Field studies were conducted in 2013 at Houlka, MS and in 2014 at Houston, MS to evaluate herbicide programs for yellow nutsedge control and sweetpotato crop response. Treatments consisted of halosulfuron-methyl at 13, 26, and 34 g ai ha−1 2 wk after transplanting (WAP) followed by (fb) S-metolachlor at 856 g ai ha−1 4 WAP; a sequential application of 13 g ha−1 halosulfuron-methyl at 2 and 4 WAP; and halosulfuron-methyl at 13 g ha−1 plus either S-metolachlor or 1,123 g ai ha−1 alachlor 2 WAP. Crop injury in treated plots ranged from 29 to 44% at 3 WAP. Injury from the sequential application increased slightly between 3 and 5 WAP (29 to 38%), but decreased between 5 WAP and harvest (19%). Injury with all other treatments was unchanged from 5 WAP through harvest. The sequential application of halosulfuron-methyl provided the greatest control of yellow nutsedge throughout the duration of the study with ≥ 83% control from 5 WAP through harvest. Control from all other treatments ranged from 38 to 78% from 5 WAP through harvest. No. 1, canner, and marketable sweetpotato yields of the hand-weeded check were 19,900; 7,140; and 27,590 kg ha−1, respectively. No. 1, canner, and marketable yields of the weedy check were only 15, 51, and 25% of the hand-weeded check, respectively. With the exception of halosulfuron-methyl at 13 g ha−1 fb S-metolachlor, which was similar to the weedy check, no. 1 and marketable yields with all treatments were greater than the weedy check, but less than the weed-free check. A sequential application of halosulfuron-methyl and a tank mix of halosulfuron-methyl plus S-metolachlor would fit well into an overall yellow nutsedge management program in sweetpotato. However, as halosulfuron-methyl is not registered for use in sweetpotato, growers must continue to manage yellow nutsedge primarily with crop rotation and sanitation.

Purple and yellow nutsedge are problematic weeds in Florida small fruit and vegetable production. EPTC and fomesafen are PRE herbicides that suppress both nutsedge species, but field application in Florida has shown control to be erratic. Greenhouse experiments were conducted in Gainesville, FL, from May to August 2014 and in Wimauma, FL, from March to May 2015 to investigate susceptibility of purple and yellow nutsedge to EPTC and fomesafen applications. Treatments included EPTC at 2.91 kg ai ha−1 and fomesafen at 0.42 kg ai ha−1 at 0, 3, 6, 9, 12, and 15 d after planting (DAP) tubers, plus a nontreated control. EPTC and fomesafen applications averaged across timings decreased purple and yellow nutsedge emergence, shoot height, leaf number, and shoot mass compared to the nontreated control. Herbicide applications 0 DAP reduced purple nutsedge emergence greater than 65% compared to the nontreated control and caused at least 74% injury 4 wk after planting. Herbicide applications 0 DAP decreased yellow nutsedge emergence and shoot mass compared to the nontreated control by at least 86 and 93%, respectively. Applications of EPTC and fomesafen have the ability to suppress short-term purple and yellow nutsedge growth. Applications made at or prior to tuber sprouting maximize herbicide efficacy.

Clover inclusion may increase the sustainability of certain low-maintenance turfgrasses. However, selective weed control within mixed turfgrass–clover swards proves problematic because of clover susceptibility to herbicides. Research was conducted to identify common turf herbicides that are tolerated by three Trifolium species, including white clover, ball clover, and small hop clover, within low-maintenance turfgrass. Leaf and flower density, as well as plant height, were measured 4 wk after treatment as indicators of clover response to 14 herbicides. The three Trifolium spp. were moderately tolerant of bentazon (< 35% decrease in leaf density, height, or flowering). Simazine was well tolerated by white clover (< 5% decrease in all response variables), yet moderate injury to ball clover and small hop clover was observed (> 32% decrease in leaf density and > 27% decrease in flower density). Pronamide was well tolerated by white and ball clovers, with no effect on measured response variables; however, pronamide decreased small hop clover height and flower density (38 and 42%, respectively). Imazethapyr and imazamox were moderately well tolerated by white clover and small hop clover (< 39% decrease by all response variables), yet ball clover may be more susceptible to these herbicides than was anticipated based on previously reported tolerance. The herbicides 2,4-DB, halosulfuron, and metribuzin were well tolerated by white clover, with no effect on measured response variables; however, results suggest ball and small hop clovers were less tolerant. Clopyralid, 2,4-D, glyphosate, imazaquin, metsulfuron-methyl, and nicosulfuron resulted in varying degrees of injury across clover species and response variables, but, in general, these herbicides may not be viable options when attempting to maintain any of the three clover species tested. Further research is needed to quantify long-term effects of herbicide application on sward composition and clover succession.

