Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract only
Publication Acceptance Date: 2/9/2007
Publication Date: 2/13/2007
Citation: Kremer, R.J., Kim, S., Park, K. 2007. Occurrence of weed-suppressive microorganisms in soils of crop production fields [abstract]. Weed Science Society of America Meeting. February 4-9, 2007, San Antonio, Texas. 2007 CDROM. Interpretive Summary:
Technical Abstract: The effectiveness of growth-suppressive microorganisms as bioherbicides has been extremely limited for management of annual weeds in row-cropping systems. Bioherbicides based on growth-suppressive microorganisms require further improvements in efficacy of microbial strains and formulations. A more profound understanding is also needed of environmental factors influencing the interactions of microbial agents with weed seeds and seedlings. Alternatively, naturally-occurring biocontrol microorganisms in soils could be manipulated through various management practices to enhance weed seed mortality or growth suppression of weed seedlings. Various weed seed and seedling characteristics were determined in several cropping systems to demonstrate the occurrence of potential weed-suppressive microorganisms under diverse management practices. Soils under sustainable agricultural practices including organic amendment applications and cover cropping generally decreased weed seedbanks and seedling densities by promoting indigenous, weed growth-suppressive microorganisms. For example, foxtail (Setaria spp.) and redroot pigweed (Amaranthus retroflexus) seeds had higher fungal and bacterial infections, respectively, in soil receiving composts compared with non-compost, tilled soils. Similarly, weed seedling density and biomass on soils amended with a municipal compost for two years was reduced by >50% compared with non-treated soil. We also found that management may not always be the major factor influencing development of growth-suppressive microorganisms. This was shown by the discovery of bacteria able to produce very high concentrations of auxins that colonized either conventional or glyphosate-resistant soybean (Glycine max). Previous research has demonstrated plant growth-inhibitory properties of high auxin concentrations. One high auxin-producing bacterium was identified as Bradyrhizobium japonicum, which suppressed growth of ivyleaf morningglory (Ipomoea hederacea), yet has growth-promoting properties as the nitrogen-fixing microsymbiont of soybean. Phyllobacterium spp., described for the first time on each soybean variety, produced ‘ultra-high’ auxin concentrations suggesting this trait might inhibit weed seedling growth within the soybean rhizosphere. Our results demonstrate that weed-suppressive microorganisms are important components of the overall soil or rhizosphere communities in a variety of agricultural soils. Identification of specific cultural practices that enhance these microbial components to produce measurable weed seed mortality and seedling growth suppression in the field is a critical next step for developing a conservation biological control approach as an effective strategy in weed management.