New options for Poa annua control in bentgrass greens

The fight against annual bluegrass in creeping bentgrass continues with two new herbicides.

Read this story in GCM's digital edition

Robert B. Cross; Bert McCarty, Ph.D.; and Alan G. Estes
July 2013 Poa annua control: plot 1

This untreated control plot had heavy annual bluegrass infestations with a combination of perennial and annual biotypes.
Photos by R. Cross

Annual bluegrass (Poa annua L.) is the most troublesome winter annual weed in managed turfgrasses, especially creeping bentgrass (Agrostis stolonifera L.) putting greens (11). In the southeastern United States, a perennial habit tends to dominate the annual bluegrass population in creeping bentgrass greens, as high input requirements during summer (syringing, fans, fungicide applications) allow annual bluegrass to survive.

Control of annual bluegrass in creeping bentgrass is difficult to attain. Because both creeping bentgrass and annual bluegrass are cool-season species, herbicide selectivity is a challenge. In addition, golf greens are under constant cultural and environmental stresses, which herbicide applications often increase.

Currently, plant growth regulators are the most viable labeled option for annual bluegrass seedhead control in creeping bentgrass greens. However, multiple applications are required throughout the fall and spring to suppress annual bluegrass growth and seedhead production.

Two new herbicides have recently generated interest for their potential annual bluegrass control. Xonerate (amicarbazone, Arysta LifeScience Corp.) is a triazolinone herbicide that inhibits photosynthesis by disrupting electron flow at photosystem II (5). It is currently labeled for annual bluegrass control in creeping bentgrass tees and fairways, but not in putting greens.

July 2013 Poa annua control: Walker

The research sites used in the studies for annual bluegrass control in creeping bentgrass greens were at Walker GC and Cross Creek Plantation (below).

Methiozolin (proposed trade name PoaCure) is a new isoxazoline compound from Moghu Research Center in South Korea with proposed modes of action being inhibition of cell-wall biosynthesis and/or inhibition of tyrosine aminotransferase (7,10). Regardless, this is a new mode of action for annual bluegrass control in turfgrass. Methiozolin is currently labeled in several Asian countries for annual bluegrass control in creeping bentgrass putting greens. It has not been registered for use in the United States, but U.S. registration is expected in 2015.

July 2013 Poa annua control: Cross Creek

The purpose of this study was to evaluate various herbicide and plant growth regulator treatment programs for annual bluegrass control and seedhead suppression, including several recently developed herbicides, and to determine their safety for use on creeping bentgrass putting greens.

Materials and methods

From 2010 to 2013, several studies were conducted on creeping bentgrass greens built to USGA recommendations in order to analyze multiple herbicides and plant growth regulators for annual bluegrass control.

Treatments were applied at Walker Golf Course at Clemson University, Clemson, S.C., from fall 2010 to spring 2012 on Crenshaw creeping bentgrass and at Cross Creek Plantation, Seneca, S.C., from fall 2011 to spring 2013 on L-93 creeping bentgrass. The same treatments were applied to the same plot area at each site during the two-year study.

All treatments were applied using a CO2-pressurized backpack sprayer calibrated to deliver 20 gallons/acre (187 liters/hectare) through 8003 flat-fan nozzles. Methiozolin treatments were sprayed at one-third of the herbicide rate three times to achieve the recommended spray volume of 60 gallons/acre (561 liters/hectare). Treatment programs, active ingredients, rates and timings are presented in Table 1.

July 2013 Poa annua control: table 1

Throughout the studies, sites were maintained by the golf course maintenance staffs to normal putting green standards. Greens were mowed daily at 0.125 inch (0.32 centimeter) with irriga- The research sites used in the studies for annual bluegrass control in creeping bentgrass greens were at Walker GC (top) and Cross Creek Plantation (bottom). tion applied as needed to prevent wilt. Study sites received 6 pounds nitrogen/1,000 square feet (29 grams nitrogen/square meter) yearly and were treated with fungicides as needed to prevent disease pressure. Plots at both sites contained more than 50% annual bluegrass density at the initiation of the studies, and more than 80% of these populations were determined to be perennial biotypes (visual observation).

July 2013 Poa annua control: plot 2

After two years of treatment, methiozolin applied six times in the fall and spring at 32 ounces/acre provided greater than 90% annual bluegrass control at the Walker GC site.

Data collected

Subsequent performance ratings were recorded monthly from trial initiation until late spring, including annual bluegrass density, seedhead suppression and creeping bentgrass turf quality. Additionally, data were obtained for creeping bentgrass phytotoxicity, plot density and bermudagrass green-up, but these data are not presented.

