Emergent diseases of plants, a high proportion of which are caused by phytoviruses, are a significant burden on the food security and economic stability of societies. However, no studies have provided a comprehensive view of the geographical distribution of phytovirus diversity to date, including both the numbers or richness of virus species and the evenness of their distribution in any individual environment on Earth. Our capacity to detect phytoviruses in the early phases of emergence is strongly dependent on our ability to determine the frequencies and geographical distributions of both new introductions of virus and plant species to environments, and the new virus-host encounters that ensue within these environments. This gap in our knowledge undermines our understanding of virus adaptation and limits our capacity to derive truly general predictive models of phytovirus emergence. Recent viral metagenomics studies, which have leveraged methodological innovations to achieve the relatively unbiased sampling and sequencing of viral genomes within natural environments have paved the way towards the analyses of phytovirus biodiversity in these environments in sufficient detail to drive major advances in our understanding of the evolutionary processes that underlie the emergence of phytoviruses as agricultural pathogens. These studies have already revealed that uncultivated areas within agricultural settings are key-players in the ecology and evolution of agriculturally relevant phytoviruses. The two overarching hypotheses that we propose testing in this project will extend the findings of these pioneering virus biodiversity studies: - Plant community structure influences phytoviral community structure: We hypothesize that plant community species richness, composition, density and biomass are predictors of phytovirus species richness. - The rate of molecular evolution of viruses is slower in uncultivated areas than in cultivated areas: We hypothesize that land uses changes and cropping practices are likely to select for fast-growing, early-transmitted, and more virulent viruses. The PHYTOVIRUS project has three scientific and technological objectives that will aim at testing whether plant species richness influences phytovirus species richness in natural and cultivated areas (objective 1), studying experimentally the effect of plant communities on phytoviral species richness (objective 2), and searching for evolutionary fingerprints associated with emergence within phytovirus genomes (objective 3). The first work package (WP1) will yield a substantially expanded inventory of known phytoviruses and provide detailed comparative data on the species richness of plants and phytoviruses in several natural unmanaged ecosystems and managed agricultural systems. WP2 will experimentally test whether associations exist between the species richness of phytovirus and plant communities. Finally, WP3 will explore sequence data generated in WP1/2 to detect and characterise evolutionary footprints (evolutionary rates, recombination patterns and natural selection patterns) that are associated with emergence. Besides providing the first assessments of phytovirus species richness in selected environments, this project also aims at defining and demonstrating a standardized experimental approach to measure phytovirus species richness that could then be universally used at scales ranging from defined ecosystems through to entire continents. The project brings together research groups that are specialized in plant virology, viral metagenomics, plant ecology, and the computational analysis of virus evolution. This multidisciplinary consortium has the ability to implement a holistic research program that is without equivalent at the international levels with respect to its focus on phytovirus species richness and the plant community and viral evolutionary parameters that have shaped this species richness.