Crop disease remains a major problem to global food production. Excess use of pesticides through chemical disease control measures is a serious problem for sustainable agriculture as we struggle for higher crop productivity. The use of plant growth promoting rhizobacteria (PGPR) is a proven environment friendly way of controlling plant disease and increasing crop yield. PGPR suppress diseases by directly synthesizing pathogen-antagonizing compounds, as well as by triggering plant immune responses. It is possible to identify and develop PGPR that both suppress plant disease and more directly stimulate plant growth, bringing dual benefit. A number of PGPR have been registered for commercial use under greenhouse and field conditions and a large number of strains have been identified and proved as effective biocontrol agents (BCAs) under environmentally controlled conditions. However, there are still a number of challenges before registration, large-scale application, and adoption of PGPR for the pest and disease management. Successful BCAs provide strong theoretical and practical support for application of PGPR in greenhouse production, which ensures the feasibility and efficacy of PGPR for commercial horticulture production. This could be pave the way for widespread use of BCAs in agriculture, including under field conditions, to assist with both disease management and climate change conditions.

Tomato bacterial canker disease, caused by Clavibacter michiganensis subsp. michiganensis (Cmm) is a destructive disease and has been a serious concern for tomato industries worldwide. Previously, a rhizosphere isolated strain of Pseudomonas sp. 23S showed antagonistic activity toward Cmm in vitro. This Pseudomonas sp. 23S was characterized to explore the potential of this bacterium for its use in agriculture. Pseudomonas sp. 23S possesses ability to solubilize inorganic phosphorus, and to produce siderophores, indole acetic acid, and hydrogen cyanide. The strain also showed antagonistic activity against Pseudomonas syringae pv. tomato DC 3000. A plant assay indicated that Pseudomonas sp. 23S could promote growth of tomato seedlings. The potential of treating tomato plants with Pseudomonas sp. 23S to reduce the severity of tomato bacterial canker by inducing systemic resistance (ISR) was investigated using well characterized marker genes such as PR1a [salicylic acid (SA)], PI2 [jasmonic acid (JA)], and ACO [ethylene (ET)]. Two-week-old tomato plants were treated with Pseudomonas sp. 23S by soil drench, and Cmm was inoculated into the stem by needle injection on 3, 5, or 7 days post drench. The results indicated that plants treated with Pseudomonas sp. 23S, 5 days prior to Cmm inoculation significantly delayed the progression of the disease. These plants, after 3 weeks from the date of Cmm inoculation, had significantly higher dry shoot and root weight, higher levels of carbon, nitrogen, phosphorus, and potassium in the leaf tissue, and the number of Cmm population in the stem was significantly lower for the plants treated with Pseudomonas sp. 23S. From the real-time quantitative PCR (qRT-PCR) analysis, the treatment with Pseudomonas sp. 23S alone was found to trigger a significant increase in the level of PR1a transcripts in tomato plants. When the plants were treated with Pseudomonas sp. 23S and inoculated with Cmm, the level of PR1a and ACO transcripts were increased, and this response was faster and greater as compared to plants inoculated with Cmm but not treated with Pseudomonas sp. 23S. Overall, the results suggested the involvement of SA signaling pathways for ISR induced by Pseudomonas sp. 23S.

Biopesticides, which consist of antagonistic microorganisms, or the bioactive compounds they produce offer attractive alternatives to synthetic agrochemicals as a means of reducing plant diseases in crop production systems. In our previous study, a strain of Pseudomonas entomophila 23S was reported to have a strong antagonistic activity toward Clavibacter michiganensis subsp. michiganensis (Cmm), a pathogen of tomato bacterial canker disease, which causes serious damage to the tomato production industry. This study focused on purification and characterization of the anti-Cmm compound(s) produced by P. entomophila 23S. The cell-free supernatant was first subjected to butanol extraction, fractioned by solid-phase extraction (SPE), and purified by high-performance liquid-chromatography (HPLC). The semi-preparative purified compounds retained anti-Cmm activity. These compounds were then characterized by liquid chromatography coupled with high-resolution tandem mass spectrometry (LC-HRMS/MS) and nuclear magnetic resonance (NMR). Two anti-Cmm compounds, C15H16N2O and C16H18N2O, were isolated by this procedure. From the LC/MS spectra, the compounds were predicted to have an isoquinoline ring, the presence of which was confirmed by NMR. The compounds differed with respect to the presence of a methyl group in a side chain. In addition, P. entomophila 23S grown in different culture media produced the greatest amounts of these anti-Cmm compounds when in Nutrient Broth (NB), where bacterial growth was the slowest. NB composition is more favorable in stimulating P. entomophila 23S to produce higher amounts of these anti-Cmm compounds.