Physicochemical Stability and In Vitro Antifungal Activity of Mentha spicata Essential Oil and L-Carvone Nanoemulsions Against Stromatinia cepivora and Sclerotinia sclerotiorum

Natural bioactive compounds, such as essential oils, have attracted increasing attention for the sustainable management of plant diseases. However, their high volatility, chemical instability, and poor water solubility limit agricultural applications, highlighting the need for nanostructured delivery systems. Although Mentha spicata essential oil and its major constituent, L-carvone, exhibit well-documented antifungal activity, their nanoemulsions remain largely unexplored for controlling the soil-borne phytopathogens Stromatinia cepivora and Sclerotinia sclerotiorum. Here, oil-in-water nanoemulsions containing M. spicata essential oil (NEOEMS) or L-carvone (NEOELC) were developed and characterized regarding physicochemical properties, colloidal stability, antifungal activity, and their effects on common bean seed germination and garlic clove emergence and early growth. Both formulations remained nanometric during the 36-day storage period, despite changes in droplet size and PDI, and a limited reduction in L-carvone content. NEOELC completely inhibited S. cepivora mycelial growth at 800 µg mL−1, whereas NEOEMS required 1600 µg mL−1. Against S. sclerotiorum, both nanoemulsions achieved complete inhibition at concentrations of 400 µg mL−1, with similar IC50 values (162.0 and 182.8 µg mL−1 for NEOELC and NEOEMS, respectively). Neither formulation significantly affected bean seed germination, and garlic clove emergence was preserved; however, shoot length was lower than that of the iprodione control under field conditions. These findings support further investigation of nanoemulsions as delivery systems for natural antifungal compounds. NEOELC’s greater activity against S. cepivora may be related, at least in part, to its higher relative L-carvone content.