We study plant-pathogen interactions, especially the host's active, if unwitting, role in disease development. We work with powdery mildew disease on model plant Arabidopsis thaliana, using mutational analysis to identify host factors required for successful disease development. We also study a new area of plant-pathogen biology, non-host resistance that protects all members of a plant species from all members of a pathogen species. Results from both these projects highlight the importance of both active and passive defenses operating in the host cell wall.