Destruction of sick plants is recommended. This is particularly effective in the case of diseases that may spread rapidly through the field.
For example plants are sometimes suffering from wilting diseases, which can be either caused by fungi or bacteria. Infested plants will die and not produce any harvest. The farmer should quickly remove and destroy the wilted plants in order to avoid the spreading of the pathogens to healthy plants.
Also the spreading of many other diseases (e.g. virus diseases in tomato) can be avoided or delayed by the destruction of sick plants.
Of course, regular visits to the field by the farmer are necessary. This will help to timely detect any sick plants and to destroy them before the disease can spread through the entire field.
While collecting sick plants care should be taken to not spread the disease through the field. Collecting the sick plants in plastic bags will help to avoid the spreading of spores.