Selective plant growth suppression by shoot application of soil bacteria

Selected rhizosphere bacterial isolates, previously determined as plant growth deleterious, were tested for their ability to suppress plant growth after foliar spray applications, for selectivity with regard to plant species, and in pilot field experiments for their potential as weed biocontrol agents. Inundative foliar applications of aqueous bacterial suspension were performed on a range of weed and crop species. Plant symptoms after spraying ranged from rapid necrosis and wilting to an overall growth suppression or stunting. Significant and selective reductions in biomass of up to 90% fresh weight, as well as large reductions in plant survival and plant height were recorded in greenhouse pot experiments. However, monocotyledonous plants were affected weakly or not at all by two isolates extensively tested. Effects of these were dose- and plant age-dependent, and were for some plants enhanced by high relative humidity. For one isolate, A153, effects were also expressed in cell-free culture filtrates pointing to involvement of specific metabolites. In pilot field experiments, strong growth suppression was observed on broad-leaved plants, while barley crop plants were unaffected.