Herein, we have displayed an easy way to produce monodisperse spinel nanoparticles (NPs) and the antifungal activity of CoFe2O4, Co0.5Zn0.5Fe2O4 and ZnFe2O4 nanostructures. Firstly, the structural, morphological and magnetic properties of each NP were investigated through x-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Vibrating Sample Magnetometer (VSM). The XRD data showed diffraction peaks related to the crystalline spinel phase. The TEM micrographs displayed monodisperse NPs with spherical morphology. The average sizes of CoFe2O4, Co(0.5)Zn0(.5)Fe(2)O(4) and ZnFe2O4 NPs were 6.87 +/- 0.05 nm, 5.18 +/- 0.01 nm and 11.52 +/- 0.09 nm, respectively. The VSM data indicated that the nanostructures are superparamagnetic at room temperature. Afterward, the antifungal properties of the Co/Zn-based ferrite NPs against Botrytis cinerea were tested. So, the inhibition of mycelial growth by different concentrations (45 - 360 ppm) of NPs was measured. The most effective nanostructure was CoFe2O4, with an EC50 value of 265 ppm. Further, to elucidate how the NPs are affecting B. cinerea, reactive oxygen species (ROS) production was measured. The results indicated that the CoFe2O4 monodisperse NPs could induce a burst of ROS in B. cinerea, promoting cellular damage.