Failure to recover Pseudomonas fluorescens D7 supports claims of ineffectiveness as biocontrol agent of Bromus tectorum

Cheatgrass (Bromus tectorum) is one of the most problematic invasive plants in the western United States. Invasion by annual grasses disrupts nutrient cycling and negatively affects above- and below-ground biodiversity. Land managers use chemical herbicides, mechanical controls, cultural practices, and bioherbicides to combat this invasive plant. Recently, the biocontrol agent Pseudomonas fluorescens D7 has been touted as a non-chemical herbicide that offers lasting control of cheatgrass. However, experimental results report limited effectiveness under field conditions. To understand the underlying cause of the variable efficacy of this commercially available bioherbicide, soil samples from an active cheatgrass invasion treated with P. fluorescens D7 were collected and screened using high-throughput sequencing. At 18 months post-application, the bioherbicide had limited lasting effects on bacterial community composition, and no reads assigned to P. fluorescens D7 were found in our sequence data. We suggest that the failure to establish may partially explain the inability of this biocontrol agent to suppress B. tectorum under field conditions.