Submitted to: Proceedings, IOBC
Publication Type: Abstract Only
Publication Acceptance Date: 6/21/2016
Publication Date: 9/19/2016
Citation: Thomashow, L.S., Weller, D.M. 2016. Suppressive soils: back on the radar screen. Proceedings, IOBC. O MES 1.
Technical Abstract: Suppressive soils are those in which a pathogen does not establish or persist, establishes but causes little or no damage, or establishes and causes disease for a while but thereafter the disease is less important, although the pathogen may persist in the soil (Weller, 2002). ‘General suppression,’ the ability of essentially all soils to suppress the growth or activity of soilborne pathogens to a limited extent, is due to the activity of the total soil microbiome competing with the pathogen and is not transferable between soils. ‘Specific suppression’ is superimposed over the background of general suppression, is highly effective, is transferable between soils, and is due to the effects of individual or select groups of microorganisms. Suppressive soils owe their activity to both general and specific suppression, with the latter the dominant force and the focus of most studies. Suppressive soils occur worldwide and are known for many pathogens (Cha et al., 2016; Mendes et al., 2011; Weller et al., 2002; 2007; Yin et al., 2013).Take-all decline (TAD), the spontaneous reduction in take-all and increase in yield that occurs with monoculture of wheat or barley following a severe attack of the disease (Weller, 2015), is the classic example of a suppressive soil. Growers in the Pacific Northwest (PNW) of the USA rely on TAD, with about 0.8 million ha of wheat suffering little damage from the disease even though the pathogen is still present (Weller, 2015). This TAD suppressiveness results from the enrichment of certain fluorescent Pseudomonas spp., now classified as P. brassicacearum (Loper et al., 2012), that produce the antibiotic 2, 4-diacetylphloroglucinol (DAPG). Biotic and abiotic factors as well as management practices influence the robustness of take-all suppression, with irrigation and wheat cultivar impacting the abundance and effectiveness, respectively, of DAPG producers (Mavrodi et al., 2012b; M-M. Yang and D. M. Weller, unpublished data). Populations of phenazine-1-carboxylic acid producers likewise are influenced by soil moisture in the PNW, where they are localized to arid soils throughout the Columbia Plateau (Parejko et al., 2013) and comprise up to 10% of the total culturable heterotrophic aerobic bacteria on the roots of dryland spring wheat (Mavrodi et al., 2012). While these bacteria are suppressive of Rhizoctonia root rot (Parejko et al., 2013), recent evidence also has implicated microbial communities of copiotrophic bacteria including members of the Oxalobacteriaceae and Sphingobacteria (Flavobacterium and Chryseobacterium) in the decline of the disease (Yin et al., 2013). One bacterium, Chryseobacterium soldanellicola, was isolated, tested in greenhouse bioassays, and shown to reduce symptoms caused by Rhizoctonia on wheat, completing the biocontrol version of Koch’s postulates and demonstrating causation for Rhizoctonia suppression. In another case, the application of DNA-based technologies revealed not only a key role for Actinobacteria in the suppression of strawberry wilt caused by Fusarium oxysporum, but also genes responsible for the ribosomal synthesis of a novel thiopeptide inhibitory to the pathogen and the antibiotic's mode of action against fungal cell wall biosynthesis (Cha et al., 2016). Since the pioneering work of Mendes et al. (2011), the use of DNA-based tools has opened the door to unprecedented new insights into the complex interactions among members of rhizosphere microbial communities and their plant hosts. These tools, in combination with classical approaches, are essential not only to unravel the network of molecular and biochemical mechanisms that underpin soil suppressiveness, but also to extend the lessons learned from these special soils to develop microbial community management strategies integral to plant health.