Research Summary
Confirming Glyphosate Resistance in an Annual Bluegrass Population Collected from Sports Turf By Sandeep S. Rana, Doctorate Student; Shawn D. Askew, Ph.D., Associate Professor and Extension Turfgrass Weed Specialist; and John R. Brewer, M.S. Student, Turfgrass Weed Science, Virginia Tech
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lyphosate is the cheapest and most commonly used herbicide for annual bluegrass (Poa annua L.) control in dormant bermudagrass turf. In 2013, a field manager at the Frank Liske Park in Concord, NC, reported an annual bluegrass population not controlled by glyphosate after six years of continuous applications.
Our research
Figure 1. Glyphosate rate needed to reduce annual bluegrass height at 7 WAT and biomass at 3 and 7 WAT by 50%. Uppercase letters within the response variable separate the means according to the Fisher’s Protected LSD at the 5% level of significance.
Plugs of suspected glyphosate-resistant annual bluegrass plants were collected from the site and screened for glyphosate resistance. The annual bluegrass population was found resistant to glyphosate (Roundup Pro Concentrate) at 13 fl. oz./A (0.42 kg ai/ha), the labeled rate for annual bluegrass control in dormant bermudagrass turf. The annual bluegrass population with glyphosate resistance selfpollinated in a greenhouse at the Glade Road Research Facility in Blacksburg to produce seeds. Two greenhouse studies were conducted with an objective to compare locally collected glyphosate-susceptible annual bluegrass to the suspected resistant population for response to glyphosate. Additional greenhouse studies were conducted to determine if resistance to glyphosate in the suspected glyphosate-resistant annual bluegrass population confers resistance to other herbicides. Data were collected for annual bluegrass visual injury and height reduction at weekly intervals and for reduction in annual bluegrass biomass at three and seven weeks after treatment (WAT). Replicate data were converted to percentage reductions compared to untreated plants and regressed against glyphosate rate using the hyperbolic function via SAS 9.2. Estimated GR50 values were then calculated and subjected to analysis of variance to test for trial and biotype effects and interactions. Significant effects were separated using Fisher’s Protected LSD test at the 5% level of significance.
Research results
Photo 1. Response of suspected glyphosate-resistant and -susceptible annual bluegrass biotypes to 1, 2 and 4 times the labeled rate of glyphosate (13 fl. oz./A) for annual bluegrass control in dormant bermudagrass turf. The four pots on the left (RRs) represent the suspected resistant population, and the four pots on the right (Ss) represent susceptible annual bluegrass population.
The suspected resistant population of annual bluegrass was found to be resistant based on significantly different GR50 values from height and biomass data at 3 and 7 WAT. Resistance factors ranged between 2 to 18, depending on measured response variable. This study confirms the first report of glyphosate-resistant annual bluegrass developed on athletic field turf. Research is currently underway to determine the mechanism of resistance in this annual bluegrass population. c
24 | Virginia Turfgrass Journal July/August 2015 www.vaturf.org