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© 2007 Plant Management Network.
Accepted for publication 13 January 2007. Published 8 May 2007.

Monthly Flurprimidol Applications Reduce Annual Bluegrass Populations in a Creeping Bentgrass Fairway

Cale A. Bigelow and Glenn A. Hardebeck, Agronomy Department, Purdue University, West Lafayette, IN 47907; and B. Todd Bunnell, SePro Corporation, Carmel, IN 46032

Corresponding author: Cale A. Bigelow. cbigelow@purdue.edu

Bigelow, C. A., Hardebeck, G. A., and Bunnell, B. T. 2007. Monthly flurprimidol applications reduce annual bluegrass populations in a creeping bentgrass fairway. Online. Applied Turfgrass Science doi:10.1094/ATS-2007-0508-02-RS.

Abstract

Annual bluegrass (Poa annua L.) (ABG) is an invasive weed in creeping bentgrass (Agrostis stolonifera L.) (CBG) golf course fairways and tees. Plant growth regulators (PGRs) like flurprimidol (FL) are often applied to these areas to manage shoot growth and minimize ABG encroachment. This two-year field study evaluated the effects of various FL, trinexepac-ethyl (TE), and FL + TE tank-mix combinations applied monthly from May through October to a mature CBG research fairway containing > 30% ABG prior to PGR initiation. After two consecutive years of PGR applications the most effective treatments for reducing ABG populations were FL alone (0.28, or 0.56 kg a.i./ha) or the FL + TE (0.56 + 0.06 kg a.i./ha) tank-mix which reduced the ABG populations from initial levels by 78, 74, and 87% respectively. PGR effects on visual color and quality were not significant when averaged across both study years. These results demonstrate that even where rather large, > 30%, ABG populations exist, monthly FL applications throughout the growing season can gradually reduce ABG to tolerable, < 10%, levels without the risk for rapid turf cover losses which can result in substantial turf voids.

Creeping bentgrass (CBG) is the cool-season turfgrass species of choice for golf course fairways and tees throughout the cool-humid region because it tolerates close mowing, remains green year-round, and possesses lateral stems that enable it to self-heal from divoting (1). Historically the most troublesome weed in CBG has been annual bluegrass (ABG) (12,13). This winter-annual is considered undesirable because of the unattractive yellow-green color and profuse seedhead production during the spring which disrupts turfgrass appearance and affects use (1). Furthermore, compared to desirable species like CBG, ABG is less tolerant of winter and summer stresses and more susceptible to chronic diseases and insect pests (8).

Although selective herbicides are available for removing ABG from CBG, they often provide erratic control and sometimes result in rapid turf cover losses resulting in large voids where ABG comprised a significant, > 25%, percentage of the turf stand (3,8,17,24). Golf course managers regularly apply plant growth regulators (PGRs) to CBG in order to suppress shoot growth and minimize ABG encroachment. Several commercially available PGRs which inhibit gibberellic acid biosynthesis, and thus cell elongation, are widely used by golf course managers to achieve these goals (18). The major PGRs used by golf course managers are fluprimidol (alpha-(1-methylethyl)-alpha-(4-(trifluoromethoxyphenyl)-5-pyrimidinemethanol) (FL), paclobutrazol ((+/-)-(R*,R*)-B-[(4-chlorophenyl)methyl]-alpha-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol) (PB), and trinexepac-ethyl (4-(cyclopropyl-hydroxymethylene)-3,5-dioxo- cyclohexanecarboxylic acid ethyl ester) (TE).

Several previous studies have evaluated PGRs for the potential to reduce or eliminate ABG from CBG turf areas. Early research with spring applied FL at 0.56, 0.84, and 1.12 kg a.i./ha on a mixed CBG/ABG turf demonstrated the ability of FL to reduce ABG cover with the best results occurring at the 0.84 and 1.12 kg/ha rates (21). Branham (2) evaluated several PGRs for their ability to suppress ABG with mixed results. However, where FL or PB was applied an overall increase in CBG was observed. On mixed CBG/ABG putting greens repeated FL or PB applications have been shown to decrease ABG populations (5,14). Woosely et al. (24) applied PB multiple times throughout the growing season to a CBG/ABG fairway and reduced ABG populations by ≥ 85%.

