Protection is the use of some protective barrier between the susceptible part of the suspect or host and the pathogen.
In most cases this is a protective spray or dust applied to the plant in advance of the arrival of the fungus spores; sometimes it means killing insects or other inoculating agents; sometimes it means the erection of a windbreak or other mechanical barrier.
Fungicidal sprays that act as protectants are used to control Cercospora leaf spot of sugar beet, especially in those fields where inoculum has carried over from the previous year. The principle of protective fungicides is to disrupt the natural sequence of infection. These fungicides act on the leaf surface to kill the newly germinated spores. Flowable sulfur is used as a protectant fungicide to control powdery mildew of sugar beet.
There is a long list of chemicals available in the literature that can be used in present-day protective spraying and dusting, along with eradicant chemicals. The commercially sold chemicals are provided with instructions or notes on compatibility and possibilities of injury.
A commercial grower can do his marijuana plants irreparable harm instead of the good he intends if he doesn’t follow the instructions supplied. Spraying is never to be undertaken lightly or thoughtlessly. Read all of the fine print on the label; be sure of the dosage and the safety of that particular chemical on the plant species to be protected.