Screening ofBacillusstrains in biocontrol of pathogenDickeya dadantiicausing stem and root rot disease of sweet potato

Outbreaks of stem and root rot disease of sweet potato caused byDickeya dadantiirecently occurred in major sweet potato planting areas in China. This study aimed to screen effectiveBacillusstrains in biocontrol of the bacterial soft rot pathogen. We screened plant-associatedBacillusstrains capable of inhibitingD. dadantiiusing anin vitrooverlay culture assay and anin vivotuber slice assay. AllBacillusstrains inhibitedD. dadantiigrowth and maceration of sweet potato tubers. Two strainsB. amyloliquefaciensA3 andB. velezensisA2 showed significantly higher rates ofin vitroinhibition (55% and 50%) onD. dadantiigrowth andin vivoreduction (82% and 77%) of the maceration of sweet potato tuber slices byD. dadantiithan otherBacillusstrains did. They showed broad-spectrum inhibition on growth of soft rotDickeyaandPectobacteriumstrains. They protected sweet potato seed tubers from infection byD. dadantiiand promoted seedling growth. Their culture supernatants inhibitedD. dadantiigrowth, swimming and biofilms and damagedD. dadantiicells.B. amyloliquefaciensA3 is more effective thanB. velezensisA2 in producing surfactins, well-known lipopeptide antibiotics and biosurfactants, and inhibitingD. dadantii. Together, we developed an effective method to screen effectiveBacillusstrains to control the soft rot pathogenD. dadantiiand screened out a strong surfactin producerB. amyloliquefaciensA3, which can be used as a reference strain for further screening of more effectiveBacillusstrains and to develop an agent to control the stem and root rot disease of sweet potato.