Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Plant and Environmental Science


Dr. L. B. McCarty

Committee Member

Dr. W. C. Bridges

Committee Member

Dr. J. S. McElroy

Committee Member

Dr. N. Tharayil

Committee Member

Dr. T. Whitwell


Annual bluegrass (Poa annua L.) is one of the most pervasive, adaptable, and variable plant species in the world and is the most problematic winter annual weed in managed turfgrass. Its prolific seedhead production regardless of mowing height, clumping growth habit, and lack of tolerance to stress reduces turfgrass aesthetic quality and playability on golf courses. Its ability to germinate almost year-round influences cultural practices and herbicide use. Herbicides are an integral part of a weed control program for all weeds that invade a desirable turfgrass stand. However, inappropriate use of herbicides results in herbicide resistance issues. In recent years, the number of annual bluegrass populations on golf courses resistant to herbicides has increased. Currently, this weed is resistant to 9 mechanisms of action worldwide. Most recently, resistance has evolved to acetolactate synthase (ALS) inhibitors and glyphosate, both of which are integral herbicides for annual bluegrass control. Therefore, the purpose of this research was to investigate ecological, physiological, and molecular characteristics of resistance to these herbicides in order to increase the understanding of annual bluegrass herbicide resistance. Studies included diagnosing resistant biotypes, assays of enzyme activity, DNA sequencing, and simulation modeling. Beyond the issue of herbicide resistance, control remains difficult in all commercial turfgrass situations. Additional studies investigated annual bluegrass control in creeping bentgrass putting greens and bermudagrass fairways overseeded with perennial ryegrass with current and experimental herbicides and plant growth regulators to determine best management practices for controlling annual bluegrass. To diagnose resistance to ALS-inhibiting herbicides in biotypes from South Carolina and Georgia, dose-response experiments and ALS activity assays were conducted on mature annual bluegrass plants using trifloxysulfuron, foramsulfuron, and bispyribac-sodium. For dose-response experiments, I50 values for susceptible (S) biotypes were 13.6 g ai ha-1 for trifloxysulfuron, 7.0 g ai ha-1 for foramsulfuron, and 38.3 g ai ha-1 for bispyribac-sodium. Fifty percent shoot biomass reduction was not observed in either the South Carolina (CI) or Georgia (FP) biotypes at eight times the labeled field rate of all ALS-inhibiting herbicides tested. For in vivo tests of ALS activity, the CI biotype yielded I50 values 3650, 3290, and 13 times S biotypes following treatment with trifloxysulfuron, foramsulfuron, and bispyribac-sodium, respectively. Similarly, I50 values for the FP biotype were 316, 140 and 64 times greater than S biotypes following the same herbicide treatments. This confirms high levels of annual bluegrass resistance to multiple ALS-inhibiting herbicides in South Carolina and Georgia. Further investigations into enzyme and growth characteristics of ALS-resistant annual bluegrass were conducted with a biotype from Alabama (GN) containing a mutation in the ALS gene resulting in a Trp574 to Leu amino acid substitution. Compared to the susceptible (VS) biotype, the GN biotype exhibited a 27- and 10-fold resistance to trifloxysulfuron at the whole plant level and under in vitro conditions, respectively. No significant differences were observed in Km (pyruvate) or extractable ALS activity between biotypes, but the Vmax was higher for the GN biotype. The feedback inhibition of ALS activity by the branched-chain amino acids was higher for the GN biotype than the VS biotype, with leucine, valine, and isoleucine inhibiting ALS activity 20, 6, and 4% more in the R biotype, respectively. The GN biotype produced more inflorescences and seeds per plant in comparison with the VS biotype, but relative growth rates between biotypes were similar at all harvest intervals. This provides baseline information regarding ALS enzyme response, vegetative growth, and reproduction characteristics of annual bluegrass biotypes resistant and susceptible to ALS-inhibiting herbicides. Glyphosate is used in the transition zone to control annual bluegrass in fully dormant warm-season grasses. A suspected resistant (CN) biotype of annual bluegrass was identified on a golf course in South Carolina after at least 10 consecutive years of glyphosate application. Resistance was confirmed after 4.4 times more glyphosate was required to reduce growth 50% compared to a standard susceptible (VS) biotype. Further studies were conducted to determine the mechanism conferring glyphosate resistance in the CN biotype. Leaf discs of both biotypes accumulated shikimate in response to increasing