Recent advances in engineering led to the fabrication of nanomaterials with unique properties targeted toward specific applications. The use of nanotechnology in agriculture, in particular for plant protection and production, is an under-explored area in the research community. Fungal diseases are one of the leading causes of crop destruction and, in this context, the antifungal effect of nanoparticles of cobalt and nickel ferrite against phytopathogenic fungi are reported here. As a proof of concept, it was also shown how such nanoparticles can be used as fungicides in real plants. The developed cobalt and nickel ferrite nanoparticles (CoFe2O4 and NiFe2O4) were successfully tested for antimycotic activity against three plant-pathogenic fungi: Fusarium oxysporum, Colletotrichum gloeosporioides and Dematophora necatrix. In addition, it is also observed that these ferrite nanoparticles reduce the incidence of Fusarium wilt in capsicum. The study suggests that nanoparticles of CoFe2O4 and NiFe2O4 could be used as an effective fungicide in plant disease management.