This cluster of papers explores the role of mycoviruses in fungal symbiosis and pathogenesis, particularly focusing on their interactions with plant pathogenic fungi, hypovirulence induction, RNA silencing mechanisms, and potential for biological control. It also delves into topics such as viral RNA-dependent RNA polymerases, horizontal gene transfer, endogenization of viral genomes, and the specific case study of chestnut blight caused by Cryphonectria parasitica.