Integrated control of bacterial soft rot of potato caused by pectobacterium carotovorum subsp. carotovorum using bacterial biocontrol agents and lemongrass (Cymbopogon citratus L.) essential oil.

Abstract

Potato (Solanum tuberosum L.) is an economically significant staple crop widely grown and consumed worldwide. It is a relatively perishable vegetable and prone to biotic factors during postharvest storage and transportation resulting in food insecurity and poor quality. Bacterial soft rot caused by Pectobacterium carotovorum subsp. carotovorum (Pcc) is one of the most economically important diseases, causing significant losses of potatoes both preharvest and postharvest. Preventative measures used to control the disease such as planting disease-free seeds, avoidance of contamination, and disinfection machines and tools are currently used. However, these control strategies are not effective in suppressing pathogen infection. On the other hand, chemical bactericides are not recommended to be used as one of the control methods due to their toxic effect, non-persistence and development of resistance by bacteria. Therefore, there is a crucial need for the development of natural, sustainable, environmentally friendly, and generally regarded as safe (GRAS) antimicrobials such as essential oils and biocontrol agents. Thus, the aim of this study was to investigate the effect of bacterial biocontrol agents and lemongrass essential oil, individually and in combination, to control Pectobacterium carotovorum subsp. carotovorum (Pcc) in vitro and in vivo. A total of 150 bacterial isolates were isolated and screened against Pcc for their antibacterial activity using a disc diffusion assay in vitro and a preventative essay on whole potatoes. Bacillus amyloliquefaciens isolates (YMCB, BMO3, and MSAG) had the best antibacterial effect against Pcc in vivo and in vitro. They caused cell deformation of Pcc and leakage of its constituents while protecting the potato cell organelles.To evaluate the antibacterial effect of lemongrass (LE) on Pcc, LE concentrations LE0.125%, LE0.175%, LE0.5%, LE1.0%, LE2.0% and LE3.0% were prepared and tested against Pcc in vitro and on ‘Sifra potatoes. LE3.0% had the highest antibacterial activity (34.00 mm inhibition zone) against Pcc in vitro and invaded the plasma membrane of the Pcc cells, which resulted in Pcc cell deformation and leakage of the cell constituents. LE0.125% and 0.175% controlled bacterial soft rot incidence by 94.83% and 89.66%, respectively, on ‘Sifra’ potatoes compared to the control treatment (0.00%).TEM micrographs of potato wounds treated with LE0.125% showed LE essential oil deposistion in the potato cell wall and inside the cell compared to the control that showed viable Pcc cells. Furthermore, the antibacterial activity of the treatment combination was evaluated by using the best LE concentrations and biocontrol isolates and screened against Pcc using disc-diffusion and dipping methods in vivo. LE3.0% + B. amyloliquefaciens (YMCB) had the largest pathogen inhibition zone (30.00 mm) and caused cell deformation of Pcc cells. A treatment combination of LE0.125% + B. amyloliquefaciens (MSAG) showed the highest antibacterial effect and reduced bacterial soft rot incidence to 96.56%. The TEM micrographs of potato wounds treated with LE0.125% + B. amyloliquefaciens (MSAG) showed that there was an interaction between different antimicrobials from LE essential oil and B. amyloliquefaciens in the potato cell wall and amyloplast. In conclusion, the research findings from this study suggest that lower concentrations of lemongrass essential oil (LE0.125% and LE0.175%) can be combined with B. amyloliquefaciens strains to control bacterial soft rot of potatoes during storage.