Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2010
Publication Date: 8/12/2010
Citation: Mazzola, M., Zhao, X. 2010. Brassica juncea seed meal particle size influences chemistry but not soil biology-based suppression of individual agents inciting apple replant disease. Plant Soil. 337:313-324. Interpretive Summary: Apple replant disease is incited by a pathogen complex composed of multiple fungal, oomycete and nematode species. Brassicaceae seed meal amendments can provide control of numerous plant pests, including a number of the agents contributing to apple replant disease. Pest control in response to brassicaceae seed meal amendments may result in response to the generation of biologically active chemistries, such as the generation of isothiocyanate, or indirectly through transformation and activity of the resident soil biology. As reported in previous studies with brassicaceae green manures, we observed that particle size will directly affect the generation of isothiocyanate from brassicaceae seed meal soil amendments. Furthermore, this study establishes that increased cell disruption of even relatively small brassicaceae residue particles will yield higher maximal levels of isothiocyanate production in treated soils and significantly increase overall isothiocyante yields. In the instance of certain fungal pathogens, it is apparent that particle size will be a significant factor in determining the ultimate efficacy of brassicaceae seed meal amendments for disease suppression. Thus, particle size characteristics must be considered in attempting to optimize the use of these materials as a disease control measure in systems where isothiocyanate release is the dominant mechanism of pathogen suppression.
Technical Abstract: Apple replant disease is incited by a pathogen complex composed of multiple fungal, oomycete and nematode species. Rhizoctonia solani AG-5 is a significant component of this complex and is suppressed via multiple mechanisms in response to Brassica juncea seed meal (SM) amendment. These mechanisms include those of both a biological and chemical nature. The effect of seed meal particle size on the operation of these mechanisms and the resulting capacity of B. juncea SM to suppress R. solani and other components of the pathogen complex that incites replant disease of apple was examined in this study. Emission of ally isothiocyanate (AITC) from B. juncea SM-amended soil begam earlier and reached higher maximal concentrations in soils amended with fine particle (<1mm dia) than coarse particle (2-4 mm dia) size SM. This corresponded with the level of disease suppression obtained when R. solani AG-5 and SM at a rate of 0.3% (wt/wt) were introduced concurrently into soils and planted to apple; fine particle size but not coarse particle size B. juncea SM suppressed apple root infection. At the same rate, Pratylenchus penetrans and Pythium spp. were effectively controlled by B. juncea SM applications irrespective of SM particle size. AITC emission from B. juncea SM-amended soils was completed within 72 h post amendment, even at an application rate as high as 1.0% (wt/wt). In both fine and coarse particle size B. juncea SM-amended soils populations of resident Streptomyces spp. were elevated approximately five to ten-fold at eight weeks post-application relative to the non-treated control soil. When soil was infested with R. solani AG-5 subsequent to this eight week incubation period, B. juncea SM amendments effectively suppressed Rhizoctonia root rot of apple irrespective of SM particle size. Relative apple root infection by R. solani AG-5 as determined by culture-based methods and a qPCR method developed in these studies was consistent across experiments. These findings demonstrate that particle size will affect the efficacy of B. juncea SM soil amendment for the control of R. solani AG-5, and will do so through effects on chemistry-based mechanisms of pathogen suppression.