Horticulture/Excluding Plant Pathogens
The best way to control plant diseases is to simply exclude the pathogen from the garden. While this is often impossible because many diseases are wind-borne, putting some effort into exclusion is worth the time.
One way to exclude pathogens is to "quarantine" any new plants for a wile before planting them in the garden. This is especially important for some of the more serious soil-borne fungi such as Phytophthora, Verticillium, and Sclerotinia.
Removing alternate hosts is effective for many pathogens.
Controlling vectors is important for many diseases.