|Kloepper, J. W.|
|Reddy, M. S.|
Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2005
Publication Date: 8/1/2005
Citation: Burelle, N.K., Kloepper, J., Reddy, M. 2005. Population dynamics of plant growth-promoting rhizobacteria as transplant amendments and their effects on indigenous rhizosphere microorganisms. Applied Soil Ecology. Vol 31/1-2:91-100. Interpretive Summary: Plant growth-promoting rhizobacteria (PGPR) are beneficial soil bacteria that colonize plant roots and result in increased growth. Because typical disease control levels observed with PGPR are less than those achieved with chemicals, it is feasible to utilize PGPR as components in integrated management systems that include reduced rates of chemicals and cultural control practices. This approach is becoming increasingly more important as many agricultural chemicals undergo intense scrutiny with regards to their human toxicity and environmental impact. The concept of introducing these beneficial organisms into the rhizosphere through the transplant plug is based on the hypothesis that their establishment in the relatively clean environment of the planting media would afford them an opportunity to develop stable populations in the seedling rhizosphere, and that these populations would persist in the field. It was also hypothesized that early exposure to PGPR might precondition the young plants to resist pathogen attack after transplanting in the field. It is important to understand the population dynamics of the applied beneficial organisms in order to optimize their application methods and timing. It is also important to understand the effects of added strains on populations of indigenous beneficial bacteria including fluorescent pseudomonads, which commonly occur in the rhizosphere, and are known to suppress pathogen establishment and disease spread. Our results demonstrate that adding the PGPR in the BioYield formulation to the potting media establishes stable populations in the rhizosphere of pepper that persist throughout the growing season. Additional aqueous applications of the PGPR during the growing season did not increase these populations, although these applications did increase plant growth. Also, application of these strains did not adversely affect populations of beneficial indigenous rhizosphere bacteria.
Technical Abstract: Field trials were conducted in Florida on bell pepper (Capsicum annuum) to monitor the population dynamics of two PGPR strains (Bacillus subtilis strain GBO3 and Bacillus amyloliquefaciens strain IN937a) applied in the potting media at seeding and at various times after transplanting to the field during the season. In-field drenches of an aqueous formulation were used for the mid-season applications. The effects of the applied PGPR and application methods on bacterial survival, rhizosphere colonization, plant growth, yield, and indigenous rhizosphere microorganisms were assessed. The Gram-positive PGPR applied to the potting media established stable populations in the rhizosphere that persisted throughout the growing season. Additional aqueous applications of the PGPR during the growing season did not increase the population size of the applied strains compared to treatments only receiving bacteria in the potting media; however, they did increase plant growth compared to the untreated control to varying degrees in both trials. Most treatments also reduced disease incidence in a detached leaf assay, which indicates that systemic resistance was induced by the PGPR treatments. However, treatments did not result in increased yield, which was highly variable. Application of the PGPR strains did not adversely affect populations of beneficial indigenous rhizosphere bacteria including fluorescent pseudomonads and siderophore-producing bacterial strains. Treatment with PGPR increased populations of fungi in the rhizosphere but did not result in increased root disease incidence. This fungal response to the PGPR product was likely due to an increase in nonpathogenic chitinolytic strains resulting from the chitosan carrier of the PGPR used for the application in the potting media.