Generally the accepted reason for CBG population increases in mixed ABG/CBG stands subjected to PGR applications is differential growth suppression. Although PGRs substantially suppress vertical growth in both species, the CBG stolons continue to spread into areas occupied by regulated ABG and begin to crowd out this species. In addition, regular PGR applications have been shown to increase turf density (7,18), which subsequently deters the ability of annual grasses like ABG to establish from seed. Another mechanism that makes FL effective for reducing ABG is the preemergence activity of FL on ABG when applied at rates ≥ 0.56 kg/ha (11).

The increased turf density due to PGR applications not only deters weeds, but also results in a darker green appearance and subsequently superior visual turfgrass quality (4,6,10,18). Occasionally, however, during environmental stresses such as summer heat and drought or the onset of significantly cooler temperatures temporary discoloration may occur, especially with higher PGR application rates (7,18).

The most widely used PGR for general growth suppression in golf course turf is most likely TE. Although TE provides excellent growth suppression compared to PB or FL, it is not as effective for suppressing ABG (18,19). Golf course managers are always in search of improved product mixtures that provide the benefits of consistent clipping reductions, enhanced turf appearance, and excellent ABG suppression or population reductions. Thus, these managers are interested in the field performance of TE tank-mixed with PB or FL to meet these goals. The benefits of combining fungicides with PB or PB + TE for improving CBG fairway quality has been demonstrated (9). Additionally, in a recent ABG reduction study, PB and PB + TE tank-mixes were applied to CBG/ABG fairways on regular 21-day intervals (16). The authors reported that PB effectively reduced ABG; however, tank-mixing PB with TE did not enhance ABG suppression or TQ. Although the response of PB for ABG suppression under fairway conditions has been documented (8,16,18,24), less data exists for FL. In fact, no previous studies have reported the effects of season-long applications of FL or FL+TE tank-mixes over multiple years for their ability to reduce ABG populations in a CBG fairway. Therefore, the objective of this study was to evaluate the effects of monthly applications of a root absorbed PGR, FL, and a foliarly absorbed PGR, TE, and various FL + TE tank mixes when applied throughout the growing season for their effect on TQ and their ability to gradually reduce ABG populations in a mixed CBG/ABG research fairway.

Evaluating the Effects of FL and TE Combinations on ABG in CBG Fairways

This field study was conducted at the W. H. Daniel Turfgrass Research and Diagnostic Center at Purdue University, West Lafayette, IN, from May 2004 through October 2005. The study area consisted of a mixed stand of CBG (Agrostis stolonifera var. ‘L-93’) and ABG grown on a Starks-Fincastle silt-loam (fine-silty, mixed, mesic Aeric Ochraqualf) with pH of 7.2 and 1.6% organic matter. The CBG was planted in August 2003 into an area with a chronic history of ABG and was fully mature prior to initiating the PGR treatments. Based on the visual characteristics, light-green color, upright growth habit and prolific seedheads, the ABG present was deemed to generally be the annual biotype. Throughout the growing season the study area was mowed at 13 mm three times per week with clippings returned, irrigated to promote growth and fertilized with N at approximately 150 kg/ha per year from a complete fertilizer source 18-1.8-8.3 N-P-K). Insect and disease pests were treated on a curative basis with broad spectrum contact fungicides (e.g., chlorothalonil) and insecticides.

All FL (Cutless 50 WP, SePro Corp., Carmel, IN) and TE (Primo Maxx 1 EC, Syngenta Crop Protection Inc., Greensboro, NC) PGR treatments were applied with a pressurized CO₂ (270 kPa) sprayer equipped with TeeJet XR 8003 tips calibrated to deliver water at 467 liter/ha. Six PGR treatments were initiated on 15 May 2004 and subsequently applied; 14 June, 9 July, 10 August, 17 September, and 19 October 2004. In 2005, treatments were initiated on 17 May, and reapplied on 13 June, 11 July, 8 August, 6 September, and 4 October. The FL rates evaluated in this study are consistent with label recommendations for ABG reduction in CBG fairways which suggests initial early spring FL applications ranging from 0.56 to 0.84 kg a.i./ha followed by 0.28 to 0.56 kg a.i./ha on three to six week intervals until late summer or early fall. The chemical rates and dates of all applications are also footnoted in the data tables. Plots were 1.5 × 3.0-m with 0.3 m borders and arranged in a randomized complete block with four replications of each treatment (22). Following each PGR application, 6 mm of irrigation was applied via an overhead irrigation system within twelve hours of application but not before the TE had dried on the leaf surface.

