Submitted to: Journal of Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 14, 2007
Publication Date: September 5, 2007
Citation: Mazzola, M. 2007. Manipulation of Rhizosphere Bacterial Communities to Induce Suppressive Soils. Journal of Nematology. 39(3):213-220. Interpretive Summary: Soil ecosystems possess a wealth of biological resources that can be harnessed for use in control of plant diseases. Soils that naturally have low incidence of disease are commonly referred to as suppressive soils. Studies have been conducted to determine the properties of these soils that are important in this natural disease suppression, the goal being to identify those soil components that function to limit plant disease. Once the important disease-limiting components are identified, the hope would be to transfer these elements to other soils as a means to induce natural biological disease control. These studies demonstrated that growing wheat in orchard soils prior to planting apple or applying seed meal from specific brassica species, significantly depressed root infection by the fungal pathogen Rhizoctonia solani. It was shown that wheat suppressed this pathogen by enhancing populations of certain bacteria that are able to inhibit growth of R. solani. However, not all wheat varieties were able to reduce root infection by R. solani. It was shown that wheat cultivars that were best at providing disease control were the same cultivars that supported the highest populations of the bacterium Pseudomonas putida btp A. Brassica seed meal amendments were shown to provide control of R. solani by enhancing populations of Streptomyces, which were capable of inducing plant resistance to this fungal pathogen. The ability to transform resident microbial communities in a manner which induces natural soil suppressiveness will have a significant role in environmentally sustainable systems for management of soilborne plant pathogens.
Technical Abstract: Disease suppressive soils have been documented in a variety of cropping systems, and in many instances the biological attributes contributing to suppressiveness have been identified. While these studies have often yielded an understanding of operative mechanisms leading to the suppressive state, significant difficulty has been realized in the transfer of this knowledge into achieving effective field-level disease control. Early efforts focused on application of individual or mixtures of microbial strains recovered from these systems, and known to function in specific soil suppressiveness. However, introduction of biological agents into non-native soil ecosystems typically yielded inconsistent levels of disease control. An alternative strategy is the cropping of specific plant species or genotypes, or the application of soil amendments with the goal of selectively enhancing disease suppressive rhizobacteria communities. This approach has been utilized in a system attempting to employ biological elements resident to orchard ecosystems as a means to control the biologically complex phenomenon termed apple replant disease. Cropping of wheat in apple orchard soils prior to re-planting the site to apple provided control of the fungal pathogen Rhizoctonia solani AG-5. Disease control was elicited in a wheat cultivar-specific manner and functioned through transformation of the fluorescent pseudomonad population colonizing the rhizosphere of apple. Cultivars that induced disease suppression enhanced specific fluorescent pseudomonad genotypes with antagonistic activity toward R. solani AG-5. Alternatively, suppression of Rhizoctonia root rot in response to brassicaceae seed meal amendment required the activity of the resident soil microbiota, and was associated with elevated apple rhizosphere populations of Streptomyces spp. Application of individual Streptomyces spp. to soil systems provided control of R. solani to a level and in a manner equivalent to that obtained with the seed meal amendment. These and other examples suggest that management of resident plant beneficial rhizobacteria may be a viable method for control of specific soilborne plant pathogens.