Pseudomonas leaf spot (PLS) disease in peppers caused by Pseudomonas syringae pv. syringae (Pss) is an emerging seedborne phytopathogen. Pss infection can severely reduce the marketable yield of peppers in favorable environmental conditions and cause significant economic losses. The intensive use of copper-sulfate and streptomycin-sulfate to control PLS and other bacterial diseases is associated with antimicrobial-resistant Pss strains, making these control methods less effective. So, there is an urgent need to develop novel antimicrobials effective against Pss in peppers. Several studies, including those done in our laboratory, have shown that small molecule (SM) antimicrobials are ideal candidates as they can be effective against multidrug resistant bacteria. Therefore, our study aims to identify novel SM growth inhibitors of Pss, assess their safety, and evaluate their efficacy on Pss-infected pepper seeds and seedlings. Using high-throughput screening, we identified 10 SMs (PC1 to PC10) that inhibited the growth of Pss strains at 200 µM or lower concentrations. These SMs were effective against both copper- and streptomycin-resistant as well as biofilm-embedded Pss. These SMs were effective against other plant pathogens (n = 22) at low concentrations (<200 μM) and had no impact on beneficial phytobacteria (n = 12). Furthermore, these SMs showed better or equivalent antimicrobial activity against Pss in infested pepper seeds and inoculated seedlings compared with copper-sulfate (200 μM) and streptomycin (200 μg/ml). Additionally, none of the SMs were toxic to pepper tissues (seeds, seedlings, or fruits), human Caco-2 cells, and pollinator honeybees at 200 μM. Overall, the SMs identified in this study are promising alternative antimicrobials for managing PLS in pepper production.
Keywords: Pseudomonas syringae pv. syringae; bacterial pathogens; growth inhibitors; high-throughput screening; novel management method; pepper disease; small molecules.