Submitted to: North Central Weed Science Society US Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 12/3/2012
Publication Date: 12/9/2012
Citation: Rosenbaum, K.K., Miller, G.L., Kremer, R.J., Bradley, K.W. 2012. Interactions between glyphosate, Fusarium infection of waterhemp, and soil microorganisms. North Central Weed Science Society US Proceedings. p. 67-57. Interpretive Summary:
Technical Abstract: Greenhouse and laboratory experiments were conducted on waterhemp (Amaranthus rudis Sauer) and soil collected from 144 soybean fields in Missouri that contained late-season waterhemp escapes. The objectives of these experiments were to: 1) determine the frequency and distribution of glyphosate resistance in Missouri, 2) determine the effects of soil sterilization on glyphosate-resistant (R) and susceptible (S) waterhemp survival, 3) determine the effects of soil sterilization and glyphosate treatment on infection of R and S waterhemp biotypes by Fusarium spp., and 4) determine the soil microbial abundance and diversity (phospholipid fatty acid analysis [PLFA]) in soils collected from soybean fields with differences in R and S waterhemp biotypes, variable herbicide and glyphosate use histories, and differences in crop rotation. Glyphosate-resistance was confirmed in 99 out of 144, or 69% of the total waterhemp populations surveyed. Crop and herbicide use history was obtained from each of the fields surveyed. Waterhemp biotypes were treated with 1.7 kg glyphosate ae/ha when plants reached 15 cm in height or were left untreated. Waterhemp survival was visually assessed at 1, 2, and 3 weeks after treatment (WAT). To determine Fusarium infection frequency, a single intact waterhemp root was harvested from each treatment at 1, 2, and 3 WAT, surface-sterilized with 10% NaClO solution, and waterhemp root sections (10-15 mm in length) 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 the antibiotics chloramphenicol, streptomycin, and tetracycline before identification. Fusarium isolates were examined microscopically and tentatively identified to species. Identification was confirmed via genomic DNA extraction, and subsequent PCR amplification and sequence analysis of the internal transcribed spacer (ITS) region. Waterhemp plants grown in sterile soils had the highest waterhemp survival, regardless of biotype. Survival of S waterhemp grown in non-sterile soil and treated with glyphosate was only 10% at 3 WAT, while survival of S waterhemp grown in sterile soil was 29%. Similarly, R waterhemp survival was reduced from 83 to 61% when grown in sterile compared with non-sterile soil. The highest Fusarium root infection in waterhemp occurred in non-sterile soil with a glyphosate treatment. Fusarium spp. were recovered from only 14% of the assayed roots (271 treatments with Fusarium out of a total 1920 treatments). The most predominant species recovered were Fusarium solani (the group that includes the causal agent of sudden death syndrome) and Fusarium oxysporum (may be causal agent of vascular wilt). As determined by PLFA, no differences in total PLFA, bacteria, fungi, protozoa, saturated PLFA, monosaturated PLFA, and PLFA biomarkers for arbuscular mycorrhizal fungi, gram positive and gram negative bacteria were observed in field soil collected from locations with either glyphosate R or S waterhemp, and regardless of crop rotation or herbicide-use history. This research supports previous findings with other crop and weed species that indicate plants are more sensitive to glyphosate in non-sterile than sterile soils and that glyphosate may predispose plants to soilborne phytopathogens. The results from this research also suggest that continuous use of glyphosate does not significantly affect soil microbial abundance or diversity.