Host range of a deleterious rhizobacterium for biological control of downy brome

Pseudomonas fluorescens strain D7 (P. f. D7; NRRL B-18293) is a root-colonizing bacterium that inhibits downy brome (Bromus tectorum L. BROTE) growth. Before commercialization as a biological control agent, strain D7 must be tested for host plant specificity. Agar plate bioassays in the laboratory and plant–soil bioassays in a growth chamber were used to determine the influence of P. f. D7 on germination and root growth of 42 selected weed, cultivated or native plant species common in the western and midwestern United States. In the agar plate bioassay, all accessions of downy brome were inhibited by P. f. D7. Root growth of seven Bromus spp. was inhibited an average of 87% compared with that of controls in the agar plate bioassay. Root growth of non-Bromus monocots was reduced by 0 to 86%, and only 6 out of 17 plant species were inhibited 40% or greater. Among all plant species, only downy brome root growth from two accessions was significantly inhibited by P. f. D7 in plant–soil bioassays (42 and 64%). P. f. D7 inhibited root growth and germination in agar plate bioassays more than in plant–soil bioassays. Inhibition in plant–soil bioassays was limited to downy brome, indicating promise for P. f. D7 as a biocontrol agent that will not harm nontarget species. Nomenclature: Downy brome; Bromus tectorum L. BROTE; rhizobacterium; Pseudomonas fluorescens.