In sugar beet (Beta vulgaris), virus yellows disease (VY) is caused by a complex of different aphid-transmitted virus species, with Myzus persicae being the most important vector. In Europe, beet yellows virus, beet mild yellowing virus, beet chlorosis virus and beet mosaic virus are the main causatives and impact sugar beet cultivation not only in single but also in co- and multi-infection. Co-infection can enhance replication, tissue spread, vector transmission and other fitness components of at least one of the viruses involved and influence host range, cellular tropism and vector preference. Furthermore, multi-infection of closely related viruses is the starting point for RNA recombination initiating the formation of new, often more virulent strains. As natural multi-virus resistance cannot be expected to be available in the Beta genepool and conventional virus control by reducing vector populations through insecticide treatment was banned, alternative solutions controlling the disease are urgently required. Having this in mind, our project aims to understand viral interactions during host co-colonization as well as vector interactions that are altered by multi-infections and might acerbate impact on plants and increase transmission, respectively. On the virus-plant level, we aim to decipher putative synergistic interactions, identify by transcriptomics plant factors involved and characterize metabolic pathways that are manipulated by VY in single- compared to selected co-/multi-infections. On the level of vector manipulation by the virus infection, effects on aphid behaviour as well as virus transmission preferences in co-infections shall be characterized. The aim of this project is a better understanding of the tight interactions between the three components of the pathosystem (plant-virus-vector) in a multi-infection context, which is a biological reality. Ultimately, this project could identify targets for future safe and sustainable control measures.