Ethephon and Trinexapac-ethyl Influence Creeping Bentgrass Growth, Quality, and Putting Green Performance

Patrick E. McCullough, Department of Plant Biology and Pathology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901-8520; Haibo Liu and Lambert B. McCarty, Department of Horticulture, Clemson University, Clemson, SC 29634-0319; and Joe E. Toler, Department of Applied Economics and Statistics, Clemson University, Clemson, SC 29634-0319

Corresponding author: Patrick E. McCullough. mccullough@aesop.rutgers.edu

Abstract

Combining ethephon with trinexapac-ethyl effectively inhibits Poa annua L. seedheads and applying this combination during summer months may enhance growth regulation of creeping bentgrass (Agrostis palustris Huds.) relative to exclusive trinexapac-ethyl use. To test this hypothesis, a two-year field experiment investigated growth, quality, and ball-roll distances of an ‘L-93’ creeping bentgrass golf green treated with ethephon at 0, 3.8, or 7.6 kg a.i./ha with trinexapac-ethyl at 0 or 0.04 kg a.i./ha every 4 weeks over 12 weeks. Ethephon provided substantial clipping yield reductions but did not enhance ball-roll distances alone or improve trinexapac-ethyl efficacy. Trinexapac-ethyl most effectively suppressed creeping bentgrass shoot growth one to two weeks after treatments but post-inhibition shoot growth enhancement occurred which exacerbated summer root decline. Overall, clipping reductions from ethephon do not enhance ball-roll distances and ethephon does not appear to enhance growth regulation effects of trinexapac-ethyl on creeping bentgrass golf greens.

Introduction

Creeping bentgrass (Agrostis palustris Huds.) requires intensive management to sustain long-term golf course putting green turf. Successful putting green management often involves routine mowing at close heights, double cutting, and heavy rolling to reduce ball-roll friction caused by uneven shoot growth. Maintaining turfgrasses under conditions that challenge their physiological capacities, such as from these mechanical methods, may have deleterious effects on root growth, turf vigor, and recuperative capabilities from disease, traffic, heat, and other environmental stresses (12). Applying a plant growth regulator (PGR), trinexapac-ethyl (TE), reduces uneven shoot growth to improve ball-roll distances without negatively affecting turfgrass health (5). Applications of TE also enhance creeping bentgrass stress tolerances, increase tillering, and increase chlorophyll concentrations (8,9).

Trinexapac-ethyl is a late gibberellin synthesis inhibitor that does not effectively suppress Poa annua populations or control seedheads that may hinder putting green playability and surface uniformity (4,13). Superintendents often tank mix TE with other PGRs, such as ethephon, to control Poa annua growth and seedhead formation on creeping bentgrass greens (7). Ethephon is an ethylene-based PGR that delays flowering, selectively aborts flowers, and reduces stem elongation (15). Researchers have noted ethephon applied alone effectively reduces clippings of creeping bentgrass, Kentucky bluegrass (Poa pratensis L.), and Poa annua (1,2,3,14). Although combining ethephon with TE arrests Poa annua seedhead formation, consistent use of this PGR combination on creeping bentgrass golf greens has received limited comprehensive investigation.

Since ethephon plus TE use has beneficial effects for putting green management during spring months, including ethephon with TE during summer months may result in synergistic creeping bentgrass growth regulation. To test this hypothesis, growth, quality, and ball-roll distances of an ‘L-93’ creeping bentgrass golf green were measured over two growing seasons to determine if ethephon could enhance efficacy of routine TE applications.

Measuring Effects of Ethephon and Trinexapac-ethyl on Putting Greens

Experiments were conducted for 12 weeks (May to August) in 2003 and 2004 on an L-93 creeping bentgrass green at the Turf Service Center, Clemson University, Clemson, SC. The green was established in August 2002 and constructed to United States Golf Association specifications (17). Turf was mowed daily in the morning (≈ 700 h) at 3.2 mm and irrigated as needed to prevent plant wilt. During active growth, chlorothalonil, azoxystrobin, and fosetyl-al were applied preventatively for disease control. Starting 1 March each year, fertilization at 6 kg of N per ha per week was provided by an 18-1-15 (N:P:K) greens grade fertilizer. The test plots were free of Poa annua and other weeds.

