The Collaborative International Dictionary of English v.0.48:

mayapple \mayapple\, may apple \may apple\n.
   1. North American herb (Podophyllum peltatum) with
      poisonous root stock and an edible though insipid
      egg-shaped yellowish fruit; called also wild mandrake.

   Syn: May apple, wild mandrake, Podophyllum peltatum.
        [WordNet 1.5]

   2. The fruit of the mayapple[1].
      [PJC]

WordNet (r) 3.0 (2006):

mayapple
    n 1: North American herb with poisonous root stock and edible
         though insipid fruit [syn: mayapple, May apple, wild
         mandrake, Podophyllum peltatum]