Response of Eight Cultivars of Roughstalk Bluegrass to Bispyribac or Sulfosulfuron

Debbie E. Morton, Daniel V. Weisenberger, and Zachary J. Reicher, Department of Agronomy, Purdue University, 915 W. State St., West Lafayette, IN 47907-2054

Corresponding author: Zachary J. Reicher. zreicher@purdue.edu

Abstract

Bispyribac and sulfosulfuron are labeled for selective control of roughstalk bluegrass (Poa trivialis L.), but previous research indicates inconsistent control. Our primary objective was to determine whether response of bispyribac or sulfosulfuron was consistent among eight cultivars of roughstalk bluegrass. Studies were conducted in Indiana at 1.25 and 5.0 cm mowing heights in 2006 and 2007. Herbicide treatments included an untreated control, sulfosulfuron at 13 g ai/ha per application applied twice on a two-week interval, and bispyribac at 37 g ai/ha per application applied four times on a two-week interval. Herbicide treatments were chosen as the lowest effective application strategy in our previous research. Applications were made in June and July to eight cultivars of mature roughstalk bluegrass. All roughstalk bluegrass cultivars were more susceptible to control from bispyribac than to sulfosulfuron. When mowed at 5.0 cm, there was some indication that ‘Laser’ or ‘Bariviera’ may be more susceptible to herbicidal control whereas ‘Pulsar’ may be more tolerant. This variability did not occur consistently in the two years of study at either mowing height, but this may partly explain variability in control typically seen in roughstalk bluegrass from bispyribac or sulfosulfuron.

Introduction

Roughstalk bluegrass is difficult to manage in cool-season turf on golf courses, athletic fields, and home lawns. Due to poor drought and heat tolerance and susceptibility to disease, roughstalk bluegrass often goes dormant in summer and leaves thin or patchy turf (1). However, roughstalk bluegrass recovers quickly from dormancy through stolons and grows aggressively in fall and spring, out-competing desirable grasses. Increasing irrigation and fungicide use are often attempted to prevent summer dormancy, but these increase maintenance expense and are not practical for most turf managers.

Roughstalk bluegrass found as a contaminant in many fairways appears to be newer varieties with fine texture and relatively dark green color, thus seed contamination is a likely source. This was validated in a 1996 study where 90 seed lots of creeping bentgrass (Agrostis stolonifera L.) were tested for purity, and over 30% contained roughstalk bluegrass as a contaminant (5). Roughstalk bluegrass can also be found in pastures, roadsides, and along streams, probably as a result of naturalization after it was introduced to the United States from Europe (2). Therefore, it is possible that roughstalk bluegrass contamination in turf could also be from spread of pre-existing stands of roughstalk bluegrass. Regardless of the source, turf stands can be contaminated with a number of cultivars or phenotypes of roughstalk bluegrass differing in color, texture, and/or growth habit.

Nonselective control of roughstalk bluegrass may not be practical on fairways, tees, sports fields, or homelawns. However, two relatively new herbicides for potential selective control of roughstalk bluegrass are bispyribac and sulfosulfuron. Bispyribac is labeled for use in turfgrass as Velocity (Valent USA Corp., Walnut Creek, CA) for selective postemergence control of annual bluegrass (Poa annua L.) and roughstalk bluegrass in creeping bentgrass and perennial ryegrass (Lolium perenne L.) golf course fairways or sod farms. Bispyribac is an acetolactate synthase (ALS) inhibiting herbicide and belongs to the pyrimidinyl carboxy herbicide family (14). Sulfosulfuron is labeled as CertaintyTM by Monsanto (St. Louis, MO) and is a sulfonyl-urea ALS inhibitor for control roughstalk bluegrass and other weeds in turf.

