No disease can be controlled throughout a seasonal cycle by one single other species of organism. On a laboratory petri plate maybe one organism can control one disease-causing organism, but we're dealing with real world agriculture in this column. For example, Mahaffee showed that control of one disease-causing fungus on cucumber for one summer season required 12 different species of bacteria to be present. A different set of species would be required for a different set of weather conditions. The point is that without all 12 species of bacteria present and functioning, the disease-causing fungus would have caused disease.
Diversity is all important.
The food to feed all 12 species had to be present as well. The plant provided most of those foods, through leaf surface and stem exudates. In soil, a larger proportion of the food for the organisms comes from soil organic matter, and waste products from the growth of other organisms. To feed organisms in the soil requires even more diversity than on aboveground parts of plants. Someday we will know, but that day is realistically a hundred years in the future.