WordNet (r) 3.0 (2006):

defoliant
    n 1: a chemical that is sprayed on plants and causes their
         leaves to fall off

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "defoliant":
   acaricide, anthelmintic, antibiotic, antiseptic, bug bomb,
   carbamate insecticide, chemosterilant,
   chlorinated hydrocarbon insecticide, contact poison, disinfectant,
   eradicant, fumigant, fungicide, germicide, herbicide,
   insect powder, insecticide, microbicide, miticide,
   organic chlorine, organic phosphate insecticide, pesticide, poison,
   rat poison, roach paste, roach powder, rodenticide, stomach poison,
   systemic, systemic insecticide, toxic, toxicant, toxin, venin,
   venom, vermicide, virus, weed killer