Annual bluegrass density was visually estimated using a scale of 0% to 100%, where 0% = no annual bluegrass and 100% = complete annual bluegrass coverage. Annual bluegrass control values were calculated using visual estimates by determining the percent annual bluegrass density reduction for each plot. Seedhead suppression was calculated as a percentage of the untreated control and was evaluated using a scale of 0 to 9, where 0 = no annual bluegrass seedheads and 9 = bright white seedheads. Turfgrass quality was visually evaluated using a scale of 1 to 9, where 1 = brown, dead turfgrass, 9 = dark green, dense, uniform turf, and 7 was considered the minimum acceptable turf quality for a golf course green.

Experimental design

The experimental design for each study was a randomized complete block with four replications. Plot sizes were 5 feet × 26 feet (1.5 meters × 8 meters) at Walker GC and 5 feet × 6.5 feet (1.5 meters × 2 meters) at Cross Creek Plantation. Annual bluegrass control, seedhead suppression and turf quality were statistically analyzed to evaluate the effect of site on treatments. Treatment performance was not statistically equivalent at both sites with respect to all ratings, therefore, results are presented and discussed separately for each site.

July 2013 Poa annua control: table 2

Results

Annual bluegrass control, seedhead suppression and turf quality data are presented from a final spring rating date after all treatments had been applied for two years at both sites. Although statistical analysis revealed treatments did not perform equally at both sites, four treatments provided clear separation from all other treatments with respect to annual bluegrass control and seedhead suppression.

Annual bluegrass control

At Walker GC, methiozolin treatments provided greater control than all other treatments, where 93% control was achieved after two years of treatment with six applications (three fall, three spring) at 32 ounces/acre (2.3 liters/hectare) each, while three applications (two fall, one spring) at 64 ounces/acre (4.6 liters/hectare) provided 92% annual bluegrass control (Table 2). In comparison, at Cross Creek Plantation, 82% annual bluegrass control was achieved with the low-rate methiozolin program and 79% control with the high-rate methiozolin program. Only Trimmit (paclobutrazol, Syngenta) + Xonerate provided control similar to that of methiozolin (79%) at Cross Creek Plantation. This treatment produced 65% annual bluegrass control at Walker GC. Eight applications of Trimmit alone through the fall and spring provided 66% annual bluegrass control at Walker GC and 50% control at Cross Creek Plantation. All other plant growth regulator or herbicide treatments provided 13% or less annual bluegrass control regardless of site.

July 2013 Poa annua control: plot 3

At both research sites (Cross Creek Plantation is shown here), Trimmit (paclobutrazol) applied four times in the fall and four times in the spring (8-16 ounces/acre) provided excellent seedhead suppression. For effective suppression, paclobutrazol must be applied consistently throughout the fall and spring.

Seedhead suppression

Annual bluegrass seedheads were suppressed 85% or more regardless of rate of methiozolin or site; this can be attributed to effective annual bluegrass population reduction (Table 2). Both Trimmit alone (81% at Walker GC, 68% at Cross Creek Plantation) and Trimmit + Xonerate (77% at Walker GC, 86% at Cross Creek Plantation) provided seedhead suppression similar to methiozolin treatments at both sites. Only Cutless (flurprimidol, SePRO) provided greater seedhead suppression (44%) than the untreated control at Walker GC, while suppression with Xonerate (52%), Velocity (bispyribac-sodium, Valent) + Xonerate (32%), and Cutless (22%) was greater than the untreated control at Cross Creek Plantation. Other treatments did not provide seedhead suppression different from the untreated control at either location.

 Turf quality

Spring turf quality ratings (1 to 9, 9 = best) were most consistent between sites for Trimmit alone and Trimmit + Xonerate (7.9 to 8.1) (Table 2). Turf quality of methiozolin treatments at the low rate was 8.4 for Walker GC and 6.9 for Cross Creek Plantation; for the high rate, turf quality was 8.5 for Walker GC and 7.3 for Cross Creek Plantation. All other treatments evaluated in this study provided turf quality similar to the untreated control (7.1) with the exception of Xonerate alone at Walker GC (5.5).

Discussion

Turfgrass managers have used plant growth regulators for annual bluegrass control for many years (4). Currently, the standard for annual bluegrass control in creeping bentgrass is multiple applications of paclobutrazol. (We used Trimmit in this study, but other plant growth regulators such as TGR and Turf Enhancer also have the active ingredient paclobutrazol.) Results from this study confirm paclobutrazol as the best plant growth regulator and currently labeled option for annual bluegrass control.

July 2013 Poa annua control: plot 4

Methiozolin was applied three times at higher rates (64 ounces/acre) for annual bluegrass control to this plot at Walker GC. One year of this aggressive treatment removed annual bluegrass too quickly, leaving voids in creeping bentgrass greens, which reduced turf and putting quality and time to heal.

