Introduction
Annual bluegrass resistance to herbicide is widespread in intensively managed turf systems. Currently there are 18 uniquely reported cases of herbicide-resistant annual bluegrass in the United States (Heap Reference Heap2023), and that is likely underreported based on recent literature (Bowling et al. Reference Bowling, McCurdy, de Castro, Patton, Brosnan, Askew, Breeden, Elmore, Gannon, Gonçalves, Kaminski, Kowalewski, Liu, Mattox, McCarty, McCullough, McElroy, McKeithen, Osburn, Rogers, Rutland, Tang, Taylor, Unruh, Vargas and Bagavathiannan2024; Ignes et al. Reference Ignes, McCurdy, McElroy, Castro, Ferguson, Meredith, Rutland, Stewart and Tseng2023; Rutland et al. Reference Rutland, Bowling, Russell, Hall, Patel, Askew, Bagavathiannan, Brosnan, Gannon, Gonçalves, Hathcoat, McCarty, McCullough, McCurdy, Patton, Unruh and McElroy2023). Herbicide-resistant annual bluegrass has proliferated in warm-season turf systems across a variety of herbicide modes of action (MOAs). A recent survey of Tennessee golf courses identified 21%, 64%, 58%, and 97% of annual bluegrass as resistant to glyphosate, foramsulfuron, prodiamine, and simazine, respectively (Brosnan et al. Reference Brosnan, Vargas, Breeden and Zobel2020c). Additionally, 4% of these resistant populations exhibited resistance to multiple herbicide MOAs (Brosnan et al. Reference Brosnan, Vargas, Breeden and Zobel2020c). Similar surveys in Mississippi and Texas indicate that annual bluegrass herbicide resistance is widespread across the southern United States and that annual bluegrass populations with multiple herbicide resistance are prevalent (Hutto et al. Reference Hutto, Coats and Taylor2004; Singh et al. Reference Singh, Reis, Reynolds, Elmore and Bagavathiannan2021). In many instances, annual bluegrass evolves resistance to multiple herbicide MOAs when resistant populations are initially managed with a single alternate herbicide (Brosnan et al. Reference Brosnan, Elmore and Bagavathiannan2020a). Furthermore, resistant annual bluegrass populations are more prevalent in warm-season turf systems (Brosnan et al. Reference Brosnan, Vargas, Spesard, Netzband, Zobel, Chen and Patterson2020b; Cross et al. Reference Cross, McCarty, Tharayil, McElroy, Chen, McCullough, Powell and Bridges2015; Isgrigg et al. Reference Isgrigg, Yelverton, Brownie and Warren2002; Kelly et al. Reference Kelly, Coats and Luthe1999; McElroy et al. Reference McElroy, Flessner, Wang, Dane, Walker and Wehtje2013; Yu et al. Reference Yu, McCullough and Czarnota2018). Owing to the widespread nature of herbicide-resistant annual bluegrass, herbicide options with alternative MOAs are needed to sustain effective annual bluegrass control in warm-season turf systems.
Endothall is currently utilized in the United States as an aquatic herbicide for control of a variety of weed species (Skogerboe and Getsinger Reference Skogerboe and Getsinger2002). In U.S. turf, endothall was historically used for semiselective control of annual bluegrass in cool-season turfgrasses. In a creeping bentgrass (Agrostis stolonifera L.) fairway, three spring applications of endothall applied at 0.56 kg ha−1 reduced annual bluegrass coverage by 62% relative to the nontreated (Engel and Aldrich Reference Engel and Aldrich1960). In Michigan, three biweekly applications of endothall applied at 0.3 and 0.6 kg ai ha−1 to Kentucky bluegrass (Poa pratensis L.) turf reduced annual bluegrass coverage from 50% to 32% and 17%, respectively (Turgeon et al. Reference Turgeon, Meggitt and Penner1972a). Endothall’s effectiveness for annual bluegrass control, however, has been limited by cool-season turf phytotoxicity (Peppers et al. Reference Peppers, Brewer and Askew2021).
