Botrytis blight was observed in Egypt (Nov., 2016) on Poinsettia (Euphorbia pulcherrima) that caused small light brown spots at the edge of leaves and enlarged to include the whole leaf. The isolated fungus was identified as Botrytis cinerea based on its morphological and cultural characteristics. In greenhouse, artificial inoculation of plants developed blight symptoms. The effect of ultrasonication on the particle size of lemongrass and citronella essential oil nanoemulsions was determined. The results showed that the particle sizes from sonication for 30 minutes were around 90.6 and 79.8 nm, and TEM study revealed the spherical shape. Changes in the essential oil emulsions components were observed in the nanoemulsions compared with the original as the size of the oil particles become small, the stability and main component of the emulsion were significantly improved. In vitro, the nanoemulsion at different concentrations (1000 – 8000 ppm) were capable of adversely affecting the mycelial growth of Botrytis cinerea. In greenhouse experiment, poinsettia plants were sprayed with the two essential oils (emulsion and nanoemulsion) and the fungicide carbendazim 50% WP (as additional control), after and before inoculation by the fungus. The disease severity was decreased by the treatments compared to the untreated control. The lowest disease severity, being 10.0 and 13.0% with nanoemulsion of lemongrass, before and after inoculation by the fungus, respectively. The results indicate an increase in the concentrations of active substances in essential oils due to their exposure to ultrasonication and their conversion to nanoemulsions, as the results suggest the potential effects of lemongrass essential oil nanoemulsion as a novel fungicide agent against Botrytis cinerea