Glyphosate-Resistant Annual Bluegrass (Poa annua) in Missouri.

            In the transition zone, annual bluegrass is a major problem for golf courses containing warm season grasses such as zoysiagrass.  Current management includes postemergence applications of glyphosate when zoysiagrass is dormant.  During 2007, a population of annual bluegrass on a golf course in central Missouri survived sequential applications of glyphosate.  Seed was collected from surviving plants and germinated under greenhouse conditions with a potting mix in plastic flats.  On 10 cm seedlings, glyphosate was applied at an expected lethal dose (0.75 kg/ha) to select for glyphosate-resistant (Gly-R) plants.  Surviving seedlings were allowed to mature and seed collected.  Suspect Gly-R seeds and known susceptible seeds were germinated in plastic pots under the conditions described above and treated with glyphosate from 0 to 6.3 kg ae/ha and 0 to 1.6 kg ae/ha, respectively.  In repeated studies, visual evaluations 21 days after treatment (DAT) indicated glyphosate injury was 95 % or greater for rates above 1.6 kg/ha on susceptible plants.  However, on suspect Gly-R plants 6.3 kg/ha of glyphosate only resulted in 61% visual injury.  Using plant dry weights at 21 DAT, the biomass of susceptible annual bluegrass was reduced by 80% at 1.6 kg/ha compared to the untreated control.  For suspect Gly-R plants, plant biomass was reduced by only 54% at the highest rate.  Three weeks after collecting biomass, 75% of suspect Gly-R plants re-grew at the highest glyphosate rate, with no survivors for susceptible plants at the 1.6 kg/ha rate.  These results indicate that glyphosate resistance has been selected in a population of annual bluegrass.  This represents the first time glyphosate resistance has occurred for a species on a golf course, and annual bluegrass is the first annual grass identified with glyphosate resistance.