Impact of the pathogen Pyrenophora semeniperda on Bromus tectorum seedbank dynamics in North American cold deserts

Bromus tectorum is a dominant winter annual weed in cold deserts of western North America. We followed patterns of seed carry-over and abundance of the pathogen Pyrenophora semeniperda over 5 years at B. tectorum-dominated shadscale (Atriplex confertifolia) and sagebrush (Artemisia tridentata) sites in southern Idaho. We hypothesised that more seeds could potentially carry over at the drier shadscale site because of minimal autumn precipitation, but that P. semeniperda, a pathogen that primarily kills dormant seeds, would have more impact at the drier site, where a higher density of dormant seeds would likely be present in the early spring seedbank. Successful first-year seed carry-over was higher in years with below-average autumn precipitation. It was lower at the shadscale site than at the sagebrush site (9% vs.16%). The number of seeds killed during incubation by P. semeniperda averaged three times higher at the drier site and the number of field-killed seeds averaged almost five times higher. This suggests that pathogen-related mortality caused the greater decrease in seed carry-over at the drier site. Mortality risk increased dramatically with seed age. This climate-pathogen interaction apparently limits B. tectorum seedbank carry-over in cold deserts to 3 years or less. Pyrenophora semeniperda shows potential as a biocontrol agent for B. tectorum in these habitats.