Think about all the diseases that can attack a plant. The exact set of diseases actually depends on the past history of the soil, including the previous plants grown, management practices used, what diseases the neighbors might have, soil type, and climate. Basically, a minimum of 12 species of bacteria were required to control one species of pathogen on cucumber foliage during one summer. That's just one cultivar of plant, just one summer. Consider that the same thing is probably the case for each pathogen, for each part of the plant, for each set of environmental conditions.
Diversity is most probably critically important.
Each summer with different environmental conditions requires a different set of bio-control organisms. We know that there should be at least a 100 to 1000 million total individuals of all species in a healthy soil, but we're not certain how many individuals of each species is required to prevent disease. No, just maximize the diversity of beneficial bacteria and fungi in the soil. Provide as complex a set of food resources as possible. Let the exudates produced by the plants provide the selective conditions needed to grow the bacteria or fungi needed by the plants to protect their root system.