Methiozolin is a new isoxazoline herbicide that has scarcely been tested in Kentucky bluegrass turf. A field trial was conducted in Blacksburg, VA, to determine response of 110 Kentucky bluegrass varieties and winter annual weeds to sequential fall applications of methiozolin. At 1.5 and 6 mo after initial treatment (MAIT), Kentucky bluegrass injury I30 values (predicted methiozolin rate that causes 30% Kentucky bluegrass injury) ranged between 3.4 to more than 10 times the recommended methiozolin rate for annual bluegrass control. Methiozolin at all rates reduced cover of annual bluegrass, common chickweed, corn speedwell, hairy bittercress, mouseear chickweed, and Persian speedwell but increased cover of parsley-piert. For all varieties, methiozolin at 2 kg ai ha−1 increased Kentucky bluegrass cover, turf quality, and turf normalized difference vegetation index (NDVI) relative to the nontreated check at 6 MAIT. Kentucky bluegrass relative cover change (RCC) was attributed primarily to weed control but was inversely correlated with methiozolin rates because of increased weed control and reduced Kentucky bluegrass growth. Despite the decline in RCC with increasing methiozolin rates, most Kentucky bluegrass varieties treated with the highest methiozolin rate (6 kg ha−1) still had greater Kentucky bluegrass cover than the nontreated check at 6 MAIT. Results from this study indicate that two fall applications of methiozolin at rates beyond that previously reported for annual bluegrass control can safely be applied to a broad range of Kentucky bluegrass varieties spanning most of the known genetic classifications.

Flame cultivation (FC) uses brief exposures of high temperature to control weeds. Three sites in southeastern Massachusetts with dewberry present were studied over a 2-yr period to determine if seasonal timing and frequency of exposure to FC would reduce dewberry stem length and biomass, both in the year of and the year following treatment, and also to evaluate whether FC treatments altered the ratio (sugar : starch) of nonstructural carbohydrates (NSC) in dewberry roots. Dewberry plants were treated with a 9-s exposure to an open-flame hand-held torch at seven different timing regimes (one application in June, July, or August or two applications in June/July, June/August, and July/August, or nontreated). After 1 yr, all treatments showed reduced aboveground dewberry biomass compared to nontreated plots. The timing and frequency of FC treatments were not significant when the weed was growing amongst cranberry vines, but were significant when weeds were treated in the absence of cranberry. Exposure to FC did not affect the ratio of NSC in roots in the year after treatment, indicating that the relative amounts allocated to each type of storage carbohydrate did not differ from nontreated plants, even though the overall amount allocated to root biomass was reduced.

Crop losses from weed interference have a significant effect on net returns for producers. Herein, potential corn yield loss because of weed interference across the primary corn-producing regions of the United States and Canada are documented. Yield-loss estimates were determined from comparative, quantitative observations of corn yields between nontreated and treatments providing greater than 95% weed control in studies conducted from 2007 to 2013. Researchers from each state and province provided data from replicated, small-plot studies from at least 3 and up to 10 individual comparisons per year, which were then averaged within a year, and then averaged over the seven years. The resulting percent yield-loss values were used to determine potential total corn yield loss in t ha−1 and bu acre−1 based on average corn yield for each state or province, as well as corn commodity price for each year as summarized by USDA-NASS (2014) and Statistics Canada (2015). Averaged across the seven years, weed interference in corn in the United States and Canada caused an average of 50% yield loss, which equates to a loss of 148 million tonnes of corn valued at over U.S.$26.7 billion annually.

