Research sites.

Field trials were conducted at the East Tennessee AgResearch and Education Center – Plant Sciences Unit, Knoxville, TN, from Apr. through Aug. 2017 and 2018. All trials were conducted in areas of common bermudagrass (Cynodon dactylon L.) or tall fescue (Festuca arundinacea Schreb.) turf established on Sequatchie silt loam soil (fine-loamy, siliceous, semiactive, thermic humic Hapludult) with pH values ranging from 5.7 to 6.7. Separate sites were used to evaluate treatment efficacy vs. natural infestations of white clover, yellow nutsedge, and virginia buttonweed; however, wild violet and ground ivy data were collected from a single site containing a mixed population of both species. Each year virginia buttonweed data were collected from a Riviera common bermudagrass fairway maintained at a 1.5-cm mowing height with a reel-mower (Reelmaster 5500D; The Toro Co., Bloomington, MN). Wild violet and ground ivy data were collected from a stand of tall fescue utility turf maintained at 8 cm with a rotary mower (Gravely 260Z; Gravely Tractor, Brillion, WI). White clover and yellow nutsedge data were collected from mixed stands of common bermudagrass and tall fescue maintained at 6 to 8 cm with the same aforementioned rotary mower. Irrigation was applied to supplement rainfall at all trial sites except the one used to collect wild violet and ground ivy data; this location received rainfall only.

White clover control.

Treatments evaluated for control of white clover included pyrimisulfan + penoxsulam (EH1580; PBI Gordon Corporation, Kansas City, MO) at 35 + 35 g·ha⁻¹ or 70 + 70 g·ha⁻¹, as well as an untreated check. Both pyrimisulfan and penoxsulam were impregnated on a granular fertilizer at 0.025% (total herbicide load = 0.05%). The analysis of the fertilizer used for impregnation was 21% N : 0% P₂O₅ : 3% K₂O, with N included in the form of polymer-coated urea. Fertilizer particle size averaged 1.5 mm. Treatments were applied on 26 Apr. 2017 and 25 Apr. 2018 using a shaker jar. Experimental design was a randomized complete block with four replications of plots measuring 1.5 × 1.8 m in 2017 and 1.5 × 1.5 m in 2018. All treatments were irrigated into the soil within 24 h of application.

Yellow nutsedge control.

Treatments evaluated for control of yellow nutsedge included pyrimisulfan + penoxsulam at 35 + 35 g·ha⁻¹ or 70 + 70 g·ha⁻¹ applied singly, as well as sequential applications of 70 + 70 g·ha⁻¹ followed by 35 + 35 g·ha⁻¹ or 52.5 + 52.5 g·ha⁻¹ followed by 52.5 + 52.5 g·ha⁻¹. An untreated check was included for comparison each year. Treatments were applied on the aforementioned granular fertilizer carrier on 26 Apr. 2017 and 25 Apr. 2018 using a shaker jar. Sequential applications were made with the same equipment on 30 May 2017 and 6 June 2018. Experimental design was a randomized complete block with four replications of plots measuring 1.5 × 1.8 m in 2017 and 1.5 × 2.1 m in 2018. All treatments were irrigated into the soil within 24 h of application.

Wild violet and ground ivy control.

Herbicide treatments evaluated on yellow nutsedge were also assessed for wild violet and ground ivy control each year. Treatments were applied on the aforementioned granular fertilizer carrier on 4 Apr. 2017 and 25 Apr. 2018 using a shaker jar. Sequential applications were made with the same equipment on 26 Apr. 2017 and 24 May 2018. Experimental design was a randomized complete block with four replications of plots measuring 1.5 × 1.8 m in 2017 and 1.5 × 1.5 m in 2018. Rainfall occurred within 24 h of treatment application to move herbicides into the soil.

Virginia buttonweed control.

Treatments evaluated for control of virginia buttonweed included pyrimisulfan + penoxsulam at 70 + 70 g·ha⁻¹ applied singly, as well as sequential applications of 70 + 70 g·ha⁻¹ followed by 35 + 35 g·ha⁻¹ or 52.5 + 52.5 g·ha⁻¹ followed by 52.5 + 52.5 g·ha⁻¹. An untreated check was included for comparison each year. Treatments were applied on the aforementioned granular fertilizer carrier on 26 Apr. 2017 and 26 Apr. 2018 using a shaker jar. Sequential applications were made with the same equipment on 30 May 2017 and 24 May 2018. Experimental design was a randomized complete block with four replications of plots measuring 1.5 × 1.8 m in both 2017 and 2018. All treatments were irrigated into the soil within 24 h of application.

Data collection and statistical analysis.

