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The mitotic-inhibiting herbicide pronamide controls susceptible annual bluegrass (Poa annua L.) pre- and postemergence, but in some resistant populations, postemergence activity is compromised, hypothetically due to a target-site mutation, lack of root uptake, or an unknown resistance mechanism. Three suspected pronamide-resistant (LH-R, SC-R, and SL-R) and two pronamide-susceptible (BS-S and HH-S) populations were collected from Mississippi golf courses. Dose–response experiments were conducted to confirm and quantify pronamide resistance, as well as resistance to flazasulfuron and simazine. Target sites known to confer resistance to mitotic-inhibiting herbicides were sequenced, as were target sites for herbicides inhibiting acetolactate synthase (ALS) and photosystem II (PSII). Pronamide absorption and translocation were investigated following foliar and soil applications. Dose–response experiments confirmed pronamide resistance of LH-R, SC-R, and SL-R populations, as well as instances of multiple resistance to ALS- and PSII-inhibiting herbicides. Sequencing of the α-tubulin gene confirmed the presence of a mutation that substituted isoleucine for threonine at position 239 (Thr-239-Ile) in LH-R, SC-R, SL-R, and BS-S populations. Foliar application experiments failed to identify differences in pronamide absorption and translocation between the five populations, regardless of harvest time. All populations had limited basipetal translocation—only 3% to 13% of the absorbed pronamide—across harvest times. Soil application experiments revealed that pronamide translocation was similar between SC-R, SL-R, and both susceptible populations across harvest times. The LH-R population translocated less soil-applied pronamide than susceptible populations at 24, 72, and 168 h after treatment, suggesting that reduced acropetal translocation may contribute to pronamide resistance. This study reports three new pronamide-resistant populations, two of which are resistant to two modes of action (MOAs), and one of which is resistant to three MOAs. Results suggest that both target site– and translocation-based mechanisms may be associated with pronamide resistance. Further research is needed to confirm the link between pronamide resistance and the Thr-239-Ile mutation of the α-tubulin gene.

St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is one of the most common turfgrass lawn species of warm‐season climates. Like many turfgrasses, it is produced as sod. Weed control during sod production is problematic, due in part to intolerance of St. Augustinegrass to a range of useful turfgrass herbicides, and because the open canopy and growth‐conducive conditions of sod production favor certain difficult‐to‐control weeds. Although those weeds are best preventatively controlled using preemergence herbicides, many preemergence herbicides also adversely affect vegetative propagation of turfgrasses. Field research was conducted at the Mississippi State University, near Starkville, Mississippi, USA, to evaluate the effects of dithiopyr, pendimethalin, prodiamine, liquid‐ and granular‐applied oxadiazon, atrazine, S‐metolachlor, atrazine + S‐metolachlor, and indaziflam on St. Augustinegrass establishment from plugs. Plots (2.25 m2) were planted with 10 plugs (225 cm2 apiece), and preemergence herbicides were applied at standard use rates 1 day after planting. Visual estimates of percentage cover were collected weekly, and aerial imagery was collected weekly to biweekly. Visual cover and vegetation index data were regressed to estimate days to reach 50% maximum nontreated response. Liquid‐applied oxadiazon and atrazine were the only treatments that did not increase days to reach 50% cover in both replication years. Dithiopyr, prodiamine, S‐metolachlor, and indaziflam increased days to reach 50% cover in both replication years. The normalized difference vegetation index, ratio vegetation index, and chlorophyll index‐red edge derived from an aerial sensor, detected similar, but fewer, treatment differences compared to estimated visual cover. Core ideas Atrazine and liquid‐applied oxadiazon can be applied to plugs without slowing establishment compared to nontreated. Dithiopyr, prodiamine, S‐metolachlor, and indaziflam slow establishment of St. Augustinegrass. Vegetation indexes from aerial imagery fail to detect minor treatment differences compared to visual cover.