Beginning 11 May 2003 and 10 May 2004, ethephon (2L) was applied at 0, 3.8, and 7.6 kg a.i./ha every 4 weeks with TE (1EC) at 0 or 0.04 kg a.i./ha every 4 weeks. The three applications of these PGRs were made with a CO₂ sprayer calibrated to deliver 700 liters/ha. The factorial experiment was conducted in a split-block design with three replications and consisted of 1.5- × 1.8-m plots.

Turf quality was visually rated on a 1 to 9 scale with 1 equal to dead turf and 9 equal to uniform, dark green turf. Ratings below 7 were considered unacceptable. Clippings were harvested weekly, oven-dried at 80°C for 48 h, and weighed. Two 600 cm³ (20-cm² × 30-cm) root samples were obtained per plot on 4, 8, and 12 weeks after treatment (WAIT). Roots were thoroughly washed to remove soil and organic matter over sieve screens that prevented loss of root fragments. Samples were oven dried at 80°C for 72 h and then weighed. The two root samples were averaged per plot prior to data analysis. Turf quality ratings, clippings, and root samples at 4 and 8 WAIT were obtained the day of, but prior to, the second and third treatment applications, respectively.

Ball-roll distances were measured weekly in the morning (900 to 1100 h) and in the evening (> 1700 h). Six ball-roll measurements (three rolls in opposite directions) per plot were made with a 38-cm stimpmeter. The stimpmeter was raised off the ground until gravity caused the golf ball to roll off the cleft located on the opposite end. Ball-roll distances were obtained with tape measures laid parallel to the plots and the six rolls were pooled per plot. Ethephon by TE interactions were not detected for clipping yield or ball roll and thus results are presented as percent change of nontreated turf. Relative clipping yield was calculated with the following equation: percent change = 100 × [(response_X- response₀)/(response₀)], where response_X equaled response of treated turf and response₀ equaled response of nontreated turf. Change in ball-roll distances are presented as percent of nontreated turf in the morning calculated from the same equation where response_X equaled evening distances of nontreated turf, morning distances of TE-treated, or evening distances of TE-treated turf and response₀ equaled response of nontreated turf from morning measurements.

Data were subjected to analysis of variance using SAS General Linear Model procedure (SAS Institute Inc., Cary, NC) to evaluate main and interaction effects of the factors.

Turf Quality

Turf quality is presented separately for each year since treatment by year interactions were detected. In both years, non-PGR-treated plots had acceptable turf quality (≥ 7) for all 12 weeks (Table 1). In 2003, applications of TE alone enhanced turf quality by approximately 10% for the 12 weeks compared to the non-PGR-treated. Interactions between TE and ethephon generally occurred two weeks after applications and lasted for one to two weeks. Ethephon applied alone caused unacceptable turf quality on all dates except 12 WAIT while quality was acceptable on six and four dates when TE was applied with ethephon at 3.8 and 7.6 kg/ha, respectively. In 2004, creeping bentgrass treated with ethephon alone only had acceptable turf quality at 8 and 12 WAIT but applications with TE resulted in acceptable quality on seven and three dates for the low and high rate, respectively. Enhanced turf quality attributable to TE was observed for 4 of the 12 weeks in 2004 but interactions with ethephon were only detected on three dates.

Table 1. Turf quality of an L-93 creeping bentgrass putting green treated with trinexapac-ethyl and ethephon in field experiments, Clemson, SC.

