Pathological aspects of metal sulfide nanoparticles are lacunae in nanometal bioapplications. The present study is one of its own kind involving sonochemical synthesis and the phytopathological effect of aqua dispersed nanoparticles of ferrous sulfide (FeS-NPs) against the economically important fungus Fusarium verticillioides. In vitro antifungal evaluation against F. verticillioides showed the significant biopotential at 18 μg/ml, with deca-fold higher efficiency than the standard fungicide, carbendazim. Visible symptoms of conidial inhibition as well as membrane disruption in SEM nanographs rationalized the fungal inhibition. In vivo seed treatment on rice (Oryzae sativa) at 30 μg/ml revealed a significant reduction in seedling blight (76.08%) which is deca-fold greater than carbendazim (32.60% at 2000 μg/ml) with a comparable reduction in seed rot (82.12% for FeS-NPs and 80.76% for carbendazim). There were no morphologically visible signs of phytotoxicity at concentrations as high as 30 μg/ml giving a favorable edge to this novel FeS-NPs sample as antifungal nanopriming agent.