Indaziflam is a PRE herbicide for annual broadleaf and grass control in turfgrass systems and requires a 40-wk minimum interval between application and overseeding perennial ryegrass. Currently, activated-charcoal application is recommended to reduce that interval; however, preliminary evaluations determined activated charcoal alone was not a robust mitigation practice for successful establishment during perennial ryegrass overseeding. Field research was conducted in North Carolina and Tennessee to evaluate various mitigation practices to effectively overseed perennial ryegrass into indaziflam-treated turfgrass areas. Immediately following indaziflam application (53 g ai ha−1), two scenarios were created by delivering 0 or 0.3 cm H2O before mitigation practice. Irrigated plots were air-dried before conducting mitigation practices. Evaluated mitigation practices included scalping (0.6 cm cut height; debris removed), verticutting (1.25 cm depth; debris removed), and activated-charcoal application (167 kg ha−1 applied as an aqueous slurry in 3,180 L ha−1), evaluated individually and in each two-way combination in the order scalp followed by (fb) activated charcoal, scalp fb verticut, or verticut fb activated charcoal. Twenty-four hours after mitigation practice completion, perennial ryegrass was seeded (976 kg ha−1) and maintained as a golf course fairway. Overall, perennial ryegrass cover was reduced ≥ 93% at 8 and 20 wk after treatment (WAT) when no mitigation practices were performed. Stand-alone mitigation practices variably improved perennial ryegrass establishment; however, no practice provided acceptable results for end users. Combining mitigation practices improved overseeding establishment, most notably by adding activated charcoal application or verticutting to scalping before irrigation. Across experimental runs and locations, scalp fb activated-charcoal application before irrigation reduced perennial ryegrass cover 22 to 27% at 20 WAT. Results from this research suggest mitigation practices in addition to the currently recommended activated-charcoal application should be performed by turfgrass managers to improve perennial ryegrass overseeding establishment in indaziflam-treated turfgrass areas.

Drip irrigation is installed under polyethylene mulch to supply irrigation and nutrients to vegetables grown in plasticulture. This irrigation system also provides an alternative method for application of herbicides into the plant bed for control of yellow and purple nutsedge. Greenhouse and field studies were conducted to determine bell pepper tolerance to halosulfuron, imazosulfuron, and trifloxysulfuron applied POST (over the top of pepper in greenhouse study, POST-directed in the field study) or soil applied (applied by hand with water in greenhouse study or through drip irrigation in the field study). In greenhouse studies, pepper injury from halosulfuron, imazosulfuron, and trifloxysulfuron applied POST was similar at 14 and 21 d after treatment (DAT; 21 to 35% and 54 to 60%, respectively). Halosulfuron, imazosulfuron, and trifloxysulfuron soil applied in greenhouse studies caused 6 to 8% and 13 to 20% injury to pepper at 14 and 21 DAT, respectively. Pepper injury in greenhouse studies increased as rate of halosulfuron, imazosulfuron, and trifloxysulfuron increased regardless of application method (soil or POST applied). Dry pepper weight at 28 DAT followed an inverse linear response to increasing rates of halosulfuron, imazosulfuron, and trifloxysulfuron. In field studies, bell pepper height among herbicide treatments ranged from 32 to 37 cm at 14 DAT and was not different from the nontreated check (36 cm). Number one grade (7.8 to 14.7 MT ha−1) and fancy grade (2.1 to 2.8 MT ha−1) pepper fruit yield was not different in herbicide-treated pepper compared with yield of pepper in the nontreated check (10.0 to 26.6 MT ha−1, respectively). Based on these studies, pepper has excellent crop tolerance to halosulfuron, imazosulfuron, and trifloxysulfuron applied through drip irrigation or POST-directed but is not tolerant to POST applications.

