| Summary: | <i>Digitaria ischaemum</i> (Schreb.) Schreb. ex Muhl. and <i>Poa annua</i> L. are competitive, early successional species which are usually considered weeds in agricultural and turfgrass systems. Bacteria and fungi associated with <i>D. ischaemum</i> and <i>P. annua</i> seed may contribute to their competitiveness by antagonizing competitor forbs, and were studied in axenic culture. <i>Pantoea</i> spp. were the most common bacterial isolate of <i>D. ischaemum</i> seed, while <i>Epicoccum</i> and <i>Curvularia</i> spp. were common fungal isolates. A variety of species were collected from non-surface sterilized <i>P. annua</i>. Certain <i>Pantoea</i> spp. isolates were antagonistic to competitor forbs <i>Taraxacum officinale,</i> <i>Trifolium repens</i>. All bacterial isolates that affected <i>T. officinale</i> mortality were isolated from <i>D. ischaemum</i> seed while none of the <i>P. annua</i> isolates affected mortality. Two selected bacterial isolates identified as <i>Pantoea ananatis</i> were evaluated further on <i>D. ischaemum,</i> <i>T. repens</i> (a competitor forb) and <i>P. annua</i> (a competitor grass) alone and in combination with a <i>Curvularia</i> sp. fungus. These bacteria alone caused >65% <i>T. repens</i> seedling mortality but did not affect <i>P. annua</i> seedling mortality. These experiments demonstrate that <i>Pantoea</i> <i>ananatis</i> associated with <i>D. ischaemum</i> seeds is antagonistic to competitor forbs in axenic culture. The weedy character of <i>D. ischaemum</i> could at least in part stem from the possession of bacteria that are antagonistic to competitor species.
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