Black rot, caused by <i>Xanthomonas campestris </i>p<i>v. campestris</i>, is the most important and potentially destructive disease in cabbage. The objective of this study was to screen individual PGPR strains and then form mixtures of PGPR strains for their capacity to control black rot via antibiosis. Tests were conducted <i>in vitro</i>, in the greenhouse, and in the field. Twenty-four PGPR strains of <i>Bacillus</i> spp. strains were tested <i>in vitro</i>, and eight strains (AP136, 201, 213, 218, 219, 295 and 305) were chosen greenhouse tests with single strains. Seven individual PGPR strains caused significantly antibiosis ability, and the best four strains (AP218, 219, 295 and 305) were evaluated together as a mixture (mixiture-1), and together with strain AP213 (mixture-2). The two mixtures and four individual PGPR strains were then tested three times in the greenhouse and one time in the field. All the treatments resulted in significant disease suppression, with mixture-2 and AP218 causing the most consistent disease reduction in two of three tests. In the field, treatments of AP219, 305, mixture-1 and mixture-2 resulted in significantly lower disease incidence and disease severity and showed higher yield, compared to the disease control. In conclusion, mixtures of selected PGPR demonstrated enhanced consistency of biocontrol of cabbage black rot.
