Why must protective sprays be applied before a pathogen infects a plant?

Protective sprays must be applied before a pathogen infects a plant because they function as protectants, which create a chemical barrier between the plant surface and the pathogen.1

These substances are not absorbed or translocated through the plant tissue; therefore, they can only protect the specific parts of the plant that have been treated prior to the arrival and invasion of the pathogen. Once a pathogen has successfully penetrated the plant or established an infection, these surface-level protectants are ineffective at stopping the disease. Because these chemicals can be washed away by rain or irrigation, broken down by sunlight, or left behind as the plant grows new, untreated foliage, repeated applications are often necessary to maintain this preventative barrier.1