An experimental test of weed-suppressive bacteria effectiveness in rangelands in southwestern Idaho, 2016–18

Approaches and techniques for control of exotic annual grasses are a high priority in sagebrush-steppe and other rangelands. Strains of the soil bacterium Pseudomonas fluorescens (Pf) have been proposed to be selectively pathogenic to multiple species of exotic annual grasses with effects evident by the second year, and with no effect on native or desirable species including native bunchgrasses. However, scientifically defensible tests of the target and non-target/risk effects of these hypothetically weed-suppressive bacteria (WSB) strains in the field have been lacking in rangelands and other environments. We evaluated the effects of two strains of Pf WSB (D7 and MB906) sprayed on the surface in autumn 2016 at three sites in sagebrush steppe across southwestern Idaho that had cheatgrass (Bromus tectorum), medusahead (Taeniatherum caput-medusae), and other exotic annual grasses. Treatments also were replicated within each site (n=3, 8.3×8.3 meter plots) and included evaluation of the WSB strains with and without herbicides (imazapic and rimsulfuron) and with or without discing to mix surface-spray of the WSB into deeper soils. By the second year following application (spring 2018), neither strain of WSB affected exotic annual grasses, perennial bunchgrasses, or total community cover, either with WSB alone or in combination with herbicides or discing. We conclude that neither the D7 nor MB906 strains of Pf WSB have a negative effect on exotic annuals at the sites we evaluated.