Annual bluegrass is one of the most problematic weeds in the turfgrass industry, exhibiting both cross-resistance and multiple-herbicide resistance. Prodiamine, pronamide, and indaziflam are commonly used preemergence herbicides for the control of this species on golf courses in the southern United States. There have been increasing anecdotal reports of annual bluegrass populations escaping control with these herbicides, but resistance has yet to be confirmed. To evaluate the response of annual bluegrass to three herbicides, populations were collected from golf courses, athletic fields, and landscape areas in Texas and Florida, and a dose-response assay was conducted on populations that were suspected to be resistant to and known to be susceptible to prodiamine, pronamide, and indaziflam. The suspected-resistant populations showed survival to prodiamine at 32 times the recommended field rate (both populations from Florida and Texas) of 736 g ai ha–1, and to pronamide at 32 times (the Florida populations) or 16 times (the Texas populations) the recommended field rate of 1,156 g ha–1. In contrast, the known susceptible populations attained 100% mortality at rates as low as 46 and 578 g ha–1, respectively, from applications of prodiamine and pronamide. For indaziflam, the suspected-resistant populations showed reduced sensitivity up to the recommended field rate of 55 g ha–1, but they were controlled when treated with a rate twice that of the field rate. Overall, annual bluegrass populations with resistance to prodiamine and pronamide, and reduced sensitivity to indaziflam (at the recommended field rate) were confirmed from golf courses in Florida and Texas. In the presence of herbicide-resistant annual bluegrass populations, especially to commonly used herbicides such as prodiamine and pronamide, turfgrass managers should adopt integrated management strategies and frequently rotate herbicide sites of action, rather than relying solely on microtubule-assembly inhibitors or cellulose biosynthesis inhibitors, to control this species. Nomenclature: Indaziflam; prodiamine; pronamide; annual bluegrass, Poa annua L.

Dithiopyr and dinitroanilines are preemergence-applied, mitotic-inhibiting herbicides used to control goosegrass [Eleusine indica (L.) Gaertn.] in turfgrass. A suspected resistant E. indica population was collected from a golf course putting green and was evaluated for possible resistance to dithiopyr and prodiamine. After dose–response evaluation, the α-tubulin gene was sequenced for known target-site mutations that have been reported to confer resistance to mitotic-inhibiting herbicides. A mutation was discovered that resulted in an amino acid substitution at position 136 from leucine to phenylalanine (Leu-136-Phe). Previous research has indicated that Leu-136-Phe does confer resistance to dinitroaniline herbicides. The level of resistance indicated by regression models and I50 values indicates that there is 54.1-, 4.7-, >100-, and >100-fold resistance to dithiopyr, prodiamine, pendimethalin, and oryzalin, respectively, when compared with the susceptible population based on seedling emergence response and 88.4-, 7.8-, >100-, and >100-fold resistance to dithiopyr, prodiamine, pendimethalin, and oryzalin, respectively, when compared with the susceptible population based on biomass reduction response. This is the first report of less resistance to prodiamine compared with pendimethalin or oryzalin due to a target-site α-tubulin mutation and the first report of a target-site α-tubulin mutation associated with dithiopyr resistance.

Core Ideas Preemergence herbicides may increase grow‐in time for hybrid bermudagrass sprigs. Preemergence herbicide treatments did not reduce sod tensile strength. Preemergence herbicides are viable options for weed control during establishment. Preemergence herbicides often negatively affect establishment of sprigged hybrid bermudagrass (Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt‐Davy). However, limited research has quantified the effects of preemergence herbicides on establishment and tensile strength of warm‐season turfgrass species commonly grown as sod in the southeastern United States. Field research was conducted at Mississippi State University in 2016 and 2017 to evaluate the effects of atrazine, atrazine + S‐metolachlor, dithiopyr, flumioxazin, indaziflam, liquid‐ and granular‐applied oxadiazon, S‐metolachlor, pendimethalin, prodiamine, and simazine on grow‐in time and tensile strength of newly sprigged ‘Latitude 36’ hybrid bermudagrass. Plots (4.65 m2) were sprigged at a rate of 44 m3 ha−1 (468 US bushels ac−1), and preemergence herbicide treatments were applied at the recommended labeled rates 1 d after planting. Dithiopyr, flumioxazin, S‐metolachlor, and indaziflam increased days to reach 50% hybrid bermudagrass cover in both 2016 and 2017. Normalized difference vegetative index and relative chlorophyll index were similarly affected. No herbicide treatments reduced sod tensile strength in either year; however, prodiamine, pendimethalin, dithiopyr, flumioxazin, and S‐metolachlor unexpectedly increased sod tensile strength in 2017.

