The two most important herbicides in this group are paraquat and diquat, both of which are contact herbicides that damage plant tissues quickly, causing the plants to appear frostbitten because of cell membrane destruction. This rapid degeneration occurs within hours of application, making these novel herbicides also useful as preharvest desiccants for seed crops, cotton, soybeans, sugarcane, and sunflowers. Diaquat is sometimes used in aquatic weed control, and paraquat applications include stubble clearing, pasture renovation, inter-row weed control in vegetable crops, and weed control in plantation crops. Paraquat earned a degree of notoriety several years ago when the United States government collaborated with the Mexican government to treat marijuana fields with the defoliant. Many drug users who were exposed to paraquat-laced marijuana developed nosebleeds, headaches, vomiting, and breathing disorders.
Microorganisms also contribute heavily to argricultural crop destruction.
Most plant diseases can be controlled to some extent with modern synthetic fungicides. In general, fungal plant diseases are basically more difficult to control with chemicals than are insects because the fungus is a plant living in close quarters with its host. This organochlorine insecticide can be considered as the pesticide of the greatest historical significance, due to its effect on the environment, agriculture, and human health. First synthesized by a German graduate student in 1873, it was rediscovered by Dr. Paul Mueller, a Swiss entomologist, in 1939 while searching for a long-lasting insecticide for the clothes moth. DDT subsequently proved to be extremely effective against flies and mosquitoes, ultimately leading to the award of the Nobel Prize in medicine for Dr. Mueller in 1948.