Environmental nutrient supply alters prevalence and weakens competitive interactions among coinfecting viruses

• The rates and ratios of environmental nutrient supplies can determine plant community composition. However, the effect of nutrient supplies on within-host microbial interactions is poorly understood. Resource competition is a promising theory for understanding microbial interactions, because microparasites require nitrogen (N) and phosphorus (P) for synthesis of macromolecules such as nucleic acids and proteins. • To better understand the effects of nutrient supplies to hosts on pathogen interactions, we singly inoculated and coinoculated Avena sativa with two virus species, barley yellow dwarf virus-PAV (BYDV-PAV) and cereal yellow dwarf virus-RPV (CYDV-RPV). Host plants were grown across a factorial combination of N and P supply rates that created a gradient of N: P supply ratios, one being replicated at low and high nutrient supply. • Nutrient supply affected prevalence and the interaction strength among viruses. P addition lowered CYDV-RPV prevalence. The two viruses had a distinct competitive hierarchy: the coinoculation of BYDV-PAV lowered CYDV-RPV infection rate, but the reverse was not true. This antagonistic interaction occurred at low nutrient supply rates and disappeared at high N supply rate. • Given the global scale of human alterations of N and P cycles, these results suggest that elevated nutrient supply will increase risks of virus coinfection with likely effects on virus epidemiology, virulence and evolution.