glyphosate concentration, but the I50 for EPSP synthase inhibition in the CN biotype was 3.5-fold higher than the S biotype. At the whole plant level, similar levels of shikimate accumulation were observed between biotypes at 6 and 24 hours after treatment (HAT) with glyphosate, but greater shikimate accumulation occurred in the VS biotype at 72, 120, and 168 HAT. Shikimate levels decreased in the CN biotype after 72 HAT. There were no differences in 14C-glyphosate uptake between biotypes. However, more 14C-glyphosate translocated out of the treated leaf in the CN biotype and into root tissues over time compared to the VS biotype. Partial sequencing of the EPSP synthase gene revealed a heterozygous mutation at Pro106 which resulted in a substitution of Ala. These results represent the first documentation of a Pro106 to Ala substitution as the mechanism of glyphosate resistance in annual bluegrass and the first report of glyphosate-resistant annual bluegrass in South Carolina. A basic simulation model was developed for the evaluation of herbicide resistance evolution in golf course populations of annual bluegrass, to understand key biological parameters of annual bluegrass which result in high resistance risks, to evaluate several annual bluegrass management programs for golf course fairways and their relative risks for selecting resistance, and to compare two herbicides (i.e., glyphosate and ALS-inhibitors) to determine how their respective characteristics influence resistance evolution in turfgrass systems. Annual bluegrass biological characteristics, typical turfgrass weed management strategies, and several genetic parameters were used in the simulations. Values and ranges for parameters were determined via review of the literature and field observations. In these simulations, the first population with evolved herbicide resistance was predicted after 5 and 9 yr of annual use of ALS-inhibitors and glyphosate, respectively. Several herbicide use strategies were subsequently simulated to assess their potential for managing resistance and included using alternate mechanisms of action in rotation or using these herbicides in combination with PRE or POST herbicides for early annual bluegrass control. The most effective use strategy for glyphosate was rotating mechanism of action. In comparison, the most effective resistance management strategy for ALS-inhibitors was applying a PRE or POST herbicide for early control followed by rotating ALS-inhibitors with an alternate mechanism of action in late winter. Regardless of which strategy was used, >90% resistance risk was predicted for either herbicide after 40 years. Biological parameters including seed bank density, annual germination proportion, and seed removal had significant influence on resistance evolution. These simulations suggest annual bluegrass population dynamics contribute to its propensity to develop herbicide resistance and highlight the need for integrated control programs to manage resistance. Field trials in creeping bentgrass putting greens revealed the best currently labeled annual bluegrass management options is multiple applications of paclobutrazol in fall and spring. This provided 70%. All methiozolin treatments provided >70% control after two years at two locations, but this herbicide is not currently registered for use. When it becomes registered, the best program in the transition zone will be multiple applications at low rates during the fall and spring. All other treatments failed to provide satisfactory control or seedhead suppression. In overseeded ryegrass, methiozolin and amicarbazone either did not consistently control annual bluegrass or damaged ryegrass beyond an acceptable threshold. Single herbicide applications in overseeded ryegrass will not provide acceptable control. The best program consists of a preemergence herbicide, an early postemergence application with a sulfonylurea herbicide, and a postemergence application with either ethofumesate or bispyribac-sodium. In summary, annual bluegrass remains as the most problematic weed for managed turfgrasses. Herbicide resistance to many mechanisms of action increases the difficulty of control of this weed. There are only three labeled mechanisms of action for turfgrass where annual bluegrass has not evolved resistance. Thus, turfgrass managers must use integrated programs to prevent resistance from developing. Where resistance to a single herbicide exists, it is of utmost importance to develop sound herbicide use practices such that multiple resistance will not evolve. Future research should continue to investigate herbicide resistant populations of annual bluegrass, management programs which are best suited to delay or prevent resistance, and new herbicides which provide alternative mechanisms of action for its control.

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