Year	Treatment^x	Rate (kg/ha every 4 weeks)	Weeks after initial treatment
			1	2	3	4	5	6	7	8	9	10	11	12
			Turf quality (1 to 9)
2003	Ethephon (EP)	0	7.0	7.4	7.5	7.4	7.6	7.5	7.4	7.3	7.4	7.5	7.9	7.9
		3.8	6.5	6.4	6.9	6.7	6.4	6.2	5.8	6.3	5.8	6.5	7.3	7.7
		7.6	6.0	5.7	6.5	6.3	6.4	5.9	5.5	5.9	5.5	6.3	7.2	7.5
	Trinexapac-ethyl (TE)	0	6.3	5.8	6.6	6.5	6.4	6.6	6.2	6.5	6.1	6.0	7.0	7.2
		0.04	6.7	7.1	7.3	7.0	7.2	6.4	6.2	6.4	6.3	7.5	7.9	8.1
		EP	*	*	*	*	*	*	*	*	*	*	*	*
		TE	NS	*	*	NS	*	NS	NS	NS	NS	*	*	*
		EP × TE	NS	NS	*	NS	NS	*	*	NS	*	*	NS	NS
2004	Ethephon (EP)	0	7.2	7.3	7.1	7.1	7.0	7.1	7.1	7.0	7.0	7.0	7.0	7.0
		3.8	6.5	6.5	6.2	6.9	6.0	6.4	6.9	7.0	6.2	6.9	6.9	7.0
		7.6	6.2	6.3	5.9	6.6	5.2	5.4	6.1	7.0	5.4	5.8	6.5	7.0
	Trinexapac-ethyl (TE)	0	6.3	6.4	6.2	6.7	6.0	6.1	6.5	7.0	5.9	6.4	6.5	7.0
		0.04	6.9	6.9	6.5	7.0	6.1	6.4	6.9	7.0	6.5	6.7	7.1	7.0
		EP	*	*	*	NS	*	*	*	NS	*	*	*	NS
		TE	*	NS	NS	NS	NS	NS	*	NS	NS	*	*	NS
		EP × TE	*	NS	NS	NS	NS	NS	NS	NS	NS	*	*	NS

* and NS denote significance at the 0.05 probability level and not significant at 0.05, respectively.

^x Initial treatments were 11 May 2003 and 10 May 2004. Turf quality was rated on a scale of 1 to 9 with 1 = dead turf and 9 = dark green, uniform turf. Ratings below 7 were considered unacceptable.

Creeping bentgrass treated with ethephon had unacceptable turf quality resulting from a noticeably lighter green leaf color each year. Discoloration likely resulted from chlorophyll loss in leaf tissue caused by ethylene diffusion as discussed by Gelertner and Stowell (7). Enhanced turfgrass quality observed in 2003 with applications of TE alone is similar to previous research (9,13) but results were not as consistent in 2004. However, when TE was applied with ethephon, TE buffered the impact of ethephon on turf quality more consistently in 2004 than in 2003.

Growth Measurements

Treatment by year interactions were not detected and thus clipping yield results were combined over both years. Ethephon by TE interactions were not detected for clipping yield and therefore main effects of each PGR were examined separately (Fig. 1). Trinexapac-ethyl reduced clipping yield at least 20% compared to non-TE-treated turf for one to two weeks following each application (Fig. 1a). Shoot growth recovered prior to the second and third applications, and clipping yields exceeded non-TE-treated turf by 13 to 35%. Accumulated GA₂₀ from TE use has been reported to increase ≈ 150% as a result of gibberellin synthesis inhibition (16) and reduced TE efficacy has resulted in similar post-inhibition growth enhancements (PIGE) of Kentucky bluegrass (11). Ethephon suppressed weekly clipping yield 13 to 36% for 9 of the 12 weeks (Fig. 1b) without PIGE which is consistent with previously reported growth inhibition of creeping bentgrass, Kentucky bluegrass, and Poa annua (1,2,3,14). Results for the two ethephon rates were generally comparable for the first six weeks, while lower clipping yields were observed with the high ethephon rate for five of the last six weeks.

Fig. 1. Dry clipping yield of L-93 creeping bentgrass treated with ethephon and trinexapac-ethyl at 0, 4, and 8 weeks in field experiments, 2003-2004, Clemson, SC. Main effects of trinexapac-ethyl (a) and ethephon (b) are presented as percentage change relative to non-trinexapac-ethyl treated (a) and non-ethephon treated (b) (* and NS denote significantly different from 0 at the 0.05 probability level and not significantly different from 0, respectively, and † denotes rates of ethephon are different).

Dry root mass results were combined for the two years since treatment by year interactions were not detected. An interaction between TE and ethephon was observed for dry root mass 4 WAIT (Table 2). Root mass was reduced 22% from non-PGR-treated turf at the high ethephon rate alone (without TE) but reductions were masked in the presence of TE. Root mass was also reduced by 25% from the high ethephon rate 8 WAIT but TE did not mask these effects. By 12 WAIT, bentgrass treated with TE (averaged over ethephon rates) had root mass reduced by 15% compared to non-TE-treated plots while turf treated with ethephon had similar root mass to the non-ethephon treated. Total root length was similar (27 to 30 cm) for all treatments on all sampling dates (data not shown).

Table 2. Dry root mass of L-93 creeping bentgrass treated with trinexapac-ethyl and ethephon in 2003 and 2004, Clemson, SC.