Recent literature has indicated that bermudagrass may be more tolerant than cool-season turf species to endothall applications. Bermudagrass was 57 and 65 times more tolerant to endothall-contaminated irrigation water compared to annual bluegrass and annual ryegrass (Lolium multiflorum Lam.), respectively (Koschnick et al. Reference Koschnick, Haller and Fox2005). Additionally, endothall applied at 0.84 and 1.68 kg ai ha−1 was not significantly injurious to hybrid bermudagrass putting greens when applied during full winter dormancy (Peppers and Askew Reference Peppers and Askew2023). To date, no peer-reviewed literature has evaluated endothall for annual bluegrass control in bermudagrass turf. Endothall is a serine/threonine protein phosphatase inhibitor, which is a novel MOA in turf systems (Bajsa et al. Reference Bajsa, Pan, Dayan, Owens and Duke2012; Tresch et al. Reference Tresch, Schmotz and Grossman2011). Previous literature has indicated that herbicides with novel MOAs can effectively control herbicide-resistant annual bluegrass populations (Brosnan et al. Reference Brosnan, Vargas, Breeden, Boggess, Staton, Wadl and Trigiano2017). Endothall-resistant annual bluegrass has already been reported in Australia. Barua et al. (Reference Barua, Bousalis, Malone, Gill and Preston2020) found that all annual bluegrass populations screened in Australia were resistant to endothall. However, in this study, the susceptible comparison was controlled 50% by a single application of endothall applied at 0.13 kg ai ha−1, which is a much lower rate than has sublethally suppressed annual bluegrass in other studies (Engel and Aldrich Reference Engel and Aldrich1960; Peppers et al. Reference Peppers, Brewer and Askew2021; Turgeon et al. Reference Turgeon, Meggitt and Penner1972a). The now-lapsed U.S. federal label for terrestrial uses of endothall (EPA 2005) indicates a maximum terrestrial use rate of 2.24 kg ai ha−1, which is approximately ten times higher than the currently labeled rate in Australia (Anonymous 2020). On the basis of previous literature, we surmise that endothall rates for turfgrass use currently in Australia and formerly in the United States were limited due to concerns of injury to cool-season turf and the product’s historical market value. In warm-season turf, especially during dormancy, the potential market value for endothall has likely increased due to developed annual bluegrass resistance to a wide range of formerly viable herbicides. Thus we hypothesized that above-Australian-label endothall dosages may acceptably control herbicide-resistant annual bluegrass with acceptable phytotoxicity to dormant, semidormant, or actively growing bermudagrass turf.
Materials and Methods
Herbicide-Resistant Annual Bluegrass Response to Endothall
Two greenhouse studies were conducted at the Glade Road Research Facility in Blacksburg, VA (37.23°N, 80.44°W), to evaluate herbicide-resistant annual bluegrass response to endothall. Both trials were arranged as randomized complete-block designs with 42 treatments and four blocks. Treatments were arranged as a 7 × 8 factorial with seven levels of annual bluegrass populations and eight levels of herbicide treatment. The six annual bluegrass populations consisted of two known susceptible populations (S1 and S2), two acetolactate synthase (ALS)-inhibitor-resistant populations (ALS1 and ALS2), two glyphosate (5-enolpyruvylshikimate-3-phosphate; EPSP)-resistant populations (EPSP1 and EPSP2), and a population that exhibits resistance to multiple herbicide MOAs (MR). The multiple-resistant annual bluegrass population was putatively resistant to EPSP-, 4-hydroxyphenylpyruvate dioxygenase (HPPD)-, photosystem II (PSII)-inhibiting herbicides. All annual bluegrass populations were collected throughout the state of Virginia and screened for herbicide resistance to a variety of MOAs. The causal mechanisms of resistance are more thoroughly presented in Table 1. The six herbicide levels include five endothall rates (0.5, 1.0, 2.0, 4.0, and 8.0 kg ai ha−1) and a nontreated comparison for every annual bluegrass population.
