Screening of Bacillus strains in biocontrol of pathogen Dickeya dadantii causing stem and root rot disease of sweet potato

Abstract

ABSTRACT Outbreaks of stem and root rot disease of sweet potato caused by Dickeya dadantii recently occurred in major sweet potato planting areas in China. This study aimed to screen effective Bacillus strains in biocontrol of the bacterial soft rot pathogen. We screened plant-associated Bacillus strains capable of inhibiting D. dadantii using an in vitro overlay culture assay and an in vivo tuber slice assay. All Bacillus strains inhibited D. dadantii growth and maceration of sweet potato tubers. Two strains B. amyloliquefaciens A3 and B. velezensis A2 showed significantly higher rates of in vitro inhibition (55% and 50%) on D. dadantii growth and in vivo reduction (82% and 77%) of the maceration of sweet potato tuber slices by D. dadantii than other Bacillus strains did. They showed broad-spectrum inhibition on growth of soft rot Dickeya and Pectobacterium strains. They protected sweet potato seed tubers from infection by D. dadantii and promoted seedling growth. Their culture supernatants inhibited D. dadantii growth, swimming and biofilms and damaged D. dadantii cells. B. amyloliquefaciens A3 is more effective than B. velezensis A2 in producing surfactins, well-known lipopeptide antibiotics and biosurfactants, and inhibiting D. dadantii. Together, we developed an effective method to screen effective Bacillus strains to control the soft rot pathogen D. dadantii and screened out a strong surfactin producer B. amyloliquefaciens A3, which can be used as a reference strain for further screening of more effective Bacillus strains and to develop an agent to control the stem and root rot disease of sweet potato.