Introduced pests and pathogens are increasingly challenging agricultural and forest production systems, as well as natural and near-natural ecosystems. Biological control of these harmful organisms is a highly desirable plant protection measure, particularly in forests where the use of pesticides is often not applicable and resistance breeding only a long-term option. Hyperparasitism, the fundamental relationship between two parasitic organisms typically provides the basis for biological control. Mycoviruses that infect plant pathogenic fungi have the potential to be used as biocontrol agents. In this project, we will use the Cryphonectria parasitica-hypovirus pathosystem to investigate the dynamics of virus infection in a fungal pathogen.