Purpose: To investigate the effect of salicylic acid (SA) on the antimicrobial profile of Stevia leaf extracts against soybean seed-borne pathogens.

Methods: Stevia seeds were planted in a greenhouse and SA foliar applied after six weeks on the whole plant at concentrations of 0 and 0.1 g L^-1. The extracts of the plant leaf were separately obtained using four different solvents (water, acetone, ethanol and chloroform), and the oil composition of the extracts determined by gas chromatography-mass spectrophotometry (GC-MS). The antibacterial and antifungal potentials of each of the extracts were examined against 11 selected pathogens responsible for soybean seed and seedling diseases.

Results: Chloroform extracts had the highest amount of α-cadinol, spathulenol, caryophyllene oxide, methyl salicylate and safranal in the SA-treated plants, and were 8, 10, 18, 14 and 11 %, respectively, higher than the non-SA treated control. In the anti-microbial tests, chloroform extract exhibited the highest diameter of inhibition zone (max 18 mm) against all the tested microorganisms while water extract showed the least effect (max 9 mm), with no effect at all on two fungi (Phomopsis spp and Cercospora kikuchii) and two bacteria (Pseudomonas syringae and Xanthomonas campestris). All extracts with or without SA had no effect on Xanthomonas campestris. SA treatment enhanced the antimicrobial potential of all extracts in the pathogenicity test compared with untreated plant extracts.

Conclusion: Stevia leaf extract has antimicrobial effect against soybean seed-borne disease if applied on the seed before planting. Application of SA on the Stevia plant substantially enhances the antimicrobial activity of the leaf extract thus affording the seeds greater protection.

Keywords: Stevia, Antimicrobial, Salicylic acid, Soybean, Pathogens.