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Fluorescent pseudomonad isolates G309 and CW2, in combination with the resistance inducer acibenzolar-S-methyl (ASM), improved control of fungal and bacterial diseases on tomato plants. The interactions of the bacteria in the presence of ASM showed that in vitro growth of Pseudomonas fluorescens G309 and Pseudomonas sp. strain CW2 was not affected in King's B broth supplemented with 10 and 20 microM ASM. Also, the bacterial cells were not able to utilize ASM as a nutrient source. In vitro production of the two antimicrobial secondary metabolites phenazine-1-carboxylic acid and 2-OH-phenazine by the isolate CW2 was not affected within 3 days from incubation. In contrary, addition of ASM at a concentration of 20 microM to King's B liquid medium significantly increased production of salicylic acid by isolate G309. When roots of tomato plants were treated with G309 or CW2 cell suspensions containing 20 microM ASM, the number of bacterial cells recovered from the rhizosphere was significantly higher in the combined treatments than in the single applications 5, 10, and 15 days after inoculation. However, ASM at a higher concentration (50 microM) did not appreciably enhance the population sizes of either bacterial isolate in the rhizosphere. Enhanced bacterial cell densities in the rhizosphere of tomato plants were also determined following simultaneous treatments of tomato roots with 10 and 20 microM ASM in combination with the transformed isolate G309-384 (mini-Tn5gfp), which encodes the green fluorescent protein.