Macartney rose is an aggressive, thorny shrub that suppresses forage production and strongly hinders cattle grazing in southern pastures. Previous studies have found this weed to be extremely difficult to control with most pasture herbicides. We conducted two studies in central Alabama to assess several new herbicide chemistries applied at spring, early fall, and late fall timings. In the first study, we compared aminocyclopyrachlor, tank mixed with either 2,4-D, triclopyr, metsulfuron, or chlorsulfuron, with the commercial standard, picloram + 2,4-D. In the second study, we compared aminopyralid, tank mixed with either 2,4-D or metsulfuron and triclopyr + fluroxypyr, to the commercial standard, picloram + 2,4-D. Aminocyclopyrachlor + chlorsulfuron or metsulfuron and aminopyralid + 2,4-D or metsulfuron provided acceptable control and were comparable to picloram + 2,4-D at the early fall timing. Macartney rose control with treatments applied at the late fall timing tended to be less than commercially acceptable levels. We found that no herbicide treatment controlled Macartney rose at the spring timing. In addition, triclopyr + fluroxypyr did not control Macartney rose at any application timing. These results indicate that the early fall timing was optimal and that both aminocyclopyrachlor and aminopyralid can provide good Macartney rose control when mixed with certain other herbicides, including metsulfuron.

The tripling of glyphosate use in the Canadian prairies during the past decade has raised concerns over the possible accumulation of glyphosate and its main metabolite AMPA in soil over time and whether there could be any detrimental effects on crop production. A controlled environment study was conducted at two locations in Alberta, Canada, to determine glyphosate and AMPA soil concentrations that would injure wheat, field pea, and canola. Treatments included glyphosate acid or AMPA applied at 0, 10, 25, 100, 250, and 500 mg kg−1 soil. Shoot and root biomass determinations 8 wk after emergence (WAE) indicated that shoot and root biomass of all crops progressively declined with increasing soil concentrations of glyphosate at both locations. In contrast, AMPA reduced crop shoot and root biomass at only one of two sites. Estimated soil concentrations of glyphosate causing 20% reductions in shoot and root biomass ranged from 80 to 190, 90 to 350, and 120 to 320 mg kg−1 for field pea, canola, and wheat, respectively. Soil concentrations of AMPA causing 20% crop biomass reductions ranged from 40 to 70, 20 to 30, and 80 to 120 mg kg−1 for field pea, canola, and wheat, respectively. Although substantial crop injury occurred in this study, it must be noted that these rates are very high in terms of field application rates that would be required to achieve these soil concentrations. Doses causing crop injury would convert to estimated glyphosate field rates ranging from 17.6 to 77 kg ha−1. Overall results indicate that even with frequent high-dose glyphosate applications over several years, the likelihood of wheat, field pea, and canola injury from soil residues is low. Nevertheless, there may be merit in greater monitoring of glyphosate and AMPA soil residues in cropping systems with high glyphosate utilization over a long time period.

In the Pacific Northwest (PNW), feral rye is a predominant winter annual grass weed in the low-rainfall region where a winter wheat–summer fallow rotation has been practiced for more than 130 yr. Recently, producers began including winter canola in their rotation, which provides additional herbicide options to control feral rye. A 3-yr study was conducted in Washington to determine the efficacy of clethodim, quizalofop, and glyphosate on feral rye control and winter canola yield. During the first year of the study herbicides were applied in the spring, and they increased canola yield and decreased feral rye biomass, density, and seed production similarly when compared with the nontreated control. During the last 2 yr of the study, split applications of quizalofop and glyphosate were the most effective treatments for controlling rye (> 95%) and increasing canola yield. In general, clethodim was less effective than both quizalofop and glyphosate in controlling feral rye. Results from this study indicate that quizalofop in conventional or glyphosate-resistant winter canola and glyphosate in glyphosate-resistant winter canola can effectively control feral rye.

