Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2013
Publication Date: 1/15/2014
Citation: Rosenbaum, K.K., Miller, G.L., Kremer, R.J., Bradley, K.W. 2014. Interactions between glyphosate, fusarium infection of common waterhemp (amaranthus rudis), and soil microbial abundance and diversity in soil collections from Missouri. Weed Science. 62(1):71-82. Interpretive Summary: Waterhemp, a member of the pigweed family, is currently the most common weed species encountered in corn and soybean production fields in the Midwestern U.S. The weed is difficult to control due to vigorous growth habit, high seed production, and adaptability to changes in environment. Where the herbicide glyphosate is applied annually, about 70 percent of the waterhemp remaining in soybean fields at harvest in Missouri are resistant to glyphosate. Considerable previous research with crops has shown that glyphosate, which is systemically transported throughout the plant after application to foliage, may predispose the plant to infection by soil microorganisms thereby aiding the herbicide in weakening and eventually killing the plant. However, little information is available on soil microbial interactions with glyphosate affecting weeds. Describing these interactions should provide a better understanding of the herbicidal performance of glyphosate on weeds in the field including the ability of GR waterhemp to withstand glyphosate applications. Such research would allow us to begin answering questions regarding the role that plant–microbe relationships may play in the evolution of resistance to glyphosate. Our research objectives were to determine the interactions that occur between glyphosate application, Fusarium infection of waterhemp, and soil microbial abundance and diversity. We chose to monitor Fusarium because colonization of roots by this fungus appears to by stimulated by glyphosate when applied to crops and other susceptible plants. Seeds of both glyphosate-susceptible (GS) and GR waterhemp biotypes were collected from Missouri soybean fields and planted in greenhouse containers of non-treated (non-sterile) or autoclaved (sterile) field soil. Waterhemp biotypes in half of the containers were sprayed with glyphosate at the recommended application rate when plants were about 6 inches (15 cm) tall; the other half was not sprayed. Waterhemp survival was recorded 21 days after glyphosate treatment and plant samples were removed to determine Fusarium colonization of roots. Root sections of waterhemp were surface-sterilized and cultured on medium selective for Fusarium growth. Root colonization by Fusarium, appearing as white colonies on root pieces placed on the agar surface of the culture medium, was recorded 14 days after incubation. Soils from 131 soybean fields with either GS or GR waterhemp were also characterized for their microbial community composition using a fingerprinting technique based on biochemical differences in microbial cell walls. Both GS and GR waterhemp plants grown in sterile soils survived glyphosate treatment; however, survival of GS waterhemp in non-sterile soil was only 10 percent while survival of GR waterhemp was 61 percent. Fusarium root infection only occurred in non-sterile soils and was associated with 25 percent of GS and 12 percent of GR waterhemp plants. This suggests that waterhemp is more sensitive to glyphosate in the presence of soil microorganisms (as in non-sterile soils) and may be more affected by soil-borne pathogens (shown by the increased incidence of root-colonizing Fusarium). The basis for the apparent resistance of GR waterhemp to any combined effect of glyphosate plus Fusarium could not be explained and requires further research. Few differences in soil microbial community characteristics based on biochemical fingerprinting were detected in field soils regardless of presence of GR or GS waterhemp, or of herbicide and crop history. Thus, continuous use of glyphosate seems to not affect soil microbial abundance or diversity. The results are important for soil and weed scientists, extension and consulting personnel, herbicide industry scientists, and crop producers because they provide new insights to the evolution of glyphosate resistance in weeds and serve as bases for deve
Technical Abstract: Greenhouse and laboratory experiments were conducted on waterhemp (Amaranthus rudis Sauer) and soil collected from 131 soybean fields in Missouri that contained late-season waterhemp infestations that escaped weed control. The objectives of these experiments were to determine the effects of soil sterilization on glyphosate-resistant (GR) and susceptible (GS) waterhemp survival, determine the effects of glyphosate treatment on infection of GR and GS waterhemp biotypes by Fusarium spp., and determine the soil microbial abundance and diversity in soils collected from soybean fields with differences in waterhemp biotypes and herbicide and crop rotation histories. Waterhemp biotypes at approximately 15 cm in height were treated with 1.7 kg glyphosate ae/ha or left untreated. Waterhemp survival was visually assessed at 21 days after glyphosate treatment (21 DAT). To determine Fusarium infection frequency, a single intact waterhemp root was harvested from each treatment at 0, 3, 7, 14, and 21 DAT, surface-sterilized, and 10-15 mm root sections were plated on Komada culture medium. After 14 days incubation, fungal colonies were selected from colonized roots and maintained on potato dextrose agar medium amended with antibiotics. Speciation of Fusarium isolates was conducted through microscopic examination of morphological characteristics and confirmed by sequencing and analysis of ribosomal DNA. Soil samples from 131 different collections were subjected to phospholipid fatty acid (PLFA) analysis, utilizing gas chromatography to determine the soil microbial community structure. Waterhemp plants grown in sterile soils had the highest waterhemp survival, regardless of biotype. After treatment with glyphosate survival of GS waterhemp grown in sterile soil was 29% at 21 DAT, while survival of GS waterhemp grown in non-sterile soil was only 10%. Similarly at 21 DAT with glyphosate, GR waterhemp survival was 83% in sterile soil, while survival was 61% in non-sterile soil. Fusarium spp. were recovered from only 12% of the assayed roots (223 treatments with Fusarium out of a total 1920 treatments). The highest Fusarium root infection in both GR and GS waterhemp occurred in non-sterile soil with glyphosate treatment. Few differences in total PLFA were observed in field soil collected from locations with either GR or GS waterhemp, and regardless of herbicide or crop history. This research supports previous findings that plant species are more sensitive to glyphosate in non-sterile than sterile soils and indicates that glyphosate may predispose plants to soil-borne phytopathogens. This research also suggests that continuous use of glyphosate does not significantly affect soil microbial abundance or diversity.