If the spray concentration of the pesticide remains constant, submicronization of the active ingredients significantly increase the particle number concentration, thereby substantially enhancing the probability of contact with plant pathogens and is anticipated to result in reduced pesticide usage. In this study, we investigated the effect of submicronization of pesticide active ingredients on the inhibitory effect against plant pathogens. Three types of monodispersed penthiopyrad (pesticide) particles with different median diameters and minimal overlap in the particle size distribution were successfully prepared using a poor solvent dilution method. Using these penthiopyrad particles, we conducted experiments on infection of tomatoes (model plants) with Botrytis cinerea (plant pathogen). The results showed that the submicronization of pesticide particles enhanced the inhibitory effects of the sprayed pesticide on the plant pathogen, while also decreasing the required amount of pesticide sprayed to the plants. Furthermore, confocal microscopy images showed that smaller particles are taken up by B. cinerea hyphae better than larger particles. Additionally, the bacterial flora on the leaf surface was not affected by submicronized pesticide particles larger than 100 nm. These findings imply that submicronization of pesticide active ingredients increases the probability of reaching targeted plant pathogens on the leaf surface, ultimately resulting in an increase in their uptake by the plant pathogens (i.e., significantly improved delivery of pesticides).