Broadleaf weeds and nutsedge are persistent problems with limited management options for strawberry growers in Florida. Experiments were conducted in 2012 to 2013 (year 1) and 2013 to 2014 (year 2) at the Gulf Coast Research and Education Center in Wimauma, FL, to evaluate tolerance of two strawberry cultivars (‘Strawberry Festival' and ‘WinterStar'™) to PRE herbicides applied on the bed top or injected through the drip tape. The bed-top herbicides were applied to formed beds following fumigation but prior to laying the plastic mulch. Herbicides evaluated included two rates of EPTC, fomesafen, halosulfuron, and S-metolachlor as well as tank mixes of EPTC + S-metolachlor, fomesafen + S-metolachlor, and napropamide + oxyfluorfen. Drip-applied herbicides were applied through a single drip tape and strawberries were transplanted 1, 7, 15 and 30 d following application. Drip-applied herbicides included two rates of EPTC and fomesafen as well as two rates of a tank mix of napropamide + oxyfluorfen. None of herbicides applied on the bed top except S-metolachlor 214 g ai ha−1 consistently injured either strawberry cultivar or impacted yields. None of the drip-applied herbicides damaged the strawberry plants or reduced yields at all planting dates. In year 1, EPTC at 229 g ai ha−1 and EPTC at 229 g ha−1 + S-metolachlor at 107 g ha−1 resulted in a 70 to 86% reduction in purple nutsedge density compared to the nontreated control. No differences in nutsedge density among treatments were observed in year 2. The herbicides evaluated are safe for use on strawberry and would provide growers with alternative herbicide options, application timings, and application techniques. Further research is needed to evaluate efficacy.

Studies were conducted in 2007 and 2008 at Clinton, NC to determine the effect of glyphosate applied POST via a Dixie wick applicator on Palmer amaranth control and sweetpotato yield and quality. In 2007, treatments consisted of glyphosate wicked sequentially 6 and 8 wk after transplanting (WAP) and glyphosate wicked sequentially 6 and 8 WAP followed by (fb) rotary mowing 9 WAP. In 2008, treatments consisted of glyphosate wicked once 4 or 7 WAP, wicked sequentially 4 and 7 WAP, mowed once 4 WAP, and mowed 4 WAP fb wicking 7 WAP. In 2008, Palmer amaranth control 6 WAP varied by location and averaged 10 and 58% for plots wicked 4 WAP. Palmer amaranth contacted by the wicking apparatus were controlled, but weeds shorter than the wicking height escaped treatment. Palmer amaranth control 9 WAP was greater than 90% for all treatments wicked 7 WAP. Competition prior to and between glyphosate treatments contributed to large sweetpotato yield losses. Treatments consisting of glyphosate 7 or 8 WAP (in 2007 and 2008, respectively) frequently had greater no. 1 and marketable yields compared to the weedy control. However, jumbo, no. 1, and marketable yields for all glyphosate and mowing treatments were generally less than half the hand-weeded check. Cracked sweetpotato roots were observed in glyphosate treatments and percent cracking (by weight) in those plots ranged from 1 to 12% for no. 1 roots, and 1 to 6% for marketable roots. Findings from this research suggest wicking might be useful in a salvage scenario, but only after currently registered preemergence herbicides and between-row cultivation have failed to control Palmer amaranth and other weed species below the sweetpotato canopy.

Most tall fescue in the United States is infected with a fungal endophyte which imparts certain advantages to the plant, such as drought tolerance, insect feeding deterrence, and enhanced mineral uptake. However, the endophyte also produces ergot alkaloids that are harmful to livestock and contribute to fescue toxicosis. Because the alkaloids are concentrated in seed and stems, a potential way to reduce the likelihood of fescue toxicosis is by suppressing seedhead formation with herbicides. Research was conducted from 2012 to 2014 using metsulfuron applied alone and in combination with other herbicides in spring to determine the growth response of tall fescue, effects on forage quality, and ergot alkaloid concentration. Clipping or metsulfuron applied alone or in combination with aminocyclopyrachlor or aminopyralid reduced seedhead density by 36 to 55% compared to the nontreated control. Treatments containing metsulfuron reduced spring harvest yield 35 to 61%, but no differences were observed in the summer or year-after harvests. The same treatments increased crude protein levels by 1.03 to 2.14% and reduced acid detergent fiber levels by 1.60 to 2.76% compared to the nontreated control at spring harvest. Treatments containing metsulfuron reduced ergot alkaloid concentration 26 to 34% at the spring harvest, but no differences were observed in summer-harvested forage. Results from this study indicate metsulfuron applied alone or in combination with aminocyclopyrachlor or aminopyralid can potentially reduce the severity of fescue toxicosis and improve forage quality.

