Natural disease suppression of Rhizoctonia by a network of fungal players: Evidence of direct competition and competitive exclusion

Evidence of antagonism by naturally-occurring fungal and bacterial species towards Rhizoctonia bare patch disease has been demonstrated in historically disease-suppressive cropping fields in Australia. However, there is little data regarding bacterial-fungal interactions in these systems nor is there an understanding of the temporal variability or stability of these communities. While the presence of putative suppressive taxa has been indicated to drive suppression in some agricultural systems, the process would require interactions among multiple taxa in order to achieve temporal stability. Here we present a two-year study focusing on both soil bacterial and fungal communities in Rhizoctonia disease conducive and suppressive soils by targeted phylogenetic marker Illumina sequencing. Approximately 450,000 sequences from 105 samples were generated for both the fungal 28S and bacterial 16S rRNA markers. Temporal variability was evidenced by larger changes in fungal communities over annual and sowing/in-crop samples, though suppression status remained the primary driving component.  Molecular Ecological Network analysis suggested that suppressive soils are characterized by a higher functional redundancy with “small-world” interactions. In all, the data suggest that suppression ofRhizoctonia solani AG8 is due a combination of direct antagonism by soil taxa and endophytic competition for root infection sites (competitive exclusion). Thus, stable disease suppression in these cropping soils appears more complex than due to the mere presence of a few characterized suppressive taxa.