Joint effects of biotic and abiotic stressors on winter wheat suppression ofBromus tectorum

Summary In winter wheat systems in the Northern Great Plains of the United States, Bromus tectorum and wheat streak mosaic virus (WSMV) commonly co-occur. While independent effects of these pests on wheat yields have been well documented, to our knowledge, no study has investigated whether WSMV modifies interactions between wheat and B. tectorum. Furthermore, the impact that environmental stressors such as nutrient availability have on these interactions has not been addressed. We conducted a randomised split-plot field study over 2 years to investigate the effects of WSMV and nitrate (N) availability on winter wheat suppression of Bromus tectorum. The study included four N treatments (10–19, 20–31, 31–84, and 85– 207 kg ha 1 ) and two WSMV treatments (mechanically inoculated or control). Increasing soil N increased the susceptibility of wheat to WSMV infection. In 2009, wheat in the lowest and highest N levels had 24% and 65% of plants infected respectively. However, regression analysis indicated that interactive effects of wheat competition, N and WSMV did not play a consistent role in B. tectorum growth. Specifically, the effect of both wheat density and distance from row on B. tectorum biomass remained constant across inoculation treatments, suggesting that wheat inoculated with WSMV suppressed B. tectorum as effectively as healthy wheat. Furthermore, wheat had a greater impact on B. tectorum growth in higher N environments, even though incidence of WSMV infection in wheat was highest. Overall, our results suggest that WSMV infection may not change the ability of wheat to suppress B. tectorum.