Optimizing efficacy of Bromus tectorum (cheatgrass, downy brome) biological contorl in crops and rangelands

Abstract

Management of Bromus tectorum L., an annual grass invasive in western North America, has focused on single and integrated methods across crop and non-crop settings. Extensive literature does not exist on the integration of Pyrenophora semeniperda, a generalist grass pathogen for B. tectorum control, which has been used experimentally with some success to control B. tectorum. However, questions remain about (1) the risk of non-target effects on grassy species, (2) efficacy as part of an integrated management plan, and (3) efficacy under different environmental conditions and on different B. tectorum populations. I sought to answer these questions with three distinct studies. First, I assessed the risk of P. semeniperda on B. tectorum and 15 co-occurring grass species in a greenhouse setting. Pyrenophora semeniperda reduced B. tectorum density by 40% but also negatively affected density of 60% of the non-target species tested, particularly native rangeland grasses. Second, I integrated P. semeniperda as part of a two-year rangeland revegetation management plan that included an herbicide (imazapic), a fungicide seed treatment, and different perennial grass seeding rates. Application of P. semeniperda did not increase inoculum loads above ambient levels, and there was no effect of seeding rate or seed treatment on B. tectorum or seeded perennial grass density or biomass. However, B. tectorum density was reduced by 60% the first year with a single imazapic application. Lastly, I compared the effects of temperature (13°C, 17°C, 21°C, 25°C, 32°C) and B. tectorum populations (range, crop, sub-alpine) on infection and mortality rates caused by P. semeniperda using a temperature gradient table. Infection rates by P. semeniperda peaked at intermediate temperatures (17°C, 21°C, 25°C) for range and sub-alpine populations, but were generally low and not as influenced by temperature in the crop population. Overall, B. tectorum control with P. semeniperda is possible, provided (1) non-target effects are considered, especially for range species, (2) research is conducted to increase P. semeniperda inoculum loads above ambient levels and revegetation is used with other control tactics, and (3) we take into account how distinct B. tectorum populations respond to P. semeniperda.