Pennsylvania bittercress ( Cardamine pensylvanica ) and other bittercress ( Cardamine ) species are among the most common and difficult-to-control weed species in container nurseries, and they have been vouched in most counties in Florida. Preemergence herbicides can provide control, but concerns over potential resistance development, environmental issues, and crop injury problems associated with herbicide use create the need for alternative weed control methods to be explored. Previous studies have shown the potential of mulch materials for controlling weeds in nurseries, but their use along with preemergence herbicides has not been extensively investigated. To compare the effects of different mulch materials and herbicides on Pennsylvania bittercress control, a full factorial designed greenhouse study was conducted. Three mulch treatments including no mulch, pine ( Pinus sp.) bark, and rice ( Oryza sativa ) hulls were evaluated with three herbicide treatments, including water (i.e., no herbicide), isoxaben, and prodiamine applied at label rates. Twenty-five seeds of Pennsylvania bittercress were sown on the surface of each container and emergence (percent), coverage (square centimeters), seedhead number, and biomass (grams) were measured. The results showed that Pennsylvania bittercress in containers mulched with rice hulls had the lowest emergence throughout the experiment. For coverage, seedhead, and biomass parameters, Pennsylvania bittercress seeded in rice hulls treatments had significantly lower coverage, fewer seedheads, and lower biomass compared with those in nonmulched or pine bark treatments, regardless of herbicide treatment. With isoxaben and the water check, nonmulched treatments had the highest coverage/seedhead/biomass, whereas with prodiamine, Pennsylvania bittercress in pine bark mulched containers had the highest coverage/seedhead/biomass. In conclusion, applying rice hulls alone can provide better Pennsylvania bittercress control compared with isoxaben or prodiamine applied alone.

Weeds are difficult to control in potted tropical ornamentals, especially when plants are kept for extended time periods at a nursery. Management is complicated by the lack of tolerance data for many tropical species. Experiments were conducted in 2015, 2016, and 2017 at the Gulf Coast Research and Education Center in Balm, FL, to evaluate tolerance of stromanthe, croton, philodendron, arbicola, cordyline, ixora, plumbago, allamanda, bird-of-paradise, firebush, and hibiscus to granular applications of indaziflam, flumioxazin, pendimethalin + oxyfluorfen, pendimethalin + dimethenamid-P, trifluralin + oxyfluorfen + isoxaben, and trifluralin + isoxaben, and liquid applications of prodiamine + isoxaben and dimethenamid-P. Indaziflam, pendimethalin + oxyfluorfen, and trifluralin + oxyfluorfen + isoxabin were safe for use on all evaluated ornamentals except stromanthe. Dimethenamid-P and pendimethalin + oxyfluorfen were safe on all evaluated ornamentals except allamanda. Flumioxazin damaged philodendron and bird-of-paradise but was safe on all other ornamentals tested. Trifluralin + isoxaben and prodiamine + isoxaben were safe on hibiscus, firebush, and bird-of-paradise, but prodiamine + isoxaben damaged allamanda. We have identified multiple PRE herbicides that can safely be used on multiple tropical ornamentals grown in containers. Nomenclature: Dimethenamid-P; flumioxazin; indaziflam; isoxaben; oxyfluorfen; pendimethalin; prodiamine; trifluralin; allamanda; Allamanda schottii Pohl.; arbicola; Schefflera arboricola (Hayata) Merr. ‘Trinette’; bird-of-paradise; Strelitzia reginae Aiton; cordyline; Cordyline fruticosa (L.) A. Chev. ‘Red Sister’; croton; Codiaeum variegatum (L.) A. Juss. ‘Mammy’; firebush; Hamelia patens Jacq.; hibiscus; Hibiscus rosa-sinensis L. ‘Painted Lady’; ixora; Ixora coccinea L. ‘Maui Red’; philodendron; Philodendron selloum K. Koch; plumbago; Plumbago auriculata Lam. ‘Dark Blue’; stromanthe; Stromanthe sanguinea Sond. ‘Triostar’.

