Despite your best efforts at controlling soil born pathogens, they will thrive as long as a preferred food supply is present.  In a fairly short time, they can multiply from negligible numbers to a population capable of launching a devastating attack.  For instance, vinca is highly susceptible to root rot, which is caused by a fungus found in small numbers in most soils.  If allowed to feed on vinca for several years in a row, however, the fungus will become rampant.  To eliminate a pathogens that is partial to only one or a few species, try rotating your plants around the garden.  This solution isn’t practical for perennials and other plants in permanent locations, but it is easily accomplished with annuals and vegetables.  Instead of planting them in the same spot year after year, skip at least 2 years before repeating an annual or vegetable in a site.