Submitted to: Plant and Soil
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/1/2006
Publication Date: 6/27/2006
Citation: Cohen, M., Mazzola, M. 2006. Resident bacteria, nitric oxide emission and particle size modulate the effect of Brassica napus seed meal on disease incited by Rhizoctonia solani and pythium spp. Plant and Soil. 286:75-86. Interpretive Summary: Organic soil amendments often have been promoted as a means to control soilborne plant diseases. However, the effective use of such an environmentally sensitive method of disease control has been impeded by a lack of understanding of how such amendments provide disease control. Tissues from plants belonging to the Brassicacea have been promoted as a soil amendment for the control of soilborne plant diseases due to their production of glucosinolates, which yield anti-microbial compounds upon hydrolysis. In this study, control of the fungal pathogen Rhizoctonia solani, which causes Rhizoctonia root rot of apple was achieved by using a low glucosinolate content Brassica napus seed meal (RSM) as a soil amendment. If the naturally occurring soil microbiology was altered or diminished by soil pasteurization prior to application of RSM, the capacity of the amendment to control this fungal pathogen was eliminated. This finding demonstrates that RSM does not provide control of R. solani through a chemically-mediated process such as "biofumigation", but rather does so indirectly by transforming the microbial community resident to soil. Application of RSM results in a dramatic increase in the generation of nitric oxide for a period of 2-3 weeks post-amendment. When the release of nitric oxide was chemically inhibited, disease control was also abolished. These findings indicate the nitric oxide is important to disease suppression, and likely impacts the plants ability to protect itself from fungal infection by triggering plant defense mechanisms.
Technical Abstract: Brassica tissues are often promoted as a soil amendment for control of soilborne plant disease due to their production of glucosinolates, which yield anti-microbial compounds upon hydrolysis. Studies demonstrated that control of Rhizoctonia root rot of apple in response to Brassica napus seed meal (RSM) could be abolished if soil was pasteurized prior to incorporation of the amendment. Disease suppressiveness could be restored to pasteurized RSM-amended soils by inoculation with any of several different Streptomyces strains. Maximal rate of nitrification in soil was observed within two weeks of RSM amendment. The capacity of RSM to provide control of Rhizoctonia solani during this time period could be eliminated by application of the nitrification inhibitor nitrapyrin, which abolished the release of nitric oxide. Apple seedling mortality resulting from proliferation of Pythium spp. in RSM amended soils was dependent upon seed meal particle size, and could be abolished by co-application with high-glucosinolate content Brassica juncea seed meal. These findings clearly demonstrate that RSM suppresses Rhizoctonia root rot by transforming the resident soil microbiota and is not a function of "biofumigation".