Conclusions

Active management of the biological resources native to agricultural soil ecosystems is a logical progression in our studies of the basis for the function of disease suppres-sive soils. While studies of disease suppres-sive soils have provided a wealth of information concerning the source of biological activity there has been relatively little progress in our capacity to manage the effective microbial resources indigenous to agricultural ecosystems. Rather, focus has remained on isolation and identification of the numerous biologically active microbial agents resident in such soils, and subsequently resorting to the inundative release of these microorganisms into non-native soils or crop production systems. Climate change undoubtedly will have an effect on many biological processes leading to altered ecosystem function. Such changes are predicted to modulate the efficacy of certain strategies that are commonly utilized for the management of soilborne diseases. In concert with the loss or impending restricted use of numerous chemicals (e.g. methyl bromide) previously used for soilborne disease control, it seems an appropriate time to further examine the prospect for managing ecosystem microbial resources as a viable soilborne disease control strategy. While climate change is bound to have impacts on the soil biology that is functional in disease suppres-sive soils, pathogen populations will acclimatize to these changes concurrently with the resident microbial milieu and thus effective adaptive traits are likely to reside across these broad communities.

In the development of protocols for the management of disease suppressive soil biology, it may be useful to consider the similar efforts pursued within the field of bioremediation and the comparable impediments to effective use of specific plant-mediated strategies. Specific plant systems are designed not only for phyto-extraction of pollutants but also as a means to secure the value of microbial partners that possess the ability to degrade organic pollutants. Much as is the case in phytore-mediation efforts, little emphasis has been placed on breeding efforts towards the development of crop genotypes with an elevated capacity to select for specific microbial genotypes functional in disease suppression. Reluctance to pursue plant breeding programmes focused on such attributes was limited until recently by a lack of acceptance or understanding of this phenomenon as a valuable parameter to crop improvement.

Molecular plant breeding programmes are at the head in the development of plants with enhanced capacity to select for plant beneficial microbial communities, including those involved in disease suppression. Multiple recent examples demonstrate the potential of this approach. A transgenic tobacco overexpressing ferritin imposed reduced iron availability in the rhizosphere resulting in a fluorescent pseudomonad community with a greater ability to grow under iron stress conditions. This resulting bacterial population possessed a greater capacity to inhibit growth of the plant pathogen Pythium aphanidermatum (Robin et al., 2007). Genetic modification of the wheat cultivar 'Bobwhite' by insertion of the powdery mildew resistance gene Pm3b resulted in multiple transgenic lines with an enhanced capacity, relative to the parental line, to select for 2,4-DAPG-producing fluorescent pseudomonads (Meyer et al., 2009). Thus, evidence indicates that efforts to develop crop cultivars with an elevated potential to exploit the resident disease suppressive microbial community are not futile, and in fact will be worthwhile for the development of more sustainable crop production systems.

Strategies other than those discussed here have received a modicum of research investigation but may effectively serve to manage microbial resources resident in agro-ecosystems in a manner to suppress soil-borne plant diseases. In addition to the use of organic amendments or the design of specific cropping sequences, further methods such as fertility management, crop residue management and soil tillage (Peters et al., 2003) may function to enhance soil suppressiveness. While adaptation of these cultural practices to increase the level of soil suppressiveness is receiving increased attention as a potential soilborne disease control practice, there has been only minimal consideration of the effect of such treatments on overall soil biology composition and function.

It is a daunting task to envision development of management systems to yield biologically suppressive soils for diseases in which such soils have not previously been characterized. Such an undertaking requires initial focus on identification of the micro-bial attributes that function to elicit disease suppression. Although such investigations continue to be a complex and protracted process, emerging tools in molecular micro-bial ecology, including metagenomics and pyrosequencing will enable more rapid evaluation of microbial community structure, and ultimately function (van Elsas et al., 2008). Such analytical tools will enable more complete examination of resident soil biology, changes to such populations in response to specific practices, and comparative micro-bial community analysis among soils allowing one to more reliably predict microbial effectors of disease suppression. These same methods will enable more efficient analysis of microbial community structure in response to agroecosystem management practices and enable prediction of the resulting benefits to plant growth through disease suppression. A greater understanding of the consequence of such treatments on food webs that modulate the density and activity of microbial populations functional in disease control will be instrumental to the development and eventual adoption of tools for the management of disease suppressive soil biology.

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