Plots were visually assessed on a regular basis (e.g., twice monthly) during the growing season (May-October) for turfgrass color, overall quality (TQ), chemical phytotoxicity, and percentage ABG. Color was assessed on a 0 to 10 scale with 0 = brown turf, 10 = dark green, and ≥ 7 = acceptable. Quality was assessed on a 0 to 10 scale where 0 = brown, dead turf; 10 = optimum color, density and uniformity; and ≥ 7 = acceptable. Annual bluegrass populations were visually assessed on a linear 0 to 100% scale where 0 = no annual bluegrass present, and 100 = total plot coverage. Due to the large number of color and quality ratings, data for each month were pooled into a single monthly value used for statistical analysis. The percentage change in ABG from initial levels (15 May) was calculated using the formula: (P_i-P_x)/P_i × 100%, where P_i = initial ABG level and P_x = ABG percentage observed on that rating date. All data was subjected to analysis of variance using the SAS system general linear model procedure (SAS Institute Inc., Cary, NC) and treatment means separated using Fisher’s protected least significant difference (LSD) test at the (P = 0.05) level.

Annual Bluegrass Suppression

Annual bluegrass populations fluctuated from 4 to 56% depending upon specific PGR treatment and rating date (Tables 1 and 2). Annual bluegrass populations in the untreated plots fluctuated seasonally and generally followed the traditional pattern of seasonal development associated with the life-cycle of a winter-annual species. This was especially true in 2004 where the highest ABG populations were observed early in the season, lower populations in late-summer, and higher populations in the late autumn. These observations are consistent with other studies that reported seasonal ABG fluctuations (16). On 28 May, during the first year all PGR treatments significantly reduced ABG from 56% in the untreated plots to 31-40% in PGR treated turf (Table 1). On 16 July, ABG populations had declined across all treatments to 20-37%; however no treatment differences were observed. By 5 Novebmer, however, PGR effects were again evident and all PGR treatments reduced ABG to 21-35%, compared to 50% ABG in the untreated control. On the initial rating date in 2005, 28 April, three PGR treatments — FL, 0.28 or 0.56 kg a.i./ha alone, and the FL (0.56 kg a.i./ha) + TE — were different than the untreated turf ranging from 18-32% and 52% for PGR treated and untreated respectively (Table 2). On 27 May, only the FL (0.56 kg a.i./ha) + TE tank-mix treatment was different. By 18 July, ABG populations generally decreased and PGR-treated turf contained significantly lower ABG populations than the untreated turf, 39%. By 26 December, ABG populations remained low in all plots with only 17% ABG in the untreated turf. On this date, four PGR treatments resulted in < 10% ABG; however, only the FL alone (0.28 or 0.56 kg a.i./ha) and FL (0.28 kg a.i./ha) + TE treatments were significantly different from the untreated. To our surprise ABG populations did not rebound to high, > 50%, levels in late 2005 as they did in 2004. This is most likely explained by unusually hot, dry weather conditions which persisted late into October throughout the region. Although the study area was regularly irrigated to promote growth, the higher temperatures may have negatively impacted ABG re-establishment. On the initial rating date in 2006, 11 April, ABG populations were very similar to the 26 December rating date (Table 2) showing no ABG population increases during the winter months. By 5 May, however, more substantial differences from the untreated turf were apparent. Both FL alone treatments and the FL (0.56 kg a.i./ha) + TE tank-mix resulted in very low, 4 to 8.5%, ABG populations compared to the untreated, 25%. Out data is consistent with previous reports where TE alone was generally not effective for reducing ABG (18,19). In fact, slight ABG increases were observed in plots treated with TE alone (Tables 1 to 2).

Table 1. Percentage of annual bluegrass in a mixed creeping bentgrass/annual bluegrass research fairway as affected by flurprimidol (FL), trinexepac-ethyl (TE), and various FL + TE plant growth regulator (PGR) tank-mix combinations.

 ^x Annual bluegrass populations were visually assessed on a 0 to 100 percent linear scale where 100 equals complete plot coverage.

 ^y PGR treatments were applied at a spray volume of 467 liters/ha on 15 May, 14 June, 9 July, 10 August, 17 September, 19 October 2004.

 ^z Means in the same column followed by the same letter are not significantly different according to Fisher’s protected LSD t-test (P = 0.05).

Table 2. Percentage of annual bluegrass in a mixed creeping bentgrass/annual bluegrass research fairway as affected by flurprimidol (FL), trinexepac-ethyl (TE), and various FL + TE plant growth regulator (PGR) tank-mix combinations.

