The Collaborative International Dictionary of English v.0.48:

Rooter \Rooter\, n.
   1. One who, or that which, roots; one that tears up by the
      roots.
      [1913 Webster]

   2. One who roots, or applauds. [Slang, U. S.]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

rooter
    n 1: an enthusiastic devotee of sports [syn: sports fan,
         fan, rooter]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "rooter":
   adherent, admirer, amateur, applauder, appreciator, booster, buff,
   claqueur, collector, commender, devotee, dilettante, encomiast,
   eulogist, eulogizer, extoller, fan, fancier, follower, groupie,
   idolater, idolizer, lauder, panegyrist, plugger, praiser, promoter,
   puffer, tout, touter, votary, worshiper