Rice Bran Amendment Suppresses Potato Common Scab by Increasing Antagonistic Bacterial Community Levels in the Rhizosphere

Abstract

Potato common scab (PCS), caused by pathogenic Streptomyces spp., is a serious disease in potato production worldwide. Cultural practices, such as optimizing the soil pH and irrigation, are recommended but it is often difficult to establish stable disease reductions using these methods. Traditionally, local farmers in southwest Japan have amended soils with rice bran (RB) to suppress PCS. However, the scientific mechanism underlying disease suppression by RB has not been elucidated. The present study showed that RB amendment reduced PCS by repressing the pathogenic Streptomyces population in young tubers. Amplicon sequencing analyses of 16S ribosomal RNA genes from the rhizosphere microbiome revealed that RB amendment dramatically changed bacterial composition and led to an increase in the relative abundance of gram-positive bacteria such as Streptomyces spp., and this was negatively correlated with PCS disease severity. Most actinomycete isolates derived from the RB-amended soil showed antagonistic activity against pathogenic Streptomyces scabiei and S. turgidiscabies on R2A medium. Some of the Streptomyces isolates suppressed PCS when they were inoculated onto potato plants in a field experiment. These results suggest that RB amendment increases the levels of antagonistic bacteria against PCS pathogens in the potato rhizosphere.

The effect of susceptible and resistant potato cultivars on bacterial communities in the tuberosphere of potato in soil suppressive or conducive to common scab disease

“…This was not expected, as the last three families were more prevalent in CS conducive than suppressive soil in Michigan (8). Our Illumina sequencing indicated that Rhizobiales and Planctomycetales were enriched in suppressive soil, as in Cha et al (29), but Actinobacteria, Chloroflexi, Bacillaceae , and Chitinophagaceae that increased with CS suppressiveness induced by rice bran (22) were more prevalent in the conducive soil here. Therefore, different biocontrol consortia may be at work in different types of suppressive soils.…”

“…Further to that the quantity of pathogenic streptomycetes (based on numbers of txtB genes) did not change with soil suppressiveness or cultivar in tuberosphere and bulk soil, but in suppressive soil the number of pathogens decreased in potato periderm possibly due to both microbial interactions and soil chemical conditions depending on location (9, 21). In this study, however, the increased numbers of actinobacteria in periderm of susceptible cultivar did not correspond to pathogenic streptomycetes, so perhaps an antagonistic community of actinobacteria developed there as a response to pathogen infection, similarly as in Rosenzweig et al (8) or Tomihama et al (22).…”

A Rhizosphere-Derived Consortium of Bacillus subtilis and Trichoderma harzianum Suppresses Common Scab of Potato and Increases Yield

“…The effect of bioagents on the α-diversity of plant-related microbial communities varies. Some studies indicate that application of bioagents can increase the α-diversity of the bacterial rhizosphere community [ 11 , [27] , [28] , [29] ], while another study reported that the application of a rice bran amendment suppressed PCS and resulted in a significant decrease in the α-diversity of the rhizosphere bacterial community [ 30 ]. It is possible that a higher bacterial α-diversity does not necessarily lead to a lower incidence of plant disease.…”

“…Bacterial taxa in the Burkholderiales have been recently reported to be plant growth-promoting rhizobacteria (PGPR) that stimulate plant growth [ 38 ]. Bioagents in the genus, Pseudomonas , also have the potential to reduce the incidence of PCS under field conditions [ 16 , 30 ]. Collectively, our results indicate that application of the designated microbial product contributes to an increase in the relative abundance of beneficial bacteria in the rhizosphere of potato.…”

“…The effect of organic fertilizer on the soil microbial community rapidly decreases with time, thus it is unlikely that it would affect the soil microbial community after the first year of application. Our results, however, suggest that the application of 225 kg·ha −1 or 300 kg·ha −1 of a microbial product, composed of a consortium of Bacillus subtilis and Trichoderma harzianum , altered the composition of the soil bacterial community, which could be further influenced by organic fertilizers, plant root exudates, or crop debris [ 13 , 30 , 41 ]. In the current study, a significant decrease in potato tuber disease (PCS) and a significant increase in tuber yield was observed in plots treated with either 225 kg·ha −1 or 300 kg·ha −1 of the microbial consoritum.…”