Optimizing fermentation of Bacillus amyloliquefaciens 3–5 and determining disease suppression and growth in cucumber (Cucumis sativus)

Abstract

Bacillus spp. have been extensively used as biocontrol agents for controlling fungal plant diseases. In the present study, a strain of Bacillus amyloliquefaciens (strain 3–5) with antagonistic activity against several plant pathogenic fungi was identified, determined optimal fermentation conditions, and explored its ability to control Fusarium wilt in cucumber. The optimal medium for solid-state fermentation (SSF) of strain 3–5 contained wheat bran (35 %), rice husk (40 %), corn meal (20 %), bean flour (15 %), maize starch (1.5 %), beef extract (2.5 %), and MgSO4 (1.5 %). The number of viable cells obtained on this medium was up to 4.577 × 1013 CFU/g under optimal fermentation conditions which included inoculum 6 %, solid content 36 g/L, material-sterile water ratio of 1.0:1.0 and fermentation for 44 h at 32 ℃ and pH 7.0 of the initial medium. The ability of B. amyloliquefaciens 3–5 to inhibit the growth of various fungal pathogens was determined by confrontation culture assays, and greenhouse pot experiments to determine the ability of 3–5 to decrease the incidence of wilt caused by Fusarium oxysporumf. sp.cucumerinum (FOC) in cucumber. Results indicated that 3–5 had an inhibitory effect on all ten of the pathogenic fungi tested, with a 60 % level of inhibition when the mass ratio of B. amyloliquefaciens 3–5 to nutrient soil of 10 %. The thickness of main stems, root length, and plant height of cucumber were significantly higher (P＜0.05) in 3–5 treated plants than control, which the growth rates were 77.5 %, 35.8 %, and 53.3 %, respectively in pot experiments. The application of 3–5 bio-agents is beneficial to reduce the incidence of cucumber Fusarium wilt that the preventive effect and therapeutic effect were 72.1 % and 48.8 %. In summary, a newly isolated strain ofB. amyloliquefaciens 3–5 was demonstrated to control Fusarium wiltin cucumber. The strain should be further evaluated for its commercial potential.