Rhizobacteria isolated from the rhizospheres of dominant weed species in six representative cropping systems and one native prairie ecosystem in mid-Missouri were screened for phytotoxicity on Lactuca sativa seedlings and their host plants in the laboratory. The proportions of deleterious rhizobacteria (DRB) were compared among different cropping systems to determine possible effects of crop management practices on the occurrence of DRB. Phytotoxicity screening on L. sativa seedlings revealed that an integrated crop management system with a Zea mays–Glycine max–Triticum aestivum cover crop rotation under no-tillage had the highest proportion of DRB at 25.3%, followed by an organic farming system with continuous Fragaria virginiana (strawberry) and organic amendments under minimum tillage at 22.9%. A continuous cool-season grass–legume meadow with no agrochemical inputs had the lowest proportion of DRB at 13%. Crop management practices that maintained high soil organic matter had higher proportions of DRB compared to cropping systems with lower organic matter. Phytotoxicity screening on host plants greatly reduced the proportion of rhizobacteria characterized as DRB, likely because of the high sensitivity of L. sativa seedlings to phytotoxins. Although screening on L. sativa is an effective method to detect phytotoxic rhizobacteria, our research indicates that it is essential to test selected cultures on their host weed species for accurate assessment of their occurrence in the field. Using this approach, we found that crop management practices influence the occurrence of DRB naturally associated with weed seedlings. Results suggest that crop production systems can be developed to favor soil microorganisms such as DRB that affect weed growth and thereby become important considerations in overall weed management.

Nomenclature: Amaranthus retroflexus L. AMARE, redroot pigweed; Ipomoea hederacea (L.) Jacq. IPOHE, ivyleaf morningglory; Setaria faberi Herrm. SETFA, giant foxtail; Zea mays L., corn; Lactuca sativa L. ‘Blackseeded simpson’, lettuce; Glycine max (L.) Merr., soybean; Fragaria virginiana L., strawberry; Triticum aestivum L., wheat.