Comparative study between pure and nanoemulsions (NEs) monoterpenes, cinnamaldehyde, citral, geraniol pulegone, and (R)-carvone for their fungicidal activity against three of soil borne pathogenic fungi namely, Pythium digitatum, Rhizoctonia solani and Fusarium solani by using poisoned food technique were evaluated. Results indicated that all examined monoterpenes have displayed antifungal activity under different tested concentrations. Further more. NEs- monoterpenes reduced significantly the linear growth of all tested pathogens than pure monoterpenes. NEs citral and geraniol ranked the   first for their effective antifungal compounds on the tested fungi with EC₅₀ values of 31.31 and 49.41 μg/ml on R. solani, 48.21 and 54.16 μg/ml on P. digitatum and 83.61 and 100.75 μg/ml on F. solani, respectively. Carbendazim fungicide was the reference in this study. NEs citral and geraniol were induced effective defense responses in vivo in tomato plants against R. solani at a rate of 1000 mg/L. Determination in the leaves after 0, 1, 3, 7 and 15 days of inoculation for polyphenoloxidase (PPO), peroxidase (POD) as well as total phenolic content activities as defense-related enzymes were measured. A significant increase in PPO, POD activities and total phenol content were detected. Results, also, showed that to reflect the resistance and susceptibility nature of tomato cultivars against the pathogenic fungi, R. solani., POD and PPO activities can be considered as biochemical markers. The obtained results indicated that the elicitor's NEs-citral and geraniol have brought about defense reaction in tomato plants towards the pathogenic fungi. These effective NEs monoterpenes could be effective potentially and environmentally safe to control tomato damping- off disease.