Abstract

Disease suppressive soils are important for managing soil-borne diseases that cannot be controlled with chemicals. One such disease is the potato common scab caused by Streptomyces species. Suppressiveness against common scab can develop spontaneously in fields where potato is grown for years without interruption, and this has been attributed to non-pathogenic Streptomyces strains. Streptomyces spp. have been used as inoculants in biological control, but their long-term effects have gained less attention. In our previous studies, a non-pathogenic Streptomyces strain (Str272) isolated from a potato common scab lesion suppressed common scab in field trials lasting over 5 years. In this study, bacterial communities in the tuberosphere i.e. in the soil adjacent to potato tubers, were analysed by next generation sequencing (NGS). The aim was to compare bacterial communities in untreated control plots to those in which seed tubers were treated with Str272 in one or several growing seasons. Str272 applications increased soil bacterial diversity and affected the bacterial composition in the potato tuberosphere. The most pronounced differences were observed between the untreated control and the treatments in which the antagonist had been applied in three or four consecutive years. The differences remained similar until the following growing season. Bacterial composition after repeated antagonist applications was associated with lower common scab severity. The antagonist applications had no or only slight effect on the number or abundance of OTUs belonging to Actinobacteria or Streptomyces, and no differences in quantities of pathogenic Streptomyces populations were detected by qPCR. This indicates that suppression of common scab by Str272 may not be based on direct effect on the common scab pathogens but is more likely to be associated with the alterations of the soil bacterial community. The most abundant bacteria phyla in the potato tuberosphere were Actinobacteria, Proteobacteria and Acidobacteria. However, the OTUs responding greatest to the antagonist treatments belonged to Bacterioidetes and Gemmatimonadetes. Results indicate that repeated applications of Str272 can change the bacterial community in the potato tuberosphere and lead to development of soil that is suppressive against potato common scab for several growing seasons after the last application.

Highlights

  • Disease suppressive soils are important in managing soil-borne pathogens that cannot be controlled with chemicals

  • The OTUs belonging to four major phyla (Actinobacteria, Proteobacteria, Firmicutes and Acidobacteria) covered over 94% of the identified OTUs and approximately 70% of the sequence reads in the data (Table A.3)

  • Results from this study showed that the introduction of the antago­ nistic Streptomyces strain Str272, which can suppress development of potato common scab (Hiltunen et al, 2017), had an impact on the bacterial community in the potato tuberosphere

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Summary

Introduction

Disease suppressive soils are important in managing soil-borne pathogens that cannot be controlled with chemicals. Diverse bacterial communities in soil are considered necessary for the ability of soil to suppress plant diseases (Garbeva et al, 2004; Raaijmakers et al, 2009), specific groups of microbes that are antagonistic to certain pathogens may exist in soil (Mazzola, 2002). If these antagonists can be identified and propagated under controlled conditions, it may be possible to use them as biological control agents to boost the suppressive properties of soil

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