Submitted to: Journal of Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/24/2004
Publication Date: 7/1/2005
Citation: Cohen, M.F., Yamasaki, H., Mazzola, M. 2005. Brassica napus seed meal soil amendment modifies microbial community structure, nitric oxide production, and incidence of rhizoctonia root rot. Journal of Soil Biology and Biochemistry. 37:1215-1227. 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 much 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 and the parasitic nematode Pratylenchus spp. was achieved by using Brassica napus seed meal (RSM) as a soil amendment. Nematode control was also achieved when soybean meal was applied at a rate providing a nitrogen content equivalent to that of RSM. In addition, RSM did not inhibit growth of an introduced isolate of the fungal pathogen. These findings indicate that glucosinolate hydrolysis products did not contribute to the observed disease control. RSM appeared to indirectly induce plant defense responses. This may have been achieved by stimulating resident populations of Streptomyces spp. that possess the capacity to produce nitric oxide; a compound known to activate genes involved in plant defense mechanisms. A possible role for bacterial-derived nitric oxide in disease suppression warrants further investigation in this and other plant systems.
Technical Abstract: Brassica tissues are promoted as a soil amendment for control of soilborne plant diseases due to their production of glucosinolates, which yield anti-microbial compounds upon hydrolysis. Application of a low glucosinolate content Brassica napus seed meal (RSM) altered populations of soil microbes. RSM amendment reduced infection by native and introduced isolates of Rhizoctonia spp. and recovery of Pratylenchus spp. from apple roots. R. solani root infection was suppressed in split-root assays where a portion of the root system was cultivated in RSM-amended soil. R. solani hyphal growth was not inhibited by RSM amendment. Suppression of Pratylenchus was attained to an equivalent extent by amending soils with either RSM or soybean meal (SM) when applied to provide a similar nitrogen content. Thus, RSM glucosinoate hydrolysis products did not appear to have a role in the suppression of Rhizoctonia spp. or Pratylenchus spp. obtained. RSM amendment elevated populations of Pythium spp. and ammonia-oxidizing bacteria that release nitric oxide but suppressed fluorescent pseudomonad numbers. Streptomyces spp. increased in response to RSM but not SM amendment. The vast majority of Streptomyces spp. recovered from apple roots produced nitric oxide and possessed a NO synthase homolog. We propose that transformations in bacterial community structure are primarily responsible for the control of Rhizoctonia root rot, with bacterial NO production potentially having a role in the induction of plant systemic resistance.