Effect of the Bioherbicide Pseudomonas fluorescens D7 on Downy Brome (Bromus tectorum)

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Abstract

Downy brome (Bromus tectorum) is one of the most problematic invasive plant species in the United States. Downy brome invasions are associated with reductions in diversity and an increase in fire intensity. Bioherbicides that are reported to effectively control downy brome have been developed. Pseudomonas fluorescens strain D7 is one such bioherbicide that is of significant interest to land managers in Wyoming. A spatially replicated field trial was performed to identify what effect D7 has on downy brome in Wyoming. The field trial showed no response of any downy brome fitness metrics to D7; only commonly used synthetic herbicides were able to reduce downy brome cover. This work suggested D7 may not be a viable product for downy brome control, or more information may be needed about the field conditions required for a positive result when using D7.

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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This work was supported by the Pesticide Safety Education Funds Management Program (grant 0000000028) and USDA-NIFA-CPPM EIP (grant 2017-70006-27281).

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