As scientific research becomes more pervasive in agriculture, we’re learning more about the complexity of plant-soil communities and the potential for controlling plant diseases and pests through ecological management instead of the application of toxic chemicals.
Advances in understanding the microbial ecology, combined with new techniques for enumerating and isolating microbial species, have yielded the use of antagonistic soil bacteria instead of chemical fungicides, for example, to protect seed from rot-causing soil pathogens. We know that Pseudomonas fluorescens has plant growth-promoting properties, but it’s not exactly clear how these strains work. Nevertheless, we reason that:
- the bacteria might induce systemic resistance in the host plant, so it can better resist attack by a true pathogen.
- the bacteria might outcompete other (pathogenic) soil microbes, e.g., by siderophores, giving a competitive advantage at scavenging for iron.
- the bacteria might produce compounds antagonistic to other soil microbes, such as phenazine-type antibiotics or hydrogen cyanide.
We do know that when a preparation of this bacteria is applied as seeds are sown, the seeds germinate safely, even in soil known to be infested with seed-rotting fungi. Seeds sown without the treatment decay before they can germinate.
The role of soil ecology in protecting plants from disease is not limited to below-ground infections. Beneficial rhizobacteria have an intriguing mode of action called Induced Systemic Resistance, or ISR, which helps plants ward off both infections and insect pests above and below ground. The process begins when the plant’s root system is colonized by beneficial rhizobacteria that cause a buildup of a signaling chemical. The chemical signal is translocated up to the shoot, where it stimulates leaf cells to mount a chemical defense against a specific pathogen, even before the pathogen as arrived at the scene!
As we begin to harness the full potential of the microbial community, we’re going to be able to do more with less and get off the roller coaster of building our soil biology and then tearing it down with the use of toxic chemicals. Sterilization of soils kills most of the beneficial as well as pathogenic organisms. The problem is that the pathogen population tends to rebound much more quickly, and then we’re back facing the same uphill battle again.