Chemical fungicides are the backbone of modern agriculture, but an alternative formulation is necessary for sustainable crop production to address human health issues and soil/water environmental pollution. So, a green chemistry approach was used to form guar gum nanoemulsions (NEs) of 186.5–394.1 nm containing the chemical fungicide mancozeb and was characterized using various physio-chemical techniques. An 84.5% inhibition was shown by 1.5 mg/mL mancozeb-loaded NEs (GG-1.5) againstA. alternata, comparable to commercial mancozeb (86.5 ± 0.7%). The highest mycelial inhibition was exhibited againstS. lycopersiciandS. sclerotiorum.In tomatoes and potatoes, NEs showed superior antifungal efficacy in pot conditions besides plant growth parameters (germination percentage, root/shoot ratio and dry biomass). About 98% of the commercial mancozeb was released in just two h, while only about 43% of mancozeb was released from nanoemulsions (0.5, 1.0 and 1.5) for the same time. The most significant results for cell viability were seen at 1.0 mg/mL concentration of treatment, where wide gaps in cell viability were observed for commercial mancozeb (21.67%) and NEs treatments (63.83–71.88%). Thus, this study may help to combat the soil and water pollution menace of harmful chemical pesticides besides protecting vegetable crops.