Mycoviruses, or fungal viruses, represent an important biotechnological tool. Understanding the complex mutual interactions is crucial for the development of new biological control methods and the improvement of strategies for managing phytopathogenic fungi. This thesis presents their potential and mechanism of action in the control of diseases such as chestnut blight, white rot, and Fusarium head blight of cereals. Furthermore, their origin and the effects of evolution, which led to the development of numerous taxonomic groups of mycoviruses, including those with dsRNA, ssRNA, or ssDNA genomes, are explored. Only those that were able to adapt to selective pressures within the host system, such as inhibiting the host/fungus antiviral response at the RNA interference level, have successfully persisted. Like other viruses, mycoviruses require living cells for their survival, but they do not spread in the usual way. The basic principles and examples of horizontal and vertical transmission are presented. An interesting characteristic of mycoviruses is their ability to influence the virulence of the host. Some mycoviruses cause hypovirulence, where the fungus becomes less virulent, while others cause hypervirulence, where the fungus becomes pathogenic due to viral infection. These effects are often linked to mechanisms that influence changes in morphology, growth, pigmentation, sporulation, and toxin production. Additionally, other biotic and abiotic factors influencing the development of phenotypic changes are also investigated.
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