Herbicide-resistant annual bluegrass biotypes screened for endothall resistance. a
a Abbreviations: ALS, acetolactate synthase; ALS1/2, ALS-inhibitor-resistant population; EPSP, 5-enolpyruvylshikimate-3-phosphate; EPSP1/2, glyphosate-resistant population; HPPD, 4-hydroxyphenylpyruvate dioxygenase; MR, population with confirmed resistance to four herbicide modes of action; PSII, photosystem II; S1/2, susceptible population.
b Annual bluegrass ALS herbicide resistance due to a Trp574-Leu mutation was first reported by McElroy et al. (Reference McElroy, Flessner, Wang, Dane, Walker and Wehtje2013).
c Annual bluegrass glyphosate resistance due to a Pro106-Ala mutation was first reported by Cross et al. (Reference Cross, McCarty, Tharayil, McElroy, Chen, McCullough, Powell and Bridges2015).
All populations were seeded into 45 × 30-cm flats containing potting soil (Pro-Mix® BXM General Purpose Growing Medium, Premier Horticulture, Quakertown, PA, USA). After annual bluegrass plants reached the 2-tiller growth stage, the herbicide-resistant annual bluegrass populations were treated with appropriate herbicides to ensure homogeneity of resistant annual bluegrass plants. Populations ALS1, ALS2, and MR were treated with trifloxysulfuron (Monument®, Syngenta Crop Protection, Greensboro, NC, USA) applied at 27.8 g ai ha−1 + non-ionic surfactant (Induce, Helena® Chemical Company, Collierville, TN, USA) applied at 0.25% v/v; populations EPSP1 and EPSP2 were treated with glyphosate (Roundup Pro® Concentrate, Monsanto Company, Washington, DC, USA) applied at 1.12 kg ai ha−1. Surviving plants were selected and transplanted into 7.6 × 7.6 × 6.4-cm pots containing a 2:1 sand–to–native soil mixture. The native soil utilized in this study was a Groseclose-Urban land complex loam (clayey, mixed, mesic Typic Hapludults) with pH 6.0 and 3.1% organic matter. All plants were fertilized with 25 kg N ha−1 (19-6-12; Sta-Green™ Indoor and Outdoor All-Purpose Food Fertilizer, Gro Tec, Modoc, IN, USA) once approximately 1 wk after germination to maintain proper plant growth. Irrigation was supplied twice daily to prevent plant wilt. Mercury vapor lamps provided 430 µmol m−2 s−1 of photosynthetically active radiation as supplemental lighting and was set to a 14-h day length throughout the studies. Greenhouse day/night temperatures were maintained at 29/20 C. All treatments were applied using a CO2-pressurized backpack sprayer calibrated to deliver 375 L ha−1 at 330 kPa and fitted with TTI11004 TeeJet® nozzles (TeeJet® Technologies, Springfield, IL, USA). Following applications, all pots were allowed to dry for 4 to 6 h before irrigation was applied. Annual bluegrass plants had approximately 5 to 8 tillers at application. At the conclusion of the trial, 28 d after application, annual bluegrass aboveground biomass was collected and dried at 50 C for 72 h before weighing. Annual bluegrass control data were extrapolated from the aboveground biomass data by comparing all treated plants to the nontreated comparison within a given biotype and replication. The endothall rate required to control annual bluegrass 50% and 90% (C50 and C90, respectively) was modeled across all doses in each replicate for a given biotype using a three-parameter sigmoidal model with Equation 1:
$$y = a/\left( {1 + {e^{\left\{ { - [x - xo]/b} \right\}}}} \right)$$
where y represents percent annual bluegrass control, e is the natural logarithm, x represents the endothall rate, and a, b, and xo represent regression parameters calculated using the PROC NLIN procedure in SAS (version 9.4; SAS Institute, Cary, NC, USA). The resulting C50 and C90 data were subjected to analysis of variance (ANOVA) using the PROC GLM procedure in SAS 9.4 with sums of squares partitioned to reflect replicate, trial, and biotype, and means were separated with Fisher’s protected least significant difference (LSD) test at α = 0.05.
