Streptomyces spp. can cause scab symptoms on many plants, including potato and radish, which can result in significant economic loses for agricultural production. As no satisfactory control methods are currently available, biological control can be considered as a promising strategy for managing the disease. A field in Michigan showed naturally occurring suppressiveness to potato common scab. The disease suppression was potentially associated with a group of antimicrobial bacteria. A strain of Bacillus, BAC03 (Patent No. US 7,615,366 B2), was selected from the bacteria showing antimicrobial activity. In this study, BAC03 was characterized under laboratory, greenhouse, and field conditions for antimicrobial activity, common scab control, and plant growth promotion activity. BAC03 was identified as Bacillus amyloliquefaciens by analyzing sequences of fragments of the recA, recN, cheA, and gyrA genes. BAC03 displayed antagonistic activities against Streptomyces spp. on agar plates using a co-culture method. An antimicrobial substance, extracted from BAC03 by ammonium sulfate precipitation, was identified as an LCI peptide using liquid chromatography-mass spectrometry. The antimicrobial activity of either BAC03 liquid culture or the ammonium sulfate precipitate fraction was stable under a wide range of temperatures, and pH levels, as well as following incubation with several chemicals, but was removed by proteinases tested. In greenhouse assays, BAC03 applied as a drench to soil-less potting mix significantly reduced scab in both potato and radish (P < 0.05), while no effect was observed using a foliar spray or a seed treatment. BAC03 applied before radish planting completely suppressed the disease, but the later BAC03 was applied the less effective it was. BAC03 at 105 CFU cm-3 of potting mix or higher concentrations was effective at reducing radish scab (P < 0.5). Increasing the frequency of BAC03 application did not increase efficacy for disease reduction. In population dynamics, as determined by qPCR, the amount of S. scabies when BAC03 was applied before radish planting was significantly lower (P < 0.5) than that in other treatments with BAC03 application at later stages.The ability of BAC03 to promote plant growth was investigated in nine selected plants at the concentration of 105 CFU cm-3 potting mix under greenhouse conditions. Application of BAC03 at 10 days after planting produced higher radish biomass compared to applications at other stages (P < 0.5). Multiple applications of BAC03 increased the biomass of radish roots and leaves compared with a single application. BAC03 produced indole-3-acetic acid and ammonia and showed a 1-aminocyclopropane-1-carboxylate deaminase activity, which potentially could be related to plant growth promotion. Volatiles released from BAC03, with acetoin and 2,3-butanediol as the major components detected by gas chromatography-mass spectrometry analysis, might be responsible for inhibition of plant seed germination, plant seedling growth, and the growth of S. scabies.In two Michigan fields in 2011 and 2012, BAC03 (106 CFU ml-1, 1 L m-2) was applied as a drench into the root zone soil of potato four times at 2-week intervals starting at one month post planting. The BAC03 treatments significantly (P < 0.05) reduced the severity of potato common scab in all tests and had enhanced potato tuber weight in 2011 only (P < 0.05) at two locations.
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