Understanding soil inoculants
Most biopesticide organisms work by either producing a substance that inhibits or kills the pest (antagonism) or by reducing the availability of food or shelter for the pathogen (competition). The most widely used biopesticide is Bacillus thuringiensis, which produces a toxin that kills soil grubs and nematodes. Specific strains of Bacillus subtilis are widely used as a fungicide. This bacterium colonizes plant roots, competing with fungi for that niche, and prevents the rapid growth of fungal pathogens.
Protozoa and nematodes that eat bacteria are also thought to play an important role in controlling pathogens (predation). As with any ecosystem, both competition and predation tend to keep populations in balance.
Plant Resistance Stimulants
In addition to acting as a direct inhibitor of plant pathogens, some fungi and bacteria stimulate the plant to activate its own defense mechanisms. This is called induced systemic resistance. In response to chemical signals from the microorganisms, plants may change physiological responses so that there are fewer symptoms of the pathogen. This may include strengthening its cell wall to resist infection, or releasing antibiotics (such as terpenes) that reduce pathogen attack. The chemical signals that pass back and forth from microorganisms to plants are specific; consequently, microorganisms and the chemical that may cause induced systemic resistance in one plant species may not work in another.