Understanding chemical pesticides requires knowing what they are and what they are not. What they are is poisonous. What they are not is medicinal. The belief that pesticides are medicines for ailing plants is misguided and it implies a curative quality that is lacking. An infected plant can seldom be cured. Necrotic tissue cannot regain its life, holes chewed in leaves will not restore themselves, and galls will not diminish.
As noted already, injury to plants can be quantitative (reducing the number of marketable plants) and/or qualitative (rendering the plant unattractive to consumers). To be most effective, pesticides need to be on the plant before the pathogen or insect invader arrives and to kill it promptly on its arrival. These pesticides will be regarded as protectants. If the pathogen is already at the site of infection or the insects are already feeding on the plant, then the pesticides must kill them immediately. Such pesticides are termed eradicants.
Thus far, the pesticides described are active against the pest only, not the host plant. Chemical control of weeds can become more complicated since the pest, like the host, is a higher plant. With the range of genetic differences narrowed, the chemicals must be formulated more precisely. The term used to describe all the chemicals that kill plants is herbicides. Those that kill all green plants are nonselective herbicides. Those that kill some kinds of plants and not others are selective herbicides. These assorted products are also characterized by whether they kill on direct contact with the weed or after the chemical has been incorporated systemically into the weed. Herbicides may kill the weed before the crop emerges (preemergence), as the crop emerges (at emergence), or after the crop has emerged (postemergence).