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

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.