Abstract
Ecology Letters (2010) 13: 810–818 Abstract 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 (10 5 –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 (10 2 –10 3 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.