Resistance shown by an entire plant species to a wide range of bacterial, fungal and viral pathogens is already mentioned as nonhost resistance (NHR) phenomenon. Here, the defense responses of cucumber, squash and pumpkin inoculated with two different pathogens of powdery mildew of compatible, which belong to genera Sphaerotheca fuliginea (Schlecht. Fr.) and incompatible, which belong to genera Leveillula taurica, cause powdery mildew of pepper were investigated. No visible symptoms were observed as a result of inoculation of cucurbits with nonadapted (incompatible) pathogen, while severe symptoms were occurred as a result of inoculation with compatible pathogen. Disease severity percentage was increased in all host plants inoculated with compatible pathogen relative to nonhost plants inoculated by incompatible pathogen. Levels of superoxide (O₂^•−) and hydrogen peroxide (H₂O₂) which are the major forms of reactive oxygen species (ROS) were increased and accumulated early after 1, 2 and 3 days from inoculation in the nonhost as compared with host plants. Activities of antioxidant enzymes such as catalase (CAT), peroxidase (POX) and polyphenol oxidase (PPO) in all tested nonhost plants inoculated with incompatible pathogen 1, 2 and 3 days after inoculation were less than host plants inoculated with compatible pathogenat the same time intervals. It seems that accumulation of ROS early in nonhost plants has the key role of inhibiting or killing the incompatible pathogen. Understanding the resistance responses of host and nonhost plants for identifying similarities and differences is needed for the practical application in crop improvement.