Functional Biology of Disease Suppressive Soils

Disease suppressive soils have been identified for a number of plant pathogens, with a few of the more prominent examples including those soils suppressive to Fusarium wilt (Rouxel et al., 1979; Scher and Baker, 1980), potato scab (Menzies, 1959), cyst nematode (Westphal and Becker, 1999), Rhizoctonia root rot (Henis et al., 1979; Barnett et al., 2006; Garbeva et al., 2006), Pythium root rot (Adiobo et al., 2007) and take-all of wheat (Cook and Rovira, 1976). Harnessing the potential of these soils as a practical means to manage diseases in agroecosystems has long been a goal; however, there have been limited attempts to actively manage these resources in the context of an overall plant production system.

Clearly, the effective implementation of strategies to manage resident soil microbial communities for the suppression of soilborne plant pathogens requires the capacity to initially identify the biological components involved in disease suppression. As noted above, the biotic factors that contribute to specific soil suppressiveness have been elucidated for a number of plant-pathogen systems (Weller et al., 2002). In certain systems, the capacity of a soil to limit disease is elevated over time in response to the application of specific plant management systems. One of the more notable examples has been documented in soils that are suppressive to the disease take-all of wheat which is incited by the fungal pathogen Gaeumannomyces graminis var. tritici. In systems where wheat is grown under continuous monoculture, disease incidence commonly increases during the initial few years of production but at some point thereafter a spontaneous decline in disease severity is realized, termed take-all decline, and the soil remains suppressive to the disease as long as wheat monoculture is not interrupted (Gerlagh, 1968; Shipton et al., 1973). Take-all decline has been observed across geographic regions, and in multiple instances 2,4-diacetylphloroglucinol (2,4-DAPG)-producing fluorescent pseudomonads have been shown to play a prominent role in the development of take-all suppressive soils

(Weller et al., 2002; de Souza et al., 2003). Fusarium wilt suppressive soils may also develop in response to crop cultivation (Larkin et al., 1993), and non-pathogenic Fusarium spp. have repeatedly been implicated as a factor functioning in disease suppression (Alabouvette et al., 1996), at times in concert with resident siderophore or phenazine-producing fluorescent Pseudomonas spp. (Duijff et al., 1999; Mazurier et al., 2009). The commonality of functional biology and inducing agronomic practices leading to specific suppression of a disease across geographic regions supports the premise that managing these phenomena is a credible strategy to pursue for soilborne disease management.

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