In all studies, weed control was evaluated on a 0% (i.e., no control) to 100% (i.e., complete kill) scale relative to untreated check plots in each replication at 4 to 10 WAIT. On each date that weed control data were collected, turfgrass injury was assessed using a similar percentage-based scale relative to untreated check plots. Analysis of variance was conducted in R (version 3.5.1) with data from each weed species analyzed separately. No statistically significant year-by-treatment interactions were detected in white clover, yellow nutsedge, wild violet, and virginia buttonweed control data. Therefore, means from each year were combined and separated using Fisher’s protected least significant difference (lsd) test at the 0.05 level via the LSD.test function of the Agricolae package within R. Significant year-by-treatment interactions were detected in ground ivy control data, resulting in data from each year being analyzed and presented separately.

White clover control.

Significant differences in white clover control resulting from herbicide treatment were detected at 4 to 10 WAIT (Table 1). Application of pyrimisulfan + penoxsulam at 70 + 70 g·ha⁻¹ controlled white clover greater than 35 + 35 g·ha⁻¹ on all assessment dates. Overall white clover control with a single application of pyrimisulfan + penoxsulam at 70 + 70 g·ha⁻¹ ranged from 96% to 100% compared with 69% to 87% for 35 + 35 g·ha⁻¹. Responses on white clover in the current study support those reported by Loughner et al. (2006), with penoxsulam applied alone at 140 g·ha⁻¹. However, when penoxsulam was applied at 22 g·ha⁻¹ in combination with 2,4-D and dicamba, white clover control only ranged from 44% to 71% at six locations in the northern United States (Loughner and Nolting, 2010). In addition to herbicidal efficacy, white clover control following application of pyrimisulfan + penoxsulam in the current study was also likely influenced by supplemental N delivered by these treatments. Application of pyrimisulfan + penoxsulam at 35 + 35 and 70 + 70 g·ha⁻¹, delivered 29 and 59 kg·ha⁻¹ N, respectively, in the form of polymer-coated urea. Reductions in broadleaf weed cover, particularly white clover, resulting from the application of N have been reported in tall fescue turf (Voigt et al., 2001).

Table 1.White clover (Trifolium repens L.) control with single applications of pyrimisulfan and penoxsulam in Knoxville, TN. Means were combined from data collected during Summer 2017 and 2018.

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Yellow nutsedge control.

Significant differences in yellow nutsedge control resulting from herbicide treatment were detected at 4 to 10 WAIT (Table 2). Similar to observations on white clover, a single application of pyrimisulfan + penoxsulam at 70 + 70 g·ha⁻¹ controlled yellow nutsedge greater than treatment at 35 + 35 g·ha⁻¹ in this study. However, efficacy of single applications declined from 6 to 10 WAIT for both rates. For example, a single application of pyrimisulfan + penoxsulam at 35 + 35 g·ha⁻¹ controlled yellow nutsedge 91% at 6 WAIT, but only 61% by 10 WAIT; a similar trend was observed for the 70 + 70 g·ha⁻¹ application rate, with control measuring 98% at 6 WAIT and only 73% by 10 WAIT. This decline in yellow nutsedge control was not observed when pyrimisulfan + penoxsulam was applied sequentially. Both sequential application treatments controlled yellow nutsedge ≥99% by 10 WAIT. These responses support previous reports of pyrimisulfan effectively controlling Cyperaceae species, particularly tidalmarsh flatsedge (Asakura et al., 2012). Reports of penoxsulam efficacy for postemergence yellow nutsedge control have been mixed. Loughner et al. (2006) reported only 35% to 65% yellow nutsedge control with penoxsulam applications in turfgrass; however, Tehranchian et al. (2015) reported ≈90% yellow nutsedge growth reduction after applying penoxsulam (40 g·ha⁻¹) to tubers (from a crop production field) in a greenhouse study. Nevertheless, our data indicate that a mixture of pyrimisulfan + penoxsulam may provide turfgrass managers an effective option for yellow nutsedge control compared with penoxsulam alone.

Table 2.Yellow nutsedge (Cyperus esculentus L.) control with single and sequential applications of pyrimisulfan and penoxsulam in Knoxville, TN. Means were combined from data collected during Summer 2017 and 2018.

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Wild violet control.

Significant differences in wild violet control resulting from herbicide treatment were detected at 4 to 8 WAIT in this study (Table 3). Treatment with pyrimisulfan + penoxsulam at 35 + 35 g·ha⁻¹ was ineffective (≤51% control); however, increasing the application rate to 70 + 70 g·ha⁻¹ markedly increased control to 89% to 91% at 4 to 8 WAIT. There were no significant differences in wild violet control between a single application at 70 + 70 g·ha⁻¹ and either sequential application regime evaluated. It should be noted that previous research has reported minimal effects of supplemental N on wild violet in tall fescue turf (Gray and Call, 1993); therefore responses observed in the current study were likely predominately the result of herbicidal efficacy and not the 29 to 59 kg·ha⁻¹ N provided with applications of pyrimisulfan + penoxsulam in this study. In addition, although excellent efficacy was observed with pyrimisulfan + penoxsulam in this study, plots were only monitored for a limited time after application. Future research should explore long-term control of Viola spp. following treatment with pyrimisulfan + penoxsulam singly and sequentially.

