The majority of the plants in the natural conditions are resistant to most of the incompatible pathogens (viral, fungal and bacterial infections). This phenomenon is called ״non-host resistance״. This type of resistance is very important however, not enough research was conducted to explain the mechanism of this type of resistance. When the authors inoculated several non-host plants with incompatible pathogens, levels of hydrogen peroxide (H₂O₂) and superoxide (O₂^.-) were increased and elevated early after inoculation. This phenomenon was found in tomato, datura, tobacco, cucumber, squash and Chenopodium which were inoculated with tobacco powdery mildew, Papaya ringspot virus (PRSV), cucumber powdery mildew, tobacco mosaic virus (TMV), tomato powdery mildew and PRSV, respectively. Interestingly that, when some hosts from those mentioned above were inoculated, each with its compatible pathogen, no accumulation of H₂O₂ and O₂^.- was occurred. The authors concluded that reactive oxygen species (ROS) mainly H₂O₂ and O₂^.- could have a key role in inhibiting or killing the pathogens early in the non-host plants. The authors recommend giving more attention to the application of H₂O₂ and O₂^.- either with direct application or with applying compounds which induce or produce ROS against phytopathogens.