Field Evaluation of Annual Bluegrass Control and Turf Tolerance to Endothall
Four field studies were conducted in Blacksburg, VA, between February 2022 and June 2023 to evaluate annual bluegrass and warm-season turf response to endothall. Two field studies, one in 2022 (TRC1) and one in 2023 (TRC2), were conducted on a ‘Tifway 419’ hybrid bermudagrass research fairway maintained at 1.5 cm height at the Virginia Tech Turfgrass Research Center (37.21°N, 80.41°W). Two additional field studies were conducted at the Glade Road Research Facility in 2022 (GRRF1) on ‘Cavalier’ manilagrass and in 2023 (GRRF2) on a ‘Latitude 36’ hybrid bermudagrass (Table 2). Both locations at the Glade Road Research Facility were research fairways maintained at 1.3 cm height throughout the study. All field trials were arranged as randomized complete-block designs with 13 treatments and four blocks. Treatments were arranged as a 3 × 4 factorial with three levels of application timing (fully dormant, 50% visible green turf coverage, and 100% visible green turf coverage) and four levels of herbicide treatment. The four levels of herbicide treatment were endothall (Teton®, United Phosphorus, Cary, NC, USA) applied at 1.12, 1.68, and 2.24 kg ai ha−1 and trifloxysulfuron (Monument®) applied at 27.8 g ai ha−1. A nontreated comparison was included in all trials. Applications were applied via a CO2-pressurized sprayer fitted with flat-fan TeeJet® 6503 nozzles (TeeJet® Technologies) and calibrated to deliver 375 L ha−1 at 330 kPa. Plots measured 0.9 × 1.8 m in size. Fully dormant, 50% green, and 100% green applications were made on April 5, April 25, and May 12 in 2022, respectively, and on March 16, April 26, and May 10 in 2023, respectively. Annual bluegrass had 80 to 120 tillers at the earliest application timing and >150 tillers at the mid-transition and fully green turf application timings.
Trial locations and application timings of field-applied endothall.
Annual bluegrass control and turf green coverage were visually evaluated throughout the study as a percentage, where 0% equals no annual bluegrass control or no visible green turf, 100% equals complete annual bluegrass control or complete green turf coverage, and 80% annual bluegrass control was considered commercially acceptable. Turf injury was extrapolated from green coverage assessment by calculating the percent green coverage reduction in treated plots compared to the nontreated check. At the conclusion of the studies (∼June 1), annual bluegrass density was measured via line-intersect grids that included 135 intersects at 5.5-cm increments within the treated portion of each plot. Final assessments of weed control were extrapolated from the line-intersect counts by comparing weed coverage in the nontreated check in each replication with the coverage in each treated plot. Data were subjected to ANOVA with sums of squares partitioned to reflect replicate and trial as random variables; herbicide, application timing, and Herbicide × Application Timing as fixed effects; and all possible interactions of trial with fixed effects. Terms that included trial interactions were tested with the mean square of residual error. The terms herbicide, application timing, and Herbicide × Application Timing were tested with the mean square associated with each term’s interaction with trial, as described by McIntosh (Reference McIntosh1983). Main effects or interactions were pooled over trial only if trial interactions were insignificant. Appropriate means were separated with Fisher’s protected LSD test at α = 0.05.
