| The ability of three rifampicin-resistant strains of Bacillus spp. (B. subtilis M1600, B. subtilis GBO3 and B. amyloliquefaciens IN937) to improve fresh market tomato growth and productivity and reduce the intensity of diseases caused by Xanthomonas euvesicatoria, Alternaria solani, Septoria lycopersici and Pseudomonas cichorii was evaluated under greenhouse, field, and controlled environmental conditions. Rifampicin-resistant mutants of each strain were generated and used to monitor colonization of tomato roots. There were no differences in colony morphology, endospore production or ERIC fingerprint pattern between wild-type and rifampicin resistant mutants of MBI600 and IN937. The rifampicin-resistant mutant of GBO3 had the same colony morphology and ERIC-fingerprint pattern as the wild-type, but produced significantly fewer endospores. Under greenhouse conditions, no growth increase or bacterial leaf spot (X. euvesicatoria) suppression was observed in tomato plants inoculated with any of the Bacillus spp. evaluated in this study. Under field conditions, MBI600 and GBO3+IN937 were integrated in an intensive tomato management program that included mulch, drip irrigation and a forecasted fungicide spray program. Bacillus spp. population densities were 104 - 106 CFU g-1 during the seedling stage and dropped to less than 103 CFU g-1 during the flowering and fruiting stages. Significant increases in plant height were observed in all Bacillus-inoculated tomato plants, however, foliar diseases incited by A. solani and S. lycopersici were not reduced. GBO3+IN937-inoculated plants were more susceptible to bacterial stem rot caused by Pseudomonas cichorii than those inoculated with MBI600 or non-inoculated control plants. In controlled environmental studies, Bacillus spp. population densities ranged from 101-105 CFU g-1 tomato root. Neither nitrogen concentration in the nutrient solution nor activation of the systemic acquired resistance defense mechanism had a significant effect on population density of MBI600, GBO3 or IN937. In a separate experiment under controlled environmental conditions, Bacillus spp. strains MBI600, and GBO3+IN937 did not increase nitrogen concentration in tomato leaves. Lesions induced by Pseudomonas cichorii on tomato stems tended to be longer in GBO3+IN937 inoculated plants than those inoculated with MBI600 or a water-treated control. |
