Growth response of weed and crop seedlings to deleterious rhizobacteria

Selected bacterial isolates previously demonstrated to be suppressive toward weed species in the laboratory were tested for effectiveness under greenhouse conditions. Rhizobacteria varied in ability to inhibit growth of host or nonhost weed species. Some bacterial isolates caused ⩽75% growth inhibition, while some isolates did not express inhibitory effects under greenhouse conditions. Host specificity of rhizobacteria also varied, with some isolates significantly suppressing growth of host plants as well as nonhost weed species and occasionally crop plants. For example, green foxtail [Setaria viridis (L.) Beauv.] growth was suppressed by 57% of rhizobacteria isolated from several weed hosts but morningglory (Ipomoea hederacea L.) and barnyardgrass [Echinochloa crus-galli (L.) Beauv.] were suppressed by only 32 and 37% of the rhizobacterial isolates, respectively. Isolates that only inhibited growth of weed plants without negatively affecting crop plants can be considered candidates for further tests as potential biological control agents. Because potential biological control agents would encounter more complex interactions with indigenous microorganisms and environmental factors when applied in the field, the greenhouse test is an important step in documenting the effectiveness and host specificity of deleterious bacteria.