Novel Small Molecule Growth Inhibitors of spp. Causing Bacterial Spot of Tomato.

Bacterial spot (BS) of tomato, caused by , , , and , is difficult to control because of the high prevalence of copper- and streptomycin-resistant strains and the lack of resistance cultivars and effective bactericides. The objective of this study was to identify novel growth inhibitors of BS-causing (BS-X) species by using small molecules (SM;  = 4,182). Several SMs (X1, X2, X5, X9, X12, and X16) completely inhibited the growth of BS-X isolates ( = 68 , 55 , 4 , and 32 ) at ≥12.5 µM by disrupting cell integrity through weakening of the cell membrane and formation of pores. These SMs were also effective against biofilm-embedded, copper- and streptomycin-resistant strains while having minimal impact on other plant pathogenic ( = 20) and beneficial bacteria ( = 12). Furthermore, these SMs displayed equivalent antimicrobial activity against BS-X in seeds and in seedlings compared with conventional control methods (copper sulfate and streptomycin) at similar concentrations while having no detectable toxicity to tomato tissues. SMs X2, X5, and X12 reduced , , , and populations in artificially infested seeds ≤3.4-log CFU/seed 1 day postinfection (dpi) compared with the infested untreated control ( ≤ 0.05). SMs X1, X2, X5, and X12 reduced disease severity ≤72% and engineered bioluminescent populations ≤3.0-log CFU/plant in infected seedlings at 7 dpi compared with the infected untreated control ( ≤ 0.05). Additional studies are needed to increase the applicability of these SMs for BS management in tomato production.