Copper based nanoparticles (Cu-based NPs) of different composition and sizes have been hydrothermally synthesized by varying the reaction temperature and in the presence of biocompatible surfactants; polyoxyethylene (20) sorbitan laurate, polyethylene glycol 1000 and polyethylene glycol 8000. Effective control of the above synthetic parameters gave rise to Cu-based NPs of 11–55 nm. CuO, Cu₂O nanoparticles and Cu/Cu₂O composite nanoparticles of nearly similar size and shape were selected among the isolated samples and tested in the field on tomato (Lycopersicon esculentum) plants against Phytophthora infestans. Their activity has been evaluated by assessing the fungal infection on the leaves in comparison to the performance of the registered commercially used copper based products Kocide 2000, Kocide Opti, Cuprofix disperss and Ridomil Gold Plus. The results showed that the prepared Cu-based NPs can be applied in a lower formulated product and active ingredient rate and are more effective than the trade agrochemicals. Moreover, assessments regarding phytotoxicity indicated that they can be used without any deleterious effect on plants.