Summary Bromus tectorum is a winter annual grass that affects rangeland in western North America. A glasshouse pot experiment was conducted that integrated imazapic application and inoculation of the soil‐borne fungal pathogen, Pyrenophora semeniperda, for the purpose of providing greater control of B. tectorum. We hypothesised that P. semeniperda inoculation would reduce B. tectorum emergence and integration of imazapic and P. semeniperda would result in a greater reduction in B. tectorum biomass and density compared with either treatment applied alone. This study revealed that P. semeniperda significantly reduced B. tectorum emergence and density and the responses were greatest for seed placed below the soil surface. Further, B. tectorum biomass was similar between imazapic and P. semeniperda treatments. This indicates that P. semeniperda could be applied in advance of B. tectorum germination and emergence. After emergence, imazapic application could reduce B. tectorum biomass and kill seedlings. A two‐pronged approach to controlling B. tectorum that combines P. semeniperda inoculation and post‐emergent imazapic application may provide a greater opportunity to limit invasion of this weed in rangeland of western North America. Future work should be directed towards the pathogen–plant relationship and how it relates to integrating biological control with traditional methods, towards the effect of varying P. semeniperda inoculum and imazapic rates and lastly, to how environmental conditions in the field may affect implementation and efficacy of this two‐pronged approach.
Chemical control of downy brome has focused on imazapic; however, imazapic efficacy in semiarid climates is unpredictable, possibly because of variable residual soil activity. Our objective was to characterize imazapic activity over 9 mo in rangeland and a Conservation Reserve Program (CRP) site following its application in the fall as affected by rate (0, 80, 160, 240 g ai ha−1) and quantity of plant residue (reduced, ambient). Greenhouse bioassays were conducted over two seasons (2010 to 2011 and 2011 to 2012) using soil collected at multiple dates after imazapic application. Quantity of plant residue did not affect downy brome biomass or response to imazapic. Imazapic reduced downy brome biomass (P < 0.05) across all sampling dates in both seasons, and the response to rates was consistent up to 200 d post application. Imazapic activity over time conformed to a biphasic model with activity being consistent, or slightly improving, up to about 160 and 150 d post application, and then dropping rapidly to the final sampling event 287 and 272 d post application in rangeland and at CRP sites, respectively. These results indicate that fall imazapic applications in semiarid climates persist into the spring, thus providing control of both fall-emerging downy brome seedlings and seeds that overwinter and emerge the following spring.
Downy brome (Bromus tectorum L., syn. cheatgrass) is a winter annual grass that invades North American cropping, forage, and rangeland systems. Control is often difficult to achieve, because B. tectorum has a large seedbank, which results in continuous propagule pressure. Pyrenophora semeniperda (Brittlebank and Adam) Shoemaker, a soilborne fungal pathogen, has been investigated as a biological control for B. tectorum, because it can kill seeds that remain in the seedbank, thereby reducing propagule pressure. Temperature influences P. semeniperda and has not been investigated in the context of seeds collected from different B. tectorum locations, that may vary in susceptibility to infection. We compared the effects of temperature (13, 17, 21, 25 C) and B. tectorum seed locations (range, crop, subalpine) with different mean seed weights on infection rates of P. semeniperda using a temperature-gradient table. Infection differed by seed location (P < 0.001) and temperature (P < 0.001), with lighter-weight seeds (i.e., range and subalpine) more susceptible to P. semeniperda infection. Infection increased as temperature increased and was higher at 21 C (66.7 ± 6.7%) and 25 C (73.3 ± 6.0%). Germination was affected by seed location (P < 0.001) and temperature (P = 0.019). Germination was highest for the crop seed location (45.4 ± 4.2%) and overall decreased at higher temperatures (21 and 25 C). Our results suggest that B. tectorum seeds from a crop location are less affected by P. semeniperda than those from range and subalpine locations. Moreover, this demonstrates a temperature-dependent effect on all populations.
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