Green manure systems to induce biological soil suppressiveness

Green manures have been examined extensively as a means to improve soil quality, but although long studied (Millard and Taylor, 1927; Rouatt and Atkinson, 1950) this practice has been less effective or consistent when applied to a system for the control of soilborne diseases. As with certain organic residue amendments, green manuring may exacerbate disease development if used in concert with an inappropriate pathosystem (Manici et al., 2004). The lack of consistency can be attributed to various factors, most being similar to those limiting the efficacy of organic residue soil amendments detailed above, including an absence of knowledge concerning the underlying mechanisms of the organic-matter-mediated disease suppression. As a result, incorporation of green manure crops into soil with the intended goal of specifically managing disease suppressive elements of the resident soil microbial community has received minimal study.

The incorporation of green manures has been shown to increase the diversity and density of certain microbes known to have pathogen inhibitory activity, including fluorescent Pseudomonas spp., non-pathogenic Fusarium spp. and Streptomyces spp. However, within resident populations of each of these microbial communities numerous members will inherently lack one or more of the functional attributes that confer capacity to limit disease incited by any given pathogen (Larkin and Fravel, 1999; Gu and Mazzola, 2003; Zhao et al., 2009). Thus, as in the use of bio-based soil amendments, identification of operative mechanisms and the ability to

Control

BjSM fine Soil treatment

BjSM coarse

Fig. 11.3. Effect of Brassica juncea seed meal (BjSM) on apple root infection incited by Pythium irregulare when oospore inoculum of the pathogen (~2000 propagules/g soil) was introduced into the soil system 16 weeks post-seed meal amendment. Coarse (2-4 mm diameter) and fine (< 1 mm diameter) seed meal particles were used in the assay.

Control

BjSM fine Soil treatment

BjSM coarse

Fig. 11.3. Effect of Brassica juncea seed meal (BjSM) on apple root infection incited by Pythium irregulare when oospore inoculum of the pathogen (~2000 propagules/g soil) was introduced into the soil system 16 weeks post-seed meal amendment. Coarse (2-4 mm diameter) and fine (< 1 mm diameter) seed meal particles were used in the assay.

monitor the relative presence of the attribute in native soil microbial populations will be intrinsic to the successful application of green manuring as a means to induce biologically based disease suppressive soils.

Kinkel and colleagues have been at the forefront in attempts to discern the mechanisms of biologically mediated disease suppressive soils developing in response to green manuring. In particular, studies have focused on the contribution of the resident Streptomyces community towards the induction of soil suppressiveness in response to green manures. A green manure crop of buckwheat or canola increased the proportion of streptomycetes in the resident population that were antagonistic towards the potato pathogens Streptomyces scabies, Verticillium dahliae and R. solani (Wiggins and Kinkel, 2005a). The relative increase in inhibitory activity of the streptomycete community was frequently associated with a decrease in disease development and an increase in potato yields. Similar increases in the proportion of antagonistic strepto-mycetes and reduction in lucerne root rot were observed in buckwheat or sorghumsudan grass-treated soils (Wiggins and Kinkel, 2005b). Buckwheat and sorghumsudan grass green manures also increased the density and inhibitory activity of resident bacterial populations and Streptomyces spp. expressing antagonistic action towards the causal pathogen of Fusarium head blight of wheat, F. graminearum (Perez et al., 2008). As an initial step in the process towards developing a protocol for selection of appropriate green manure crops (or other resource amendments) for the generation of a highly inhibitory soil microbial community, studies were conducted to explore the effects of specific types and quantities of C compounds on resident populations of Streptomyces spp., and their antagonistic potential (Schlatter et al., 2009). Addition of complex C sources tended to yield greater Streptomyces densities than the simple sugar glucose. Higher inputs in the form of these C sources resulted in a Streptomyces community with greater antibiotic inhibitory activity than when soil was treated at a lower input level. In this system, further characterization of the means by which specific nutrient inputs influence Streptomyces inhibitory activity may enhance the ability to actively manage disease suppressive soils through the green manure management programmes.

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