Suppression of soil-borne plant pathogens

Soil borne plant pathogens considerably reduce crop yields worldwide and are difficult to control due to their ”masked” occurrence in the heterogeneous soil environment. This hampers the efficacy of chemical - and microbiological control agents. Outbreaks of crop diseases are not only dependent on the presence of pathogen propagules in the soil, but are also influenced by soil-related properties like physico-chemical characteristics, microbial activity and community composition. Strong competition for limited available carbon substrates restricts or prevents germination and pre-infective growth of pathogens. This competition can occur directly by rapid exploitation of substrates, so called resource competition, or indirectly via inhibitory secondary metabolites, called interference competition The overall effect of all competition based mechanisms and the abiotic environment on disease development is known as “general disease suppression” and is the sum of all factors that reduce disease. The aim of this thesis was to study different aspects of general disease suppression, in order to get more insight into the interplay between microbial communities, pathogen dynamics, and substrate availability in different agricultural soils. The first objective was to study the role of microbial volatile organic compounds in natural disease suppression in agricultural soils. In chapter 2 a series of simultaneous experiments were performed on a agricultural soil that received different management practises. We showed a strong correlation between root infection and -biomass production in a bioassay and the suppressive effects of microbial volatiles on the in vitro growth of the pathogen Pythium intermedium. No or weak volatile suppression coincided with significant lower root biomass and a higher disease index, whereas a strong volatile suppression related to high biomass and a low disease index. Furthermore, the composition of the original soil bacterial community showed a drastic shift due to the legacy effects of management practices, coinciding with the loss of volatile suppression. By comparing the emission profiles and the bacterial community composition of the differently managed soils, candidate inhibitory compounds and volatile producing bacterial groups could be identified. Altogether these results indicate that volatile organic compounds can have an important role in general disease suppression. To follow up on volatile suppression chapter 3 investigates the influence of soil-related (abiotic and biotic) variables on volatile mediated in vitro growth inhibition of different plant pathogens via an extensive soil survey including 50 Dutch arable agricultural fields. The volatile mediated suppression of three phylogenetic different soil borne pathogens (Rhizoctonia solani, Fusarium oxysporum and Pythium intermedium) was linked to a wide range of soil-related variables with univariate and multivariate regression models. The overall suppression of different pathogens was linked to microbial activity and organic substrates. However, different pathogens showed different sensitivity to volatile suppression. Furthermore, the soil-related factors corresponding to volatile mediated suppression were pathogen specific. In total, the results described in this chapter show that part of volatile suppression for a particular pathogen is based on general microbial activity, but our data shows as well that the individual response is pathogen specific. Chapter 4 explores the reservoir of potential plant pathogens harboring agricultural soils before the start of the growth season, together with the environmental drivers of this pool of pathogens. By investigating the pathogenic seedbank in relation to its environment we assessed which soil-related variables could explain differences among site pathogen community composition. Pathogens differing in phylogeny or mode of infection were related to different soil variables. For example the among-site differences in the presence of oomycetes could not be related to their environmental context. On the other hand the variation in root and shoot fungal pathogen community composition was linked to soil physico-chemical properties and non-pathogen microbial community composition, with potentially a significant role of litter saprophytes therein. As the presence of pathogen propagules in soil is not necessarily related to disease incidence, chapter 5 investigates the dynamics of root pathogens in the presence of a root in a model rhizosphere. We developed a qPCR based assay to test the growth response of a pathogen (Pythium intermedium) to the presence of root exudates over time. This exposure to root exudates showed soil specific pathogen dynamics. This finding may indicate that in situ (microbial) processes can successfully prevent pathogen development in some of the soils but not in others. Thus this method has the potential to provide an alternative way to assess the susceptibility of a soil to certain soil-borne diseases. The results of this thesis gave new insights into different aspects of disease suppression in agricultural soils which could serve as a fundament to develop environmentally-friendly control methods based on natural occurring ecological processes. Ideas for the implementation of this study and future research are discussed in chapter 6.