Multi-species endophytic Bacillus for improved control of potato soilborne and tuber-borne diseases in Tunisia: from laboratory to field conditions

Abstract

Abstract Background Due to the soilborne and tuber-borne nature of their causative agents, Fusarium wilt, Rhizoctonia root canker and black dot diseases are still leading to serious problems in potato production in Tunisia and worldwide. Among endophytic bacteria, the genus Bacillus is one of the most exploited microbial groups known as potent biocontrol agents against several potato diseases. In the present study, six strains belonging to five Bacillus species were screened for their abilities to inhibit root and wilt potato pathogens in vitro, in vivo and under natural conditions over three cropping seasons and to promote plant growth. Results Based on the dual-culture assays, the whole-cell suspensions of SV39 and SV104 (Bacillus tequilensis), SV41 (B. subtilis), SV44 (B. methylotrophicus) and SV65 (B. amyloliquefaciens subsp. plantarum) strains exhibited potent antifungal activity against important potato soilborne phytopathogens with ~ 65 to 70% inhibition rates. Significant inhibition rates were also induced by the cell-free culture filtrates, the butanolic and the chloroformic extracts depending on the target pathogens, the concentration used and the Bacillus strain tested. In pot experiment, a decrease in Rhizoctonia root canker severity, ranging from 43 to 65% compared to the inoculated and untreated control, was induced by all Bacillus spp. strain-based treatments, while SV39-, S42- and SV65-based treatments were the most effective in suppressing by 50–53 and 65–52%, black dot severity and the relative vascular discoloration extent induced by F. oxysporum f. sp. tuberosi, respectively, relative to positive control. This biocontrol potential was associated with an enhancement of potato growth parameters. Field studies indicated that soil treatment with the most of the Bacillus spp. strains had significantly controlled all the target fungal soilborne diseases and improved at the least two growth and/or production parameters depending on the strain used and the cropping seasons. SV39-, SV41-, SV44- and SV104-based treatments resulted in a significant increase in tuber yield in one cropping season. Conclusion These Bacillus spp. strains could be used in combinations and/or introduced with other existing practices in order to provide supplemental control of target diseases and yield promotion under organic or conventional potato production systems.