Effect of the Bioherbicide Pseudomonas fluorescens D7 on Downy Brome (Bromus tectorum)☆
Introduction
Downy brome (Bromus tectorum) is one of the most widespread and problematic invasive plant species in the United States (Knapp 1996). Downy brome is associated with reductions in plant biodiversity and fitness (Gasch et al., 2013, Parkinson et al., 2013), reductions in wildlife habitat (Dumroese et al. 2015), and increases in fire risk (Bradley et al. 2018), among many other negative impacts (e.g., Melgoza et al. 1990).
Due to the negative impacts associated with downy brome invasion, landscape managers continue to seek new strategies to remove downy brome and restore previously invaded habitats (Kelley et al. 2013). Bioherbicides have been a recent alternative tool of significant interest for downy brome control (Kremer and Kennedy, 1996, Aston and Gorton, 2015). In particular, the use of specific genetic strains of the soil bacteria Pseudomonas fluorescens has been reported to reduce the vigor and cover and downy brome among other invasive annual grasses (Gealy et al., 1996, Tranel et al., 1993, Tranel et al., 2014). One such strain, D7, has been registered by the Environmental Protection Agency as a bioherbicide for use on downy brome in rangeland systems (EPA Registration 2014). Therefore, the intent of this study was to identify the effect of P. fluorescens D7 on downy brome in Wyoming and determine if it may be a viable alternative to current management strategies.
Section snippets
Methods
To test the effect of P. fluorescens on downy brome, a field study was performed during the 2017 growing season. The product P. fluorescens D7 was used because it is the only current P. fluorescens product labeled for downy brome control. Because P. fluorescens is a living agent, prehandling of bioherbicide is more important when compared with synthetic herbicides to ensure it survives and can incorporate into the soil (D7 Label 2014). In all cases, the product was kept near freezing until just
Results
Downy brome cover was significantly affected by herbicide treatments (P = 0.0001). However, in 2017, only treatments that included imazapic significantly reduced downy brome cover below the untreated control (Fig. 1). D7 alone did not reduce downy brome cover when compared with the untreated control. In 2018, only combinations of synthetic and bioherbicides significantly reduced cover below the untreated control; however, these two treatments were not significantly different from the synthetic
Discussion
P. fluorescens strain D7 did not decrease downy brome cover regardless of the application rate. Even at a 2× rate there was no effect on cheatgrass. This is contrary to results from the Pacific Northwest (Kennedy, 2018, Kennedy, 1991).
Most applications of P. fluorescens in Wyoming are in combination with synthetic herbicides with the belief that synergies occur between the two treatments. Unfortunately, there was also no synergistic effect when combining D7 and either of the synthetic
Implications
Downy brome management is still a challenge for many managers across the western United States. Unfortunately, P. fluorescens strain D7 did not have any effect on downy brome at multiple sites in Wyoming, making it unlikely to be an effective product in diverse environmental conditions. Unfortunately, synthetic herbicides did not offer long-term control either. This research shows why land managers have and will continue to seek alternative management strategies to deal with the continuing
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2020, Rangeland Ecology and ManagementCitation Excerpt :This suggests that tank mixing bacteria and herbicide may have reduced the phytotoxicity of the herbicide, and we offer hypothetical explanations for this below. Our results for MB906 are somewhat more encouraging than results for WSB effects in other recent studies, which invariably showed no effect of various P. fluorescens strains, including MB906, on invasive annual grasses (Germino and Lazarus, 2020; Reinhart et al., 2020; Pyke et al., 2020; Tekiela, 2019). However, the inconsistent times and places where we observed MB906-induced reductions of invasive-annual grasses differ from Kennedy's (2018) report of steady declines up to 80% by the third year following ACK55 application in mixed stands.
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This work was supported by the Pesticide Safety Education Funds Management Program (grant 0000000028) and USDA-NIFA-CPPM EIP (grant 2017-70006-27281).