Results and Discussion
Herbicide-Resistant Annual Bluegrass Response to Endothall
The trial × biotype interaction was significant for annual bluegrass C50 and C90 (P = 0.0077 and 0.0008, respectively); therefore results are presented separately by trial. The trial interaction was likely due to differences in endothall efficacy between the two trials with respect to the S2 biotype, as this was the only biotype to change the mean rank between trials for either C50 or C90 (Table 3). Although differences between biotypes other than S2 were similar in both trials, 26% to 53% more endothall was required to control annual bluegrass in Trial 1 compared to Trial 2 for all biotypes except S2 and EPSP2. The average amount of endothall needed to control susceptible annual bluegrass 90% was 1.6 kg ha−1 (Table 3). It was surmised that light intensity under greenhouse conditions was less than full sunlight, and this may have reduced endothall performance given the herbicide’s MOA (Dodge Reference Dodge1982; Mayasich et al. Reference Mayasich, Karlander and Terlizzi1986). In Trial 2, plants were moved outdoors under full sunlight at treatment time and remained for 3 d following treatment, and endothall more effectively controlled all biotypes except S2 and EPSP2. Similar differences occurred between field and greenhouse performances of endothall when 4.5 kg endothall ha−1 was required to reduce annual bluegrass foliar length in greenhouse studies in 1972 (Turgeon et al. Reference Turgeon, Meggitt and Penner1972a).
Influence of biotype on endothall rate required to control annual bluegrass 50% and 90%.a,b,c,d
a Abbreviations: ALS, acetolactate synthase; ALS1/2, ALS-inhibitor-resistant population; C50, 50% control; C90, 90% control; EPSP, 5-enolpyruvylshikimate-3-phosphate; EPSP1/2, glyphosate-resistant population; HPPD, 4-hydroxyphenylpyruvate dioxygenase; MR, population with confirmed resistance to four herbicide modes of action; PSII, photosystem II; S1/2, susceptible population.
b Letters following means indicate significant differences, according to Fisher’s protected LSD test (α = 0.05), within a given column.
c Biotypes were tested for putative resistance in previous studies, and only survivors of prescreened populations were used in the current study.
d Trial 1 remained under greenhouse supplemental lighting conditions throughout the duration of the study. However, annual bluegrass plants in Trial 2 were moved outdoors for 3 d immediately following application to mimic field light intensity.
Only EPSP2 required more endothall to achieve C50 than both susceptible populations in both trials (Table 3). Interestingly, EPSP2 and EPSP1 each had a Pro-106-Ala amino acid substitution on EPSPS, but only EPSP2 exhibited endothall resistance. This disparate endothall response in similar EPSP-resistant biotypes that had not been previously exposed to endothall indicates that the Pro-106-Ala substitution does not confer endothall resistance. The endothall resistance mechanism in EPSP2 is unknown. Previous research has indicated that annual bluegrass populations with nontarget site resistance can be resistant to novel MOAs (Brosnan et al. Reference Brosnan, Vargas, Breeden, Boggess, Staton, Wadl and Trigiano2017), but there also may be other mechanisms that could explain the resistance of the EPSP2 population to endothall. It should be noted that ALS1 did not have an observed target site mutation and was not resistant to endothall. Non-target-site resistance is most observed in row crop systems but has been reported in annual bluegrass from turf systems. In an Alabama population, annual bluegrass resistance to PSII-inhibiting herbicides was conferred via reduced herbicide absorption and translocation (Syvantek et al. Reference Syvantek, Aldahir, Chen, Flessner, McCullough, Sidhu and McElroy2016). An annual bluegrass population in Tennessee resistant to multiple herbicide MOAs exhibited increased levels of transporter and metabolic enzymes (Brosnan et al. Reference Brosnan, Laforest, Soufiane, Boggess and Trigiano2019). ALS1 and EPSP2 may exhibit different resistance mechanisms, leaving open the possibility of a nontarget site mechanism of resistance to endothall in EPSP2.
