The reason why is that once the fungus is established in a plant, the fungus causes damage and is often difficult to get rid of. The most commonly used type on fruits and vegetables is protectant spray. Herbicides - are chemicals used to control unwanted plants. These chemicals are a bit different from other pesticides. Some herbicides kill every plant they contact, while others kill only certain plants.
Nonselective herbicides are toxic to all plants.
These materials are often used when no living plant is wanted in the area. For example, nonselective herbicides could be used for cleaning under guardrails or for total control of weeds in industrial areas. Selective herbicides kill some plants with little or no injury to other plants. Usually selective types will kill either mostly broadleaved plants, or mostly grassy plants, or individual types of plants. These are useful for lawns, golf courses or in areas with desirable trees.