {depreciator}, {knocker}]