Paclobutrazol does not remove (control) annual bluegrass at rates used on creeping bentgrass greens (8-16 ounces/acre [0.6-1.2 liters/ hectare]), but provides a gradual transition from annual bluegrass to creeping bentgrass by providing a competitive advantage to creeping bentgrass (9). Thus, to be effective, applications of paclobutrazol must remain constant as ceasing applications allows annual bluegrass to rebound from regulation and aggressively reinvade creeping bentgrass stands (8). For adequate annual bluegrass suppression, applications of paclobutrazol should cease only during periods of summer stress and when creeping bentgrass is not actively growing during winter.

Paclobutrazol provides good suppression of annual bluegrass populations if applications remain consistent, but selective herbicides that effectively remove annual bluegrass from creeping bentgrass greens would be more beneficial long-term.

Methiozolin

Results from this and other research (1,2,3,6,12) suggest multiple methiozolin applications are effective at selectively reducing annual bluegrass populations in creeping bentgrass putting greens. Although both low- and high-rate methiozolin programs provided similar control in this study, we observed reductions in turf quality and creeping bentgrass coverage at both sites in the first year of applications with high rates of methiozolin when heavy annual bluegrass populations were removed too quickly (data not shown). Therefore, multiple applications (6+) at lower rates (≤32 ounces/acre, 2.3 liters/hectare) over longer periods of time (14-17 days) may be safer and more effective. Although not different from the untreated control, turf quality of methiozolin treatments was lower at Cross Creek Plantation compared to Walker GC. This suggests some differences may exist in the susceptibility of creeping bentgrass cultivars to methiozolin.

Research suggests methiozolin can reduce creeping bentgrass root length (15). Therefore, methiozolin should not be applied immediately following summer heat stress when creeping bentgrass roots are weakest.

Two modes of action of methiozolin have been proposed and provide a new mode of action for annual bluegrass control in turfgrass (7,10). Annual bluegrass is highly susceptible to herbicide resistance development, and thus, methiozolin should be used judiciously to delay the onset of resistance issues. Results from this study suggest two years of applications of methiozolin significantly reduces annual bluegrass populations. Thus, once these populations are reduced to acceptable levels, an aggressive paclobutrazol regime could be maintained until further applications of methiozolin would be needed.

July 2013 Poa annua control: plot 5

Two years of treatment with methiozolin applied six times in the fall and spring (32 ounces/acre) at Cross Creek Plantation provided more than 80% annual bluegrass control.

Xonerate

Xonerate is a newly developed herbicide for annual bluegrass control in warm- and cool-season grasses, but creeping bentgrass greens are not listed on the product label. Four spring applications of Xonerate at 1 ounce/acre (70 grams/hectare) failed to provide adequate (>70%) annual bluegrass control at either location. Spring turf quality was unacceptable (5.5) at Walker GC, but was 7.0 at Cross Creek Plantation, suggesting some variability in the safety of Xonerate at these rates on creeping bentgrass greens in the Southeast as previous research has reported (3,16). This could be attributed to environmental conditions at the time of application as Xonerate efficacy is highly temperature-dependent (13), or reduced photosynthetic efficiency as Xonerate is a photosystem II-inhibiting herbicide (14).

Because Xonerate is potentially phytotoxic to creeping bentgrass (14,16) and because paclobutrazol does not control annual bluegrass at the reduced rate for greens, reduced rates of Xonerate were evaluated in combination with Trimmit and Velocity. Annual bluegrass control was significantly increased at Cross Creek Plantation compared to Trimmit alone (50%) when four spring applications of Xonerate at 0.5 ounce/acre (35 grams/hectare) were added to the Trimmit program (79%). However, similar control (~65%) was observed at Walker GC with these programs. Therefore, low rates of Xonerate may increase annual bluegrass control compared to Trimmit alone, but this was not consistently shown in this study. Xonerate + Velocity combinations did not provide comparable annual bluegrass control or seedhead suppression to other treatments. Low rates of these products were used to achieve creeping bentgrass safety, but these results suggest higher rates are probably needed to control annual bluegrass.

Summary

Results from this study confirm multiple paclobutrazol (Trimmit, TGR, Turf Enhancer) applications remain the most effective option cur- rently labeled for managing annual bluegrass in creeping bentgrass putting greens in the southeastern United States.

Methiozolin appears to be extremely effective for reducing annual bluegrass populations while maintaining acceptable safety on creeping bentgrass putting greens. If methiozolin were registered in the United States, low rates (≤32 ounces/ acre, 2.3 liters/hectare) applied multiple times (6+) in the fall and spring would be most effective for slow removal of annual bluegrass. This herbicide should be used judiciously to maintain its effectiveness for annual bluegrass control as resistance has developed to almost all modes of action used to control annual bluegrass.

