The Collaborative International Dictionary of English v.0.48:

Phytologist \Phy*tol"o*gist\ (f[imac]*t[o^]l"[-o]*j[i^]st), n.
   One skilled in phytology; a writer on plants; a botanist.
   --Evelyn.
   [1913 Webster]

WordNet (r) 3.0 (2006):

phytologist
    n 1: a biologist specializing in the study of plants [syn:
         botanist, phytologist, plant scientist]