Our previous research in four states showed that various rates and application strategies of bispyribac and sulfosulfuron are effective for controlling roughstalk bluegrass, but control can be variable (10). Bispyribac outperformed sulfosulfuron at two locations in this study, sulfosulfuron outperformed bispyribac at another location, and neither herbicide controlled roughstalk bluegrass in two other locations. Other reports also describe variability in effects of these herbicides among locations or years (16,17,18). Efficacy of bispyribac and sulfosulfuron is also temperature sensitive, which may further contribute to observed variation. Bispyribac is more effective on annual bluegrass in temperatures > 21°C (4,6), whereas sulfosulfuron is more effective on roughstalk bluegrass at 15 or 25°C compared to 20°C (7).

Variation in cultivars or phenotypes could have contributed to the differential herbicide response in our earlier study because ‘Laser’ roughstalk bluegrass was used in one location while infestations of unknown cultivars or phenotypes were used in the other locations (10). Rajasekar et al. (11) found tremendous genetic variability within the species of roughstalk bluegrass and even within the subspecies sylvicola. McCullough and Hart (8) suggested that variability in their control of roughstalk bluegrass populations on golf course fairways may have been due to genetic diversity, but had no data to verify their suspicion. In another preliminary study, sulfosulfuron effects were variable across fourteen roughstalk bluegrass phenotypes collected from golf courses in Indiana (12). Furthermore, effects of bispyribac are variable across another Poa species, Kentucky bluegrass (Poa pratenis L). Responses of 173 Kentucky bluegrass cultivars to bispyribac ranged from little or no damage to complete kill (13).

Though selective control of roughstalk bluegrass has potential for finely managed turf areas, it may not be practical unless control is consistent and predictable for the turf manager. Though a number of factors could contribute to observed inconsistency in control from bispyribac or sulfosulfuron, we focused on the genetic aspects in this paper. Therefore, our objective was to determine if response to sulfosulfuron or bispyribac is consistent among cultivars of roughstalk bluegrass.

Evaluating Cultivar Response

Our study was conducted in 2006 and 2007 at the W.H. Daniel Turfgrass Research and Diagnostic Center, West Lafayette, IN. Soil types were Mahalasville-Treaty clay loam (fine-silty, mixed, mesic Typic Argiaquolls) with a pH of 7.0 and organic matter content of 7.2% and Starks-Fincastle silt loam (fine-silty, mixed, mesic Aeric Ochraqualfs) with a pH of 7.3 and organic matter content of 4.9%. Adjacent experiments were performed at either 1.25 cm or 5.0 cm mowing heights. All experimental areas were seeded at 98 kg/ha on 30 August 2005 and were mature when applications were begun in 2006. A three by eight factorial was used in this study with three herbicide treatments and eight cultivars of roughstalk bluegrass. Statistical design was a split-plot with cultivars of roughstalk bluegrass as main plots and herbicide treatments as subplots. Three replications of main plots were used and main plots were 5.5 × 1.5 m while subplots were 1.5 × 1.5 m with 0.5 m untreated alleyways between each subplot. Roughstalk bluegrass cultivars included ‘Sabre II,’ ‘Sun-up,’ ‘Bariviera,’ ‘Winterlinks,’ ‘Racehorse,’ ‘Pulsar,’ ‘Proam,’ and Laser. All cultivars were commercially available in 2005 and no attempt was made to select cultivars from different parental lines. Herbicide treatments included an untreated control, sulfosulfuron (Certainty 75WDG) at 13 g ai/ha per application applied twice on a two-week interval, and bispyribac (Velocity 80WSP) at 37 g ai/ha per application applied four times on a two-week interval. Sulfosulfuron treatments included MON 0818 (Monsanto, St. Louis, MO) nonionic surfactant at 0.25% v/v. Herbicide were applied at the lowest effective rate and application interval based on our earlier research (10). Initial applications were made on 13 June 2006, and the experiments were repeated on adjacent but previously untreated sites beginning with applications on 13 June 2007. All herbicides were applied in 814 liter/ha water with a CO₂-pressurized backpack sprayer using a three-nozzle (Tee Jet XR8001.5VS, Spraying Systems Co., Wheaton, IL) boom at 207 kPa. Throughout the experiment, turf received N at 196 kg/ha/year and was sufficiently irrigated to ensure optimal growth. Experiments maintained at 1.25 cm were mowed three times per week and turf maintained at 5.0 cm was mowed two times per week with clippings returned at both heights. Experimental areas were on a preventative fungicide program, mainly targeting dollar spot, brown patch, and Pythium blight, with applications made every 10 to 14 days during June, July, and August of each year.