Field Evaluation of Annual Bluegrass Control and Turf Tolerance to Endothall
Over 90% of bermudagrass winter damage occurred at GRRF2. Therefore turf injury data were not collected at this location. Owing to differences in herbicide speed of activity between endothall and trifloxysulfuron, turf injury data were converted to the maximum observed injury over the assessment period. Maximum turf injury did not differ due to trial (P ≥ 0.1854). However, the interaction of Herbicide × Application Timing was significant (P = 0.0002). Therefore data are pooled over trials, and the Herbicide × Application Timing interaction is shown (Table 4). Herbicides did not unacceptably (>30%) injure fully dormant turf, and only endothall applied at 2.24 kg ai ha−1 injured actively growing turf >30%. In general, semidormant turf was more sensitive to endothall than was fully dormant or green turf. These results align with previous literature, as warm-season turf is generally injured more by herbicides applied during postdormancy transition (Dernoeden Reference Dernoeden1994; Johnson Reference Johnson1976; Peppers and Askew Reference Peppers and Askew2023; Reed and McCullough Reference Reed and McCullough2014; Reed et al. Reference Reed, McCullough, Grey, Czarnota, Vencill and Waltz2015). Turf injury during postdormancy transition generally increased with endothall rate to as much as 60% (Table 4).
Maximum observed turfgrass injury and end-of-season annual bluegrass control as influenced by herbicide at three application timings (dormant, mid-transition, and 100% green turf).a,b
a Letters following means indicate significant differences, according to Fisher’s protected LSD test (α =0.05), within a given column.
b Annual bluegrass control was derived via line-intersect counts that included 135 intersects at 5.5-cm increments within the treated portion of each plot. Final assessments of weed control were extrapolated from the line-intersect counts by comparing weed coverage in the nontreated check in each replication with the coverage in each treated plot.
c Statistical significance, according to Fisher’s protected LSD test (α =0.05), within rows for a given rating metric.
d Non-ionic surfactant was included at 0.25% v/v in all trifloxysulfuron treatments.
Despite unacceptable maximum injury at any endothall rate during postdormancy transition and the highest rate on green turf, turf recovered to acceptable levels by 14 d after treatment (DAT) for all treatments except the highest endothall rate applied to transitioning turf (Table 4). By 28 DAT, treatments did not injure turf more than 13% (data not shown). Hybrid bermudagrass putting greens responded similarly to endothall application timings and exhibited rapid recovery whenever bermudagrass was injured in other studies (Peppers and Askew Reference Peppers and Askew2023). Additionally, warm-season turf demonstrated endothall tolerance relative to historical reports of cool-season turf response (Engel and Aldrich Reference Engel and Aldrich1960; Peppers et al. Reference Peppers, Brewer and Askew2021; Turgeon et al. Reference Turgeon, Meggitt and Penner1972a), which is consistent with studies that evaluated turf tolerance to endothall-contaminated irrigation water (e.g., Koschnick et al. Reference Koschnick, Haller and Fox2005).
Although the three-way interaction of Trial × Herbicide × Application Timing on annual bluegrass control at the conclusion of the trial was significant (P = 0.0495), the low F-value for the interaction (2.6) compared to the Herbicide × Application Timing main effect (F = 11.4, P = 0.002) suggests that the interaction explains a small amount of the variance. The three-way interaction was likely significant due to variability in annual bluegrass control with trifloxysulfuron in relation to endothall treatments. For example, at GRRF1, trifloxysulfuron applied during mid-transition controlled annual bluegrass 93% and more than all endothall treatments. At GRRF2, trifloxysulfuron applied during mid-transition controlled annual bluegrass 72% and less than all endothall treatments (data not shown). Additionally, when trifloxysulfuron treatments were removed from the analysis, the trial interaction was no longer significant (P = 0.7041), indicating that trifloxysulfuron was the primary cause of the three-way interaction. For these reasons, data were pooled to show the Herbicide × Application Timing interaction, which was also significant (P < 0.0001) for clarity and brevity (Table 4).
