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

Abstract

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 SMamended 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 culturebased 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.

Differential impacts of brassicaceous and nonbrassicaceous oilseed meals on soil bacterial and fungal communities

“…The major bioactive compounds associated with mustard and other brassicaceous plants are isothiocyanates, the hydrolysis products of glucosinolates (Gimsing & Kirkegaard, 2006). Past work has shown that majority of isothiocyanate emissions, including allyl isothiocyanate, occurs within 24-72 h following addition to the soil system (Mazzola et al, 2007;Mazzola & Zhao, 2010). Isothiocyanates are thought to be effective in suppressing fungal pathogens, such as Rhizoctonia solani, during this relatively short window of time; however, soils amended with mustard seed meal still harbor the ability to suppress Rhizoctonia after 4 weeks of incubation (Mazzola et al, 2007).…”

“…Although it has been shown that isothiocyanates degrade rapidly in the environment (i.e. > 99% of emissions occur within 24-72 h) (Mazzola et al, 2007;Mazzola & Zhao, 2010), soils amended with brassicaceous plant materials often retain the ability to suppress weeds and patho-gens for much longer periods of time, up to several weeks postamendment (Mazzola et al, 2007;Rice et al, 2007).…”

Biofumigation for Fighting Replant Disease- A Review

“…Nevertheless the conversion rate detected in that study was still low (303 µmol glucosinolates/g seed meal, 3 mg seed meal/g soil applied = 909 nmol glucosinolates/g soil applied; = conversion rate of 7.7%). However, it has to be kept in mind that in that study, only the head space above the treated soil was sampled and not the soil itself [69]. Further, waterlogging to enhance isothiocyanate release [62] and soil tarping with plastic foils to keep the volatile compounds in the soil [67] are recommended.…”

Impact of Indian Mustard (Brassica juncea) and Flax (Linum usitatissimum) Seed Meal Applications on Soil Carbon, Nitrogen, and Microbial Dynamics

“…This appeared to be largely due to greater fungal biomass in the mustard and sorghum-sudangrass treatments. For the mustard, this is especially interesting given the number of studies documenting its adverse effects on various soil fungi [11,15,16]. Our initial presumption was that the mustard would broadly inhibit soil fungal populations.…”

“…These compounds have been documented to have broad biocidal effects and suppress a number of soil pathogens, insects, and weeds [11][12][13][14]. Although numerous studies have documented the beneficial usage of brassicaceous plants or green tissue as biofumigants, only a limited number of studies have been conducted on use of brassicaceous oilseed meals for this purpose [11,[15][16][17]. Additionally, although the impacts of isothiocyanates and related compounds on microbial pathogens have been well documented, the impacts of nonpathogenic, soil microorganisms are largely unknown.…”