 ^x Annual bluegrass populations were visually assessed on a 0 to 100 percent linear scale where 100 equals complete plot coverage.

 ^y PGR treatments were applied at a spray volume of 467 liters/ha on 15 May, 14 June, 9 July, 10 August, 17 September, 19 October 2004; and 17 May, 13 June, 11 July, 8 August, 6 September, and 4 October 2005.

 ^z Means in the same column followed by the same letter are not significantly different according to Fisher’s protected LSD t-test (P = 0.05).

Turf Appearance

Several studies have reported enhanced color and TQ following PGR applications (5,10). In our study, however, these PGR effects were inconsistent for color (data not presented) and TQ (Table 3). There were no differences among treatments for overall study or mean annual TQ in either study year. However, a general increase in mean annual TQ was observed in 2005 compared to 2004. These lack of TQ differences are most likely due to several factors such as the seasonal variability in ABG appearance (e.g., the presence of spring seedheads), non-uniform plot color due to the presence of ABG patches or the occasional negative effects of PGR applications. In September 2005, however, PGR treated turf TQ was generally superior to the untreated turf. Additionally, occasional declines in TQ due to PGR applications were also recorded. This response was worse where high PGR rates were applied during October of both study years. These low TQ values were primarily due to temporary leaf bronzing/purpling following the PGR applications. This phenomenon is not uncommon (6,14,21,23) and the turf normally recovers within three to four weeks.

Table 3. Turfgrass quality ratings of a mixed creeping bentgrass/annual bluegrass research fairway as affected by flurprimidol (FL), trinexepac-ethyl (TE), and various FL + TE plant growth regulator (PGR) tank-mix combinations.

 ^x Turfgrass quality was visually assessed on a 0 to 10 scale where 0 = brown, dead turf; and 10 = dark green color, optimum uniformity; and shoot density values ≥ 7 equals minimally acceptable golf course fairway quality.

 ^y PGR treatments were applied at a spray volume of 467 liters/ha on 15 May, 14 June, 9 July, 10 August, 17 September, 19 October 2004; and 17 May, 13 June, 11 July, 8 August, 6 September, and 4 October 2005.

 ^z Means in the same column followed by the same letter are not significantly different according to Fisher’s protected LSD t-test (P = 0.05).

Table 4. Percentage reductions of annual bluegrass in a mixed creeping bentgrass/annual bluegrass research fairway as affected by flurprimidol (FL), trinexepac-ethyl (TE) and various FL + TE plant growth regulator (PGR) tank-mix combinations.

 ^x Percentage annual bluegrass reductions were calculated based on initial populations recorded prior to PGR treatment on 15 May 2004.

 ^y PGR treatments were applied at a spray volume of 467 liters/ha on 15 May, 14 June, 9 July, 10 August, 17 September, 19 October 2004; and 17 May, 13 June, 11 July, 8 August, 6 September, and 4 October 2005.

 ^z Means in the same column followed by the same letter are not significantly different according to Fisher’s protected LSD t-test (P = 0.05).

Summary and Recommendations

These data suggest that monthly applications of FL and FL + TE during the growing season for two consecutive years is an effective management practice which can significantly reduce ABG populations in a mixed CBG/ABG golf fairway. In this study, three PGR treatments were most effective for suppressing ABG to tolerable levels, < 10%. These treatments were: FL alone (0.28 or 0.56 kg a.i./ha), and FL (0.56 kg a.i./ha) tank-mixed with TE. However, the most consistently effective treatment for reducing ABG was FL (0.56 kg a.i./ha) tank mixed with TE which provided significantly lower ABG relative to the untreated turf on seven of eleven rating dates (Tables 1 and 2). On several dates throughout the study a desirable darker green color and superior TQ was associated with PGR applications (data not presented). This response, however, was not consistent for any PGR treatment during the study. Although the FL (0.56 kg a.i./ha) + TE tank-mix effectively suppressed ABG, serious caution should be exercised when considering this treatment as temporary discoloration, and a subsequent reduction in TQ may be observed during early spring and mid- to late-autumn months (Table 3). This response appears related to cooler air temperatures and slow seasonal turf growth but the temporary discoloration may be worthwhile since this treatment gradually reduced ABG populations. Future studies should evaluate the most effective treatments from this study tank-mixed with safeners such as low rates of soluble N-sources or liquid chelated iron plus N-sources to mask potential discoloration while reducing ABG populations.

Acknowledgments

This research was made possible by grant-in-aid support from the Midwest Regional Turf Foundation and the SePro Corporation.

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