Plant—pathogen interactions are often omitted as a topic in most introductory and upper-level biology courses. The infection process of the plant pathogen Pythium irregulare on the moss Mnium cuspidatum can be observed and exploited to provide lessons on host—pathogen responses, as well as introduce other biological topics such as microscopy, spectrophotometry, and enzymes. Students can qualitatively analyze plant responses to pathogen infection using microscopy and observe quantitative enzyme responses to draw conclusions. Students are also encouraged to generate hypotheses and test them using this biological system as a method to develop scientific skills.