In both years and at both mowing heights, percent cover was rated visually every two weeks during the growing season. Percent control was calculated by:

[ 1 − (% cover in treated plot / % cover of untreated control) ] * 100.

Data from cultivars were omitted from the analysis when cover in untreated control plots averaged < 20% on the rating date. Error variances between years were not homogenous, and thus data were not combined across years. Statistical analysis was done with PROC ANOVA (version 9.1, SAS Institute Inc., Cary, NC) and means are presented and separated with LSD at P < 0.05.

Effects of Herbicides on Cultivars

At 1.25-cm mowing height, there was no main effect of cultivar at either 8 or 12 WAIT in 2006. All cultivars exhibited 90% or greater control when averaged over herbicide treatments at 8 or 12 WAIT in 2006 (Table 1). The exception to this was Pulsar, which was dropped from the analysis on 8 and 12 WAIT because the untreated control plots averaged less than 20% cover. Though overall control was lower and less consistent in 2007, there was again no main effect of cultivar. Proam and Pulsar were dropped from the analysis in 2007 because their untreated control plots averaged less than 20% cover. There was no main effect of herbicide in 2006, but our bispyribac treatment provided higher control than sulfosulfuron on both reported rating dates in 2007 (Table 1). Even though we selected marginally effective application strategies for this study, this is similar to our previous results where bispyribac tends to provide slightly higher control than sulfosufuron (9,10). There was no significant cultivar*herbicide interaction at anytime during the two years at 1.25-cm mowing height. This indicates these cultivars should respond consistently to a specific herbicide, and some cultivars should not be more susceptible to bispyribac while others are more susceptible to sulfosulfuron at 1.25-cm heights.

Table 1. Percent control^t of 1.25 cm-mowed roughstalk bluegrass at 8 and 12 weeks after initial treatment in 2006 and 2007.

		2006		2007
		8 WAIT^u	12 WAIT	8 WAIT	12 WAIT
Cultivar	Bariviera	100^v	97	82	85
	Laser	96	90	51	60
	Proam	99	99	−	−
	Pulsar	−^w	−	−	−
	Racehorse	99	96	85	89
	Sabre II	98	98	49	37
	Sun-Up	100	98	82	93
	Winterlinks	96	96	41	63
Herbicide	Sulfosulfuron	97^x	93^x	41^y	55^y
Herbicide	Bispyribac	100	99	89	87
ANOVA	Cultivar (C)	NS^z	NS	NS	NS
	Herbicide (H)	NS	NS	**	**
	C × H	NS	NS	NS	NS

^t Percent control =
[ 1 − (% cover in treated plot / % cover of untreated control) ] * 100.

^u WAIT = weeks after initial treatment.

^v Cultivar means are averaged over three herbicides and three replications.

^w Cultivars were omitted from the analysis when cover in untreated control plots averaged ≤ 20% on the rating date.

^x Herbicide means are averaged over seven cultivars and three replications.

^y Herbicide means are averaged over six cultivars and three replications.

^z NS or *, nonsignificant or significant at P ≤ 0.05, respectively.

