Diverse microbial communities in non-aerated compost teas suppress bacterial wilt.

Non-aerated compost teas (NCTs) are water extracts of composted organic materials and are used to suppress soil borne and foliar disease in many pathosystems. Greenhouse trials were used to test the effectiveness of NCTs to suppress potato bacterial wilt caused by Ralstonia solanacearum on plants grown in soils inoculated with a virulent isolate of the pathogen (biovar II). NCTs prepared from matured compost sources: agricultural waste (AWCT), vermicompost (VCT) and solid municipal waste (SMWCT) were evaluated at three initial application times (7 days before inoculation, at time of inoculation and 7 days after inoculation) prior to weekly applications, in a randomized complete-block design. AWCT applied initially at the time of inoculation resulted in the greatest disease suppression, with the disease severity index 2.5-fold less than the non-treated plants and the "area under the disease progress curve" (AUDPC) 3.2-fold less. VCT and SMWCT were less suppressive than AWCT regardless of initial application time. Next generation sequencing of the v4 region of 16S rRNA gene and the internal transcribed spacer region (ITS1) revealed that diversity and composition of the bacterial and fungal communities across the NCTs varied significantly. Dominant bacterial phyla such as Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria, Verrucomicrobia, Chloroflexi, Planctomycetes, Acidobacteria, and a fungal phylum Ascomycota were detected in all NCTs. AWCT had optimum physico-chemical measurements with higher bacterial Shannon diversity indices (H) and fungal richness (S) than the other treatments. We conclude that bacterial wilt of potatoes grown in controlled conditions can be suppressed by a non-aerated compost tea with a high microbial diversity when applied at planting and weekly thereafter.