Treatment^y	Rate (kg/ha every 4 weeks)	WAIT^x
		4	8	12
		Dry root mass (g/m²)
Ethephon (EP)	0	112	96	68
	3.8	112	93	82
	7.6	110	73	75
Trinexapac-ethyl (TE)	0	106	88	81
	0.04	117	86	69
	EP	NS	*	NS
	TE	*	NS	*
	EP × TE	*	NS	NS

* and NS denote significance at the 0.05 probability level and not significant at 0.05, respectively.

^x WAIT = weeks after initial treatments.

^y Initial treatments were 11 May 2003 and 10 May 2004.

L-93 creeping bentgrass root mass generally declined from June (4 WAIT) to August (12 WAIT) which agrees with previous research on ‘Penncross’ creeping bentgrass golf greens (6). Ethephon induced root growth restrictions on creeping bentgrass are consistent with previous experiments on Kentucky bluegrass and perennial ryegrass (2,10). Shoot growth inhibition from TE generally does not restrict bentgrass rooting (6) but post-inhibition shoot growth enhancements from reduced TE efficacy apparently exacerbated summer root mass decline in these experiments. Applications of TE appear to initially mask rooting restrictions from ethephon but these effects were inconsistent throughout the experiment.

Ball-Roll Measurements

Ball-roll results were combined over the two years since treatment by year interactions were not detected. Time of day (AM versus PM) by treatment and ethephon by TE interactions were not detected (Fig. 2). Turf treated with TE had increased ball-roll distances compared to the non-TE-treated for one to three weeks following each treatment application. Significant change in ball-roll distance from ethephon was not observed on any sampling date (data not shown).

Fig. 2. Ball-roll distances obtained with a 38-cm stimpmeter on L-93 creeping bentgrass treated with trinexapac-ethyl (TE) at 0.04 kg/ha every 4 weeks in field experiments, 2003-2004, Clemson, SC. Trinexapac-ethyl results (morning and evening) and evening results of nontreated turf are presented as percent change relative to morning ball-roll distances of the nontreated. (* and NS denote significant at the 0.05 probability level and not significant at the 0.05 probability level, respectively).

Morning ball-roll distances were consistently greater than evening ball-roll distances on all dates for nontreated and TE-treated turf. Ball-roll distances of non-TE-treated turf were reduced 4 to 9% from morning to evening. Applications of TE did not mask diurnal ball-roll distances but evening distances of TE-treated turf differed no greater than 4% from morning distances of the non-TE-treated on 8 of 12 dates. Ball-roll distances of TE-treated turf were similar but not reduced from non-TE-treated on dates when post inhibition shoot growth enhancement occurred.

Ball-roll distances were not enhanced by ethephon despite substantial clipping yield reductions. Results suggest other physiological responses of creeping bentgrass leaf tissue, such as change in water content, from ethephon applications likely masked potential ball-roll enhancements that would have otherwise resulted from shoot growth inhibition (10). Eggens and Wright (1) noted ethephon at 2, 4, and 8 kg/ha inhibited Poa annua growth but increased internode elongation. Responses, such as these, would likely negatively influence creeping bentgrass surface uniformity and may explain a lack of ball-roll distance enhancements despite clipping yield reductions. In experiments on ‘Penncross’ creeping bentgrass greens, Fagerness et al. (5) noted TE enhanced ball-roll distances while applications of paclobutrazol, an early gibberellin synthesis inhibitor, did not. These experiments suggest shoot growth inhibition following applications of various PGRs may not necessarily enhance ball-roll distances since other physiological responses to growth regulation may influence putting green surface uniformity.

Conclusions

Each TE application reduced clipping yield and enhanced ball-roll distances but post inhibition shoot growth enhancements exacerbated summer root mass decline. Thus, turf managers should be cautious when using TE on creeping bentgrass putting greens especially during summer months. Applying lower TE rates at more frequent intervals would likely be a viable option to reduce post inhibition growth enhancements and warrant further investigation on creeping bentgrass putting greens. Ethephon effectively reduced creeping bentgrass clippings but shoot growth inhibition did not significantly enhance ball-roll distances. Ethephon had deleterious effects on root growth and turf quality but these effects were periodically mitigated by TE. Overall, ethephon use in routine putting green management does not appear to improve ball-roll distances or enhance growth regulation effects of TE.

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