Other treatments evaluated in this study did not provide annual bluegrass control or seedhead suppression comparable to that achieved by methiozolin. Rates and timings of the products used in these treatments may need to be adjusted to achieve annual bluegrass control and selectivity in creeping bentgrass. Future research should continue to evaluate all of these products and others to find optimal rates and timings for annual bluegrass control in creeping bentgrass greens.

Acknowledgments

We thank 24-year GCSAA member Don Garrett, superintendent at Walker Golf Course at Clemson University, and 14-year GCSAA member Tom Grundy, superintendent at Cross Creek Plantation, for accommodating and assisting in this research project.

Literature cited

  1. Askew, S., and S.J. Koo. 2012. Annual bluegrass control on golf putting greens with spring applications of methiozolin. Proceedings of the Annual Meeting of the Northeastern Weed Science Society 66:94. Online. www.newss.org/proceedings/proceedings_2012.pdf (verified June 4, 2013).
  2. Brosnan, J.T., and G.K. Breeden. 2012. Methiozolin programs for annual bluegrass control in creeping bentgrass putting greens in Tennesee. Proceedings of the Annual Meeting of the Northeastern Weed Science Society 66:93. Online. www.newss.org/proceedings/proceedings_2012.pdf (verified June 4, 2013).
  3. Brosnan, J.T., S. Calvache, G.K. Breeden and J.C. Sorochan. 2013. Rooting depth, soil type, and application rate effects on creeping bentgrass injury with amicarbazone and methiozolin. Crop Science 53:655-659.
  4. Christians, N.E. 1996. A historical perspective of annual bluegrass control. Golf Course Management 64(11):49-57.
  5. Dayan, F.E., M. Trindade and E.D. Velini. 2009. Amicarbazone, a new photosystem II inhibitor. Weed Science 57:579- 583.
  6. Flessner, M.L., G.R. Wehtje and J.S. McElroy. 2013. Methiozolin absorption and translocation in annual bluegrass (Poa annua). Weed Science 61:201-208.
  7. Grossman, K., J. Hutzler, S. Tresch, et al. 2012. On the mode of action of the herbicides cinmethylin and 5-benzyloxymethyl- 1, 2-isoxazolines: putative inhibitors of plant tyrosine aminotransferase. Pest Management Science 68:482-492.
  8. Johnson, B.J., and T.R. Murphy. 1995. Effect of paclobutrazol and flurprimidol on suppression of Poa annua spp. reptans in creeping bentgrass (Agrostis stolonifera) greens. Weed Technology 9:182-186.
  9. Kageyama, M.E., L.R. Widell, D.G. Cotton and G.R. McVey. 1989. Annual bluegrass to bentgrass conversion with a turf growth retardant (TGR). Proceedings of the International Turf Research Conference 6:387-390.
  10. Lee, J.N., S.J. Koo, K.H. Hwang et al. 2007. Mode of action of a new isoxazoline compound. Proceedings of the Asian Pacific Weed Science Society Conference 21:591-601.
  11. McCarty, L.B. 2011. Best Golf Course Management Practices, 3rd ed. Prentice-Hall, Upper Saddle River, N.J.
  12. McCullough, P.E., D.G. de Barreda and J. Yu. 2013. Selectivity of methiozolin for annual bluegrass (Poa annua) control in creeping bentgrass as influenced by temperature and application timing. Weed Science 61:209-216.
  13. McCullough, P.E., S.E. Hart, D. Weisenberger and Z.J. Reicher. 2010. Amicarbazone efficacy on annual bluegrass and safety on cool-season turfgrasses. Weed Technology 24:461-470.
  14. Perry, D.H., J.S. McElroy and R.H. Walker. 2009. The influence of amicarbazone on creeping bentgrass (Agrostis stolonifera) photochemical efficiency. Proceedings of the Southern Weed Science Society 62:386. Online. www.swss.ws/NewWebDesign/Proceedings/Archives/2009%20Proceed ings-SWSS.pdf (verified June 4, 2013).
  15. Venner, K.A., S. Askew and S.J. Koo. 2012. Effects of methiozolin rates on creeping bentgrass and annual bluegrass root growth. Proceedings of the Annual Meeting of the Northeastern Weed Science Society 66:14. Online. www.newss.org/proceedings/proceedings_2012.pdf (verified June 4, 2013).
  16. Warren, L.S., F.H. Yelverton and T.W. Gannon. 2009. The effect of various rates and timings of amicarbazone on bentgrass cultivar tolerance and annual bluegrass control. Proceedings of the Southern Weed Science Society 62:387. Online. www.swss.ws/NewWebDesign/Proceedings/Archives/2009%20Proceed ings-SWSS.pdf (verified June 4, 2013).

Robert B. Cross is a graduate student, Lambert B. McCarty (bmccrty@clemson.edu) is a professor and Alan G. Estes is a turfgrass research technician in the school of agriculture, forestry and environmental sciences, Clemson University, Clemson, S.C.