Rice is the daily food for more than half of the world's population. This crop is threatened by various diseases, including rice sheath rot which results in rotting of the rice sheath and sterility of the resulting grains. The major pathogen causing this disease is a fungus, called Sarocladium oryzae. This fungus produces two toxic compounds, helvolic acid and cerulenin. Helvolic acid is not only toxic for the plant, but also for other bacteria, while cerulenin is toxic for other fungi. We believe that by producing these toxins, the fungus creates a specific niche in the rice leaf sheath in which few other microorganisms can survive, a hypothesis that we will test in this project. In our research about rice sheath rot in Rwanda, we found that specific fluorescent Pseudomonas bacteria are associated with rice sheath rot. These bacteria are very similar to well-known biocontrol bacteria that normally live on plant roots and protect plants to soilborne pathogens by producing antifungal toxins. These bacteria have never been found in leaves of plants before. In this project we want to study whether these bacteria help the plant to control the fungal disease or rather enhance the disease symptoms. We will also investigate how these Pseudomonas bacteria and Sarocladium oryzae interact with each other in the rice leaf sheath and how they are influenced by each other's toxins. Results of this project may lead to strategies to conquer this disease by biological control.