The Collaborative International Dictionary of English v.0.48:

Picker \Pick"er\, n. [From Pick.]
   1. One who, or that which, picks, in any sense, -- as, one
      who uses a pick; one who gathers; a thief; a pick; a
      pickax; as, a cotton picker. "Pickers and stealers."
      --Shak.
      [1913 Webster]

   2. (Mach.) A machine for picking fibrous materials to pieces
      so as to loosen and separate the fiber.
      [1913 Webster]

   3. (Weaving) The piece in a loom which strikes the end of the
      shuttle, and impels it through the warp.
      [1913 Webster]

   4. (Ordnance) A priming wire for cleaning the vent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

picker
    n 1: a person who chooses or selects out [syn: picker,
         chooser, selector]
    2: someone who gathers crops or fruits etc.

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "picker":
   Bauer, agriculturalist, agriculturist, agrologist, agronomist,
   banjoist, coffee-planter, collective farm worker, crofter, cropper,
   cultivator, dirt farmer, dry farmer, farm laborer, farmer,
   farmhand, fiddler, folk guitarist, gentleman farmer, granger,
   grower, guitar-picker, guitarist, harper, harpist, harvester,
   harvestman, haymaker, husbandman, kibbutznik, kolkhoznik, kulak,
   lute player, lutenist, muzhik, peasant, peasant holder, planter,
   plowboy, plowman, psalterer, raiser, rancher, ranchman, reaper,
   rustic, sharecropper, sower, string musician, strummer,
   tea-planter, tenant farmer, thrummer, tiller, tree farmer,
   truck farmer, twanger, violinist, yeoman, zitherist