Brassica juncea seed meal particle size influences chemistry but not soil biology-based suppression of individual agents inciting apple replant disease

Apple replant disease is incited by a pathogen complex composed of multiple fungal, oomycete and nematode species. Rhizoctonia solani AG-5 can be a significant component of this complex and is effectively suppressed via multiple functional 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 was initiated earlier and reached higher maximal concentrations in soils amended with fine particle (< 1 mm 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 8 weeks post-application relative to the non-treated control soil. When soil was infested with R. solani AG-5 subsequent to this 8 week incubation period and planted to apple, B. juncea SM amendments effectively suppressed Rhizoctonia root rot irrespective of SM particle size. Relative R. solani AG-5 apple root infection as determined by culture-based methods and a qPCR method developed in these studies was consistently similar 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, but will do so only through effects on chemistry-and not biologically-based mechanisms of pathogen suppression.