Mulch is often applied in landscape planting beds for weed control, but little research has focused specifically on mulch and preemergence (PRE) herbicide combinations. The objectives of this research were to determine the efficacy of herbicide + mulch combinations and which factors significantly affected weed control, including herbicide formulation and posttreatment irrigation volumes. Additional objectives were to determine efficacy derived from mulch or herbicides used alone under herbicide + mulch combinations and to identify differences in the additive (herbicide + mulch combinations) or singular (herbicide or mulch) effects compared with the use of herbicides or mulch only. Large crabgrass ( Digitaria sanguinalis ), garden spurge ( Euphorbia hirta ), and eclipta ( Eclipta prostrata ) were used as bioassay species for prodiamine, dimethenamid-P + pendimethalin, and indaziflam efficacy, respectively. The experiment consisted of a factorial treatment arrangement of two herbicide formulations (granular or spray applied), three mulch types [hardwood chips (HWs), pine bark (PB), and pine straw (PS)], two mulch depths (1 and 2 inches), and three levels of one-time, posttreatment irrigation volumes (0.5, 1, and 2 inches). Three sets of controls were used: the first set included three mulch types applied at two depths receiving only 0.5-inch irrigation volume, the second set included only two herbicide formulations and three one-time irrigation volumes, whereas the last set received no treatment (no herbicide or mulch) and only 0.5-inch irrigation volume. High levels of large crabgrass and garden spurge control (88% to 100%) were observed with all herbicide + mulch combinations evaluated at mulch depths of 1 inch or greater. When comparing mulch types, the best eclipta control was achieved with hardwood at 2 inches depth. The spray formulation of indaziflam outperformed the granular formulation in most cases when used alone or in combination with mulch. Overall, the results showed that spray formulations of prodiamine and dimethenamid-P + pendimethalin were more effective than granular formulations when applied alone, whereas indaziflam was more effective as a spray formulation when used both alone and in combination with mulch. Increasing irrigation volume was not a significant factor for any of the herbicide + mulch combinations when evaluating overall weed control.

Goosegrass biotypes from golf courses in Richmond, VA (CCV) and New Bern, NC (RB) historically treated with oxadiazon were identified as resistant compared to susceptible standard (PBU) based on comparisons of oxadiazon applied preemergence at increasing rates (0.03 to 2.24 kg ha-1). Oxadiazon at rates ≤ 2.24 kg ha-1 rate did not prevent emergence of suspected resistant CCV and RB seedlings. PBU emergence was completely prevented at 0.14 kg ha-1. Based on percent seedling emergence relative to non-treated and percent above-ground biomass reduction relative to nontreated, the oxadiazon rate at which emergence would be reduced 50% (I50) or 90% (I90) ranged from 0.12 to 0.18 kg ha-1 or 10.83 to 85.57 kg ha-1, respectively for suspected resistant CCV and RB, compared to 0.03 to 0.4 kg ha-1 or 0.12 to 0.19 kg ha-1, respectively for susceptible standard PBU. Seedling emergence data predicted 7.9 and 3.0 times greater I90 values for CCV and RB, respectively compared to biomass data. All three biotypes were completely controlled by preemergence applied labeled rates of prodiamine and indaziflam. This is the first peer-reviewed report of evolved weed resistance to oxadiazon. Nomenclature: Oxadiazon; goosegrass, Eleusine indica (L.) Gaertn. ELEIN.

Praxelis is an annual to short-lived perennial herb in the Asteraceae family and an emerging problematic weed species in Florida nurseries. The objective of these experiments was to determine efficacy of selected PRE and POST herbicides for control of praxelis. An additional experiment was conducted to determine efficacy of the same PRE herbicides for control of bluemink, a weed similar in appearance to praxelis that is also common in Florida. The granular herbicides dimethenamid + pendimethalin, flumioxazin, oxyfluorfen + pendimethalin, oxyfluorfen + prodiamine, and trifluralin + isoxaben were evaluated, along with spray-applied formulations of dimethenamid, indaziflam, and prodiamine + isoxaben. Flumioxazin consistently provided over 90% control of praxelis at both sites. Indaziflam control was inconsistent between the two sites, providing 100% control in Apopka but only a 22% reduction in weed counts in Balm. Oxyfluorfen + pendimethalin, oxyfluorfen + prodiamine, and prodiamine + isoxaben also provided control (57% to 97% reduction in shoot weight), albeit generally to a lesser degree than was observed with flumioxazin. All PRE herbicides provided similar control of both praxelis and bluemink, with the exception of dimethenamid and dimethenamid + pendimethalin, which reduced bluemink shoot weight more than praxelis. Clopyralid, glyphosate, and triclopyr all provided effective POST control of mature and flowering praxelis through 12 wk after treatment (WAT) and resulted in greater praxelis control than glufosinate. Results suggest that many commonly used PRE and POST herbicides would provide control of praxelis, but use of PRE and POST herbicides as well as sequential applications may be needed for long-term management. Nomenclature: Clopyralid; dimethenamid; flumioxazin; glufosinate; glyphosate; indaziflam; isoxaben; oxyfluorfen; pendimethalin; prodiamine; triclopyr; trifluralin; bluemink, Ageratum houstonianum Mill.; praxelis, Praxelis clematidea (Kuntze) R.M. King and H. Rob