Endothall applied to fully dormant turf controlled annual bluegrass 49% to 72%, depending on rate, and less at any given rate than when applied during or after postdormancy transition (Table 4). This reduction in endothall efficacy when applied during full dormancy may be attributed to a variety of factors. Temperature and sunlight intensity were inherently lower during the fully dormant applications. Average temperatures at the time of the fully dormant applications were 8 to 9 C, and average temperatures at the time of both later applications were 18 to 23 C (data not shown). Previous research has indicated that herbicide activity is positively correlated to temperature and light intensity (Dodge Reference Dodge1982; Kells et al. Reference Kells, Meggitt and Penner1984; Mayasich et al. Reference Mayasich, Karlander and Terlizzi1986; McWhorter and Azlin Reference McWhorter and Azlin1978; Wills and McWhorter Reference Wills and McWhorter1981). Endothall applied at 1.68 and 2.24 kg ai ha−1 to postdormant turf controlled annual bluegrass 88% to 95% and equivalently to or better than trifloxysulfuron (Table 4).
These results indicate that endothall controls annual bluegrass acceptably at any of the rates tested when applied to fully green turf and injures bermudagrass not more than 21% at rates of 1.68 kg ha−1 or lower. Endothall does not injure fully dormant turf, but single treatments are unlikely to acceptably control annual bluegrass at this timing. Endothall should not be applied to bermudagrass during postdormancy transition unless severe but transient injury can be tolerated. To avoid turf injury, it may be possible to make multiple applications of endothall or include other herbicide admixtures. Previous research has indicated that multiple endothall applications at lower rates control annual bluegrass more effectively than do single endothall applications at higher rates (Turgeon et al. Reference Turgeon, Meggitt and Penner1972a). All annual bluegrass evaluated in this study had >80 tillers at the time of application, and endothall may more effectively control relatively less mature annual bluegrass, as weed growth stage regularly affects herbicide activity (Bellinder et al. Reference Bellinder, Arsenovic, Shah and Rauch2003; Busey Reference Busey2004; Peppers et al. Reference Peppers, McElroy, Orlinski, Baird, Petelewicz, Joseph, Sierra Augustinus, Schiavon and Askew2024; Steckel et al. Reference Steckel, Wax, Simmons and Phillips1997). Additionally, surfactant was not included with endothall in these studies. Adjuvants like non-ionic surfactants or crop oil concentrates can increase herbicide efficacy (Norsworthy and Grey Reference Norsworthy and Grey2004; Riechers et al. Reference Riechers, Wax, Liebl and Bush1994; Sherrick et al. Reference Sherrick, Holt and Hess1986) and may increase annual bluegrass control by a given endothall rate.
Practical Implications
Results from these studies indicate that endothall can provide selective annual bluegrass control in warm-season turf species, even on herbicide-resistant annual bluegrass populations if mid-transition treatments are avoided. Endothall applied at rates of 1.12 and 1.68 kg ai ha−1 controls annual bluegrass when applied to fully green turf; however, annual bluegrass is unlikely to be controlled more than 75% when single treatments are applied during dormancy. Turf was most tolerant to endothall when it was applied during full dormancy. To improve weed control while capitalizing on turf safety during dormancy, future research should evaluate herbicide and surfactant admixtures with endothall applied at earlier application timings. To reduce the potential for phytotoxicity on fully green turf, postapplication irrigation should be investigated as a means to reduce foliar injury because endothall is translocated primarily acropetally (Turgeon et al. Reference Turgeon, Penner and Meggitt1972b). Sequential herbicide applications at lower rates are commonly more effective than single applications at higher rates (Willis et al. Reference Willis, Beam, Barker and Askew2006) and should be investigated for their potential to improve annual bluegrass control in dormant or green turf and to reduce phytotoxicity in green turf.
Acknowledgments
The authors thank John Hinson for maintaining turf plots at the Virginia Tech Turfgrass Research Center for the duration of these studies.
Competing interests
The authors declare no conflicts of interest.
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