Management of Biologically Mediated Soil Suppressiveness

Various attributes of a cropping system including plant species (Garbeva et al., 2006), input system (organic versus conventional) (Workneh et al., 1993; van Bruggen, 1995; Liu et al., 2007), tillage (Peters et al., 2003) and fertilization (Smiley, 1978), among others, will influence ecological processes that determine microbial community structure and function, including its capacity to induce suppression of soilborne plant pathogens. These observations imply that, given knowledge of the operative biological mechanisms, there exists the ability to enhance or diminish the suppressive nature of a resident microbial community through timely application of the appropriate agronomic practices (Workneh and van Bruggen, 1994; Hoeper and Alabouvette, 1996; Pankhurst et al., 2002). As the induction of soil suppressiveness is often mediated through transformations in soil microbial communities over time (Liu and Baker, 1980; Larkin et al., 1993; Raaijmakers et al., 1997; Mazzola and Gu, 2002; Weller et al., 2002), there may be a significant opportunity to manage the phenomenon, and perhaps accelerate the onset of the disease-suppressive state, a notable prerequisite to the economic viability and adoption of such a disease control strategy.

In the management of soil suppressive-ness, it may be argued that enhancement of overall general suppression would be the easier course to pursue as this can be achieved simply through elevation of general micro-bial activity in a soil. In certain instances such a tactic may be a viable means to achieve disease suppression. However, as is true for all but the most drastic control methods (e.g. soil fumigation), it is unlikely to be a universal solution to the management of soilborne diseases. For example, the general suppression phenomenon is reported to function in certain soils suppressive to Pythium root rot (Adiobo et al., 2007). In addition, control of diseases incited by Pythium spp. in response to addition of organic residues to soils is often attributed to and dependent upon an overall elevation in general soil microbial activity (Hoitink and Boehm, 1999). While the level of disease control attained will be dependent upon substrate composition and state at the time of soil incorporation (Mandelbaum and Hadar, 1990), pursuing this strategy for control of diseases incited by Pythium would appear to possess significant potential. In contrast, suppression of other soilborne diseases, such as Rhizoctonia root rot (Henis et al., 1979; Mazzola and Gu, 2002), may function through 'specific suppression' and rely upon the activity of a defined subset of the total soil microbial community. Substrate-induced generation of soil suppressiveness to Rhizoctonia root rot was dependent upon specific microorganisms or communities resident in the organic substrate (Kuter et al., 1983; Kwok et al., 1987) or the capacity of the amendment to selectively amplify the functional populations resident to the soil (Cohen et al., 2005; Wiggins and Kinkel, 2005a). Thus, in pathosystems where specific suppression is the primary determinant of disease control, even where overall enhancement of microbial activity realized in response to a management practice, in the absence of the specific functional microbial population disease control may not be realized (Aryantha et al., 2000; Cohen et al., 2005).

Efforts to direct development of specific soil suppressiveness as a management tool requires knowledge of the biological consortia conferring disease suppression as well as an understanding of how any particular strategy will influence the activity of the functional population. Many biologically based 'alternative' practices have failed to live up to their potential owing to an inability to identify the functional population(s) leading to pest suppression. When such information is available, functional groups can be monitored enabling the prediction of pest control efficacy. For instance, the natural development of soils suppressive towards take-all of wheat in response to wheat monoculture was shown to be dependent upon native 2,4-DAPG-producing fluorescent pseudomonads attaining a threshold population of 105 colony forming units (cfu)/g root or greater in order to achieve effective disease control (Raaijmakers and Weller, 1998; de Souza et al., 2003). This functional population can now serve as a biological indicator to predict the efficacy of practices (e.g. continuous wheat monoculture) that lead to take-all suppressive soils.

Within the scope of a 'functional micro-bial population' exists an extraordinary level of complexity that may not be apparent nor routinely considered in the application of such a disease management strategy. For instance, populations of 2,4-DAPG-producing fluorescent pseudomonads are genetically diverse and differ in capacity to suppress take-all of wheat (Raaijmakers and Weller, 2001). Wheat cultivars also differ in both relative density and genetic composition of the 2,4-DAPG population selected from indigenous soil populations of these bacteria (Mazzola et al., 2004). The capacity of the plant host to seize the benefit from a particular functional group, and diversity in susceptibility of the universal pathogen population to the mode(s) of action contributing to disease suppression will also influence disease development. For instance, a particular plant growth promoting rhizo-bacteria that elicits defence responses in one plant species (Tran et al., 2007) may yield no such response in a different plant species (Mazzola et al., 2007b). Although cyclic lipopeptide-producing rhizobacteria function to provide control of diseases incited by certain Pythium spp. through the zoosporo-

cidal activity of these metabolites (de Souza et al., 2003), this mechanism obviously will not contribute to suppression of diseases incited by Pythium spp. for which zoospore production is not a functional or important component of the disease cycle (Mazzola et al., 2007b). Thus, various attributes of an agricultural ecosystem are likely to modulate the development and efficacy of a disease suppressive soil.

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