Only on one evaluation date during the two years of study did the cultivars vary in their response to herbicides at the 5.0-cm mowing height. Though overall control was very low at 8 WAIT in 2006, control varied from Laser at 28% to Pulsar at −29% (Table 2). This differential control was not evident at 12 WAIT in 2006 and was not evident when overall control increased to ≥ 50% at 8 or 12 WAIT in 2007. However, this does suggest some variability in cultivar response to herbicides and Pulsar may exhibit more tolerance to herbicidal control while Laser or Bariviera may be more susceptible to these herbicides. Similar to the 1.25 cm mowing height study, our bispyribac treatment provided 79 to 92% control while sulfosulfuron provided 50 to 55% control in 2007, but there was no herbicide main effect in 2006 on either rating date (Table 2). Similar to the 1.25 cm study, there was no cultivar × herbicide interaction in percent control on any of the rating dates. It is important to note the dramatically increased control in 2007 compared to 2006 at the 5 cm mowing height. Maximum air temperatures were markedly higher in 2007 compared to 2006, especially at 7 to 9 WAIT and again 10 to 13 WAIT (Fig. 1). Anecdotal observations as well as a report by Sifers and Beard (15) indicate roughstalk bluegrass is among the least heat tolerant of commonly used cool-season grasses. These herbicides tend to be more effective at higher temperatures (6,7), plus additional summer stress on these plants appears to increase roughstalk bluegrass control from bispyribac or sulfosulfuron. This has been seen in other studies with control increasing with higher temperatures or increased stress of roughstalk bluegrass (4,6,7,9,10).

Table 2. Percent control^u of 5.0 cm-mowed roughstalk bluegrass maintained at 5.0 cm at 8, and 12 weeks after initial treatment in 2006 and 2007.

		2006		2007
		8 WAIT^v	12 WAIT	8 WAIT	12 WAIT
Cultivar	Bariviera	25 ab^wx	5	62	59
	Laser	28 a	3	53	68
	Proam	13 abc	−7	82	85
	Pulsar	−29 d	2	68	71
	Racehorse	−10 cd	2	65	50
	Sabre II	5 abcd	5	78	84
	Sun-Up	−4 abcd	1	72	68
	Winterlinks	−9 bcd	−1	82	50
Herbicide	Sulfosulfuron	1^y	1	50	55
Herbicide	Bispyribac	14	2	92	79
ANOVA	Cultivar (C)	*^z	NS	NS	NS
	Herbicide (H)	NS	NS	**	**
	C × H	NS	NS	NS	NS

^u Percent control =
[ 1 − (% cover in treated plot / % cover of untreated control) ] * 100.

^v WAIT = weeks after initial treatment.

^w Cultivar means are averaged over three herbicides and three replications.

^x Cultivar means followed by the same letter or no letter are not significantly different within each rating date with LSD (0.05).

^y Herbicide means are averaged over eight cultivars and three replications.

^z NS or *, nonsignificant or significant at P ≤ 0.05, respectively.

Fig. 1. Maximum air temperatures 2006 and 2007 recorded 0 to 13 weeks after initial treatment (WAIT) of herbicides. Air temperatures were recorded at Purdue University Regional Airport, which is approximately 1 km from study site.

Though we cannot make a statistical comparison between the two mowing heights, it is also important to note the dramatic difference in herbicide efficacy between the two mowing heights in 2006. Bispyribac or sulfosulfuron provided almost complete control (≥ 93% control at 12 WAIT) at 1.25 cm and almost no control (≤ 2% control at 12 WAIT) at 5.0 cm, even though the check plots at 1.25 and 5.0 cm averaged 77% and 94% cover, respectively (data not shown). Since these experiments were maintained identically other than mowing height and frequency, it appears the extra stress imposed by the 1.25 cm mowing height improved roughstalk bluegrass control in the relatively unstressful growing season of 2006. We consistently observe improved roughstalk bluegrass control from these herbicides in more stressful summers for cool-season grasses compared to less stressful summers (9,10), and lower mowing may increase stress on these plants leading to improved control. To further elucidate this effect, we are currently evaluating the effect of high temperatures on physiology and survival of roughstalk bluegrass.

Regardless of cultivar, bispyribac at 37 g ai/ha applied four times on a two-week interval was more effective controlling roughstalk bluegrass than sulfosulfuron at 13 g ai/ha applied twice on a two-week interval. When mowed at 5.0 cm, there was some indication that Laser or Bariviera are more sensitive to these herbicides than Pulsar. This trend did not occur consistently in the two years of study at either mowing height, but this may add to the variability typically seen in roughstalk bluegrass control from bispyribac or sulfosulfuron.

Literature Cited

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