What bad does pesticides do to a plant?

Pesticides are designed to kill pests. If a plant is a pest (weed) a pesticide intended to control it will kill the plant.

Pesticides that are designed to kill insects and beetles that damage crops nevertheless may leave a residue of the poison on the plant itself. This may not hurt the plant but may be hazardous to other animals that eat the plant. This is why such food should be washed before being eaten