In the current study, we formulated and characterized bio-based oil in water nanoemulsions incorporating monoterpenes; (R)-carvone, cinnamaldehyde, citral, geraniol and pulegone and tested their antimicrobial activity against two plant pathogenic bacteria, Pectobacterium carotovorum sub carotovorum and Ralstonia solanacearum in vitro and in vivo studies. Nanoemulsions (NEs) were prepared by adding dropwise monoterpenes at concentration of 5% in an aqueous solution containing 10% a surfactant (tween 80) with constant stirring and then ultrasonication. NEs were confirmed by dynamic light scattering and transmission electron microscopy. Physical stability and viscosity were, also, investigated. NEs monoterpenes had a poly dispersity index ranged from 0.130 to 0.630 and droplet size in the range of 56.64-176.00 nm. Results revealed that NE -cinnamaldehyde had great antibacterial activity against P. carotovorum and R. solanacearum (MIC = 60 and 100 mg/L, respectively). NE cinnamaldehyde was induced effective defense responses in vivo in potato plant and tubers against two tested bacteria at a rate 1000 and 3000 mg/kg. Higher activities of polyphenoloxidase (PPO), peroxidase (POD) activities and total phenolic content of tubers and leaves were significantly recorded   compared to control at all tested times. This was evident from reduced soft and brown rot diseases symptoms of potato spunta cv treated with elicitors subsequently with tested bacteria. These results showed that the PPO, POD and total soluble phenols play a role in instructing resistance to potato soft and brown mold infections. The observed relationship between formulation and activity can lead to the rational design of nanoemulsion based systems for monoterpenes for applications in antimicrobial and agrochemical industries. Amalgamation of such economical treatments might lessen management costs and minimize the environmental pollution.