New options for Poa annua control in bentgrass greens
The fight against annual bluegrass in creeping bentgrass continues with two new herbicides.
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B. Cross; Bert McCarty, Ph.D.; and Alan
This untreated control plot had heavy annual bluegrass infestations with a combination of perennial and annual
Photos by R. Cross
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
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
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.
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.
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.
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).
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.
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.
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.
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.
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.
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
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.
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).
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.
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.
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
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.
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 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
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.
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.
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Robert B. Cross is a graduate student, Lambert B. McCarty (firstname.lastname@example.org)
is a professor and Alan G. Estes is a turfgrass research technician in the
school of agriculture, forestry and environmental sciences, Clemson University,