Ecology Letters (2010) 13: 810–818
Host characteristics commonly determine infection risk, but infection can also be mediated by regional- or local-scale variation in the biotic and abiotic environment. Experiments can clarify the relative importance of these factors. We quantified drivers of infection by barley and cereal yellow dwarf viruses (B/CYDV), a group of generalist, vector-borne grass pathogens, at hierarchically nested spatial scales (105–1 m) by planting individuals of six common grass species into five Pacific Coast grassland sites spanning 7° of latitude (> 5000 total hosts) and applying a factorial combination of nitrogen and phosphorus fertilizer. Infection differed most among experimental blocks (102–103 m scale), suggesting that local factors control infection risk; infection increased with cover of long-lived hosts and phosphorus, but not nitrogen, fertilization. For B/CYDV, local context more strongly predicts infection risk than host species traits or regional context; such spatially nested experiments can clarify the factors underlying variation in infection risk.