Table 3.Wild violet (Viola spp.) control with pyrimisulfan and penoxsulam applications in Knoxville, TN. Means were combined from data collected during Summer 2017 and 2018.

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Ground ivy control.

Significant year-by-treatment interactions were detected in ground ivy control data (Table 4). In 2017, no significant differences in ground ivy control were detected among treatments at 6 or 8 WAIT. Overall control with single and sequential applications of pyrimisulfan + penoxsulam ranged from 70% to 100%.

Table 4.Ground ivy (Glechoma hederacea L.) control with pyrimisulfan and penoxsulam applications in Knoxville, TN, during Summer 2017 and 2018.

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In 2018, the 35 + 35 g·ha⁻¹ rate of pyrimisulfan + penoxsulam was ineffective vs. ground ivy (≤28% control); increasing the application rate of pyrimisulfan + penoxsulam to 70 + 70 g·ha⁻¹ markedly improved ground ivy control at both 6 and 8 WAIT. Sequential applications of pyrimisulfan + penoxsulam were highly effective in both years of the study, controlling ground ivy 93% to 100% in 2017 and 89% to 95% in 2018. These findings support reports by Patton et al. (2017) that ALS-inhibiting herbicides can be used to control ground ivy effectively. Moreover, the fertilizer carrier used to apply pyrimisulfan + penoxsulam in these studies may have affected ground ivy control positively. Herbicides were applied on a fertilizer carrier that included 21% N in the form of polymer-coated urea. To that end, applications of pyrimisulfan + penoxsulam at 70 + 70 g·ha⁻¹ delivered 59 kg·ha⁻¹ N. Improvements in ground ivy control when combining herbicide applications with N fertilizer have been reported previously (Kohler et al., 2004).

Virginia buttonweed control.

Similar to what was observed for ground ivy, significant differences in virginia buttonweed control were detected among treatments, with sequential applications of pyrimisulfan + penoxsulam improving control compared with applying these herbicides singly (Table 5). A single application of pyrimisulfan + penoxsulam at 70 + 70 g·ha⁻¹ controlled virginia buttonweed 85% at 4 WAIT, but declined to 53% by 10 WAIT. Comparatively, both sequential applications resulted in 76% to 87% virginia buttonweed control by 10 WAIT with no significant differences detected between the 70 + 70 g·ha⁻¹ followed by 35 + 35 g·ha⁻¹ or the 52.5 + 52.5 g·ha⁻¹ followed by 52.5 + 52.5 g·ha⁻¹ treatments. Overall, virginia buttonweed control with pyrimisulfan + penoxsulam could be improved by applying treatments sequentially over multiple seasons, as has been shown with auxin herbicides including 2,4-D, mecoprop, and dicamba in warm-season turfgrass (Kelly and Coats, 2000). Future research should explore this concept in further detail.

Table 5.Virginia buttonweed (Diodia virginiana L.) control with pyrimisulfan and penoxsulam applications in Knoxville, TN. Means were combined from data collected during Summer 2017 and 2018.

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Overall results of this research highlight that pyrimisulfan + penoxsulam may provide turfgrass managers a new option for managing sedge and broadleaf weeds of warm-season turfgrass systems. This is significant given that granular herbicides offer many advantages to turfgrass managers, including reduced application equipment costs and minimal concern over pesticide drift (Christians et al., 2017). Moreover, there are few effective granular herbicide options in turfgrass with most active ingredients performing optimally when formulated as liquids (Patton and Weisenberger, 2012). However, active ingredients such as penoxsulam and pyrimisulfan that can enter susceptible weeds via root tissue (Senseman, 2007; D. Sanson, personal communication) can provide effective weed control similar to liquid formulations (Patton and Weisenberger, 2012).

No turfgrass injury was observed in any of the aforementioned experiments (data not presented), which aligns with previous reports by Hoyle (2017). However, we did not explore tolerance of cool-season species such perennial ryegrass (Lolium perenne L.) to pyrimisulfan + penoxsulam in this research. When applied alone, penoxsulam can be used to suppress overseeded stands of perennial ryegrass from bermudagrass (Cynodon spp.) golf courses and sports fields (Anonymous, 2014). Given that cool-season turfgrass golf courses, sports fields, and lawns often contain mixtures of Festuca, Lolium, and Poa species, use of pyrimisulfan + penoxsulam may be better suited for warm-season turfgrass systems of the southern United States. However, additional research on cool-season turfgrass tolerance to pyrimisulfan + penoxsulam, as well as pyrimisulfan alone, is warranted. Furthermore, reports of sulfonylurea herbicides selecting for ALS resistance in both broadleaf weed and sedge species of warm-season turfgrass (McCullough et al., 2016a, 2016b) warrants further research on efficacy of pyrimisulfan + penoxsulam for managing these resistant biotypes.