Management effects on the incidence of jointed goatgrass inhibitory rhizobacteria

Jointed goatgrass (Aegilops cylindrica Host., JGG), a winter annual grass weed, is a major threat to small grain production in the United States. It infests an estimated 2 million hectares in the US and causes annual losses of $145 million in crop yield and quality. Selective herbicides are available only for certain herbicide-resistant crop varieties and alternative management practices are tillage, burning, or rotation to less profitable crops. We evaluated bacteria that were isolated from winter wheat (Triticum aestivum L., WW), downy brome (Bromus tectorum L., DB), and JGG roots to determine their potential as biological control agents for JGG. The bacteria were sequentially screened in laboratory bioassay, growth chamber and field studies for their effect on JGG and WW root growth. In laboratory studies, 1236 isolates (50%) of the 2450 naturally occurring soil bacteria tested were inhibitory to JGG seedling growth. Only 76 isolates (3.1%) inhibited JGG, but not WW in the in vitro bioassays. In the growth chamber, only 7 (0.3%) of the isolates inhibited JGG growth and not WW. In field studies, 4 of the 7 isolates selected after bioassay and growth chamber tests inhibited JGG more than 30%. Conventional tillage systems maintained higher populations of inhibitory bacteria than no-tillage systems. The number of bacteria inhibitory to JGG was greater in the spring sampling and at the highest level of nitrogen fertilization. Crop rotation had little effect on the incidence of jointed goatgrass inhibitory bacteria. Different JGG accessions from the western US were diverse in their response to inhibitory bacteria. Weed-suppressive bacteria isolated from plant roots and soil have the potential to be used as biological control agents for JGG.