The tolerance of bermudagrass and stargrass to the relatively new herbicide, aminocyclopyrachlor (ACP), must be known before it can be recommended for weed control in these forage systems. Field experiments were conducted in 2012 and 2013 in south-central Florida to determine the tolerance of established bermudagrass and stargrass to various rates and combinations of ACP, chlorsulfuron, 2,4-D, triclopyr, and metsulfuron. Overall, bermudagrass and stargrass injury was transient and was minimal by 60 d after treatment (DAT). Similarly, biomass production was negatively affected at 30 DAT when treated with ACP at rates of 70 g ae ha−1 or greater, but was similar to the nontreated control by 60 DAT. Tank-mixing ACP with chlorsulfuron, 2,4-D amine, triclopyr, or metsulfuron did not increase injury compared with ACP alone applied at equivalent rates. Forage nutritive values were unaffected by herbicides. These data suggest that long-term effects of ACP on bermudagrass and stargrass are negligible, and this herbicide could be an important component of weed management programs in these forage systems.

Volunteer horseradish plants that emerged from root segments remaining after harvest can reduce yields of rotational crops as well as provide a host for pathogens and insects, thus reducing the benefits of crop rotation. POST applications of halosulfuron in corn can be an effective component to improve management of volunteer horseradish, but the replant interval from application to safe planting of commercial horseradish has not been determined. Fall herbicide applications are another possible volunteer horseradish management strategy than can be implemented once crops are harvested. Therefore, field experiments were conducted to evaluate the safe replant interval of horseradish following halosulfuron applications and to determine the efficacy of fall herbicide applications for volunteer horseradish control. Visual estimates of horseradish injury were greatest (85%) in plantings made zero months after halosulfuron applied at two times the approved rate; moreover, for all rates, injury decreased as the time after halosulfuron application increased. No herbicide injury or root biomass reduction occurred on horseradish at any halosulfuron rate from replanting beyond 4 mo after halosulfuron application. Control of volunteer horseradish was 91% or greater for all fall herbicide applications that included 2,4-D. Furthermore, volunteer horseradish shoot density was the lowest following combinations of 2,4-D tank-mixed with halosulfuron or rimsulfuron : thifensulfuron (0.2 and 0.4 shoots m−2, respectively) compared with the nontreated control (5.1 shoots m−2). This research demonstrates the effectiveness of both halosulfuron and 2,4-D as components of an integrated management strategy for volunteer horseradish control and the potential for halosulfuron applications without soil persistence beyond 4 mo affecting subsequent commercial horseradish production.

Studies were conducted in 2012 and 2013 to determine the effect of fomesafen based Palmer amaranth control program in ‘Covington' and ‘Evangeline' sweetpotato cultivars. Treatments consisted of fomesafen pretransplant alone at 0.20, 0.28, 0.36, 0.42, 0.56, and 0.84 kg ai ha−1 or followed by (fb) S-metolachlor at 1.12 kg ai ha−1 0 to 7 d after transplanting (DAP), fomesafen at 0.28 kg ha−1 fb S-metolachlor at 1.12 kg ha−114 DAP, flumioxazin pretransplant at 0.105 kg ai ha−1, S-metolachlor at 1.12 kg ha−1 0 to 7 DAP, clomazone at 0.63 kg ha−1 0 to 7 DAP, napropamide at 2.24 kg ha−1 0 to7 DAP, flumioxazin fb S-metolachlor 0 to 7 DAP, and flumioxazin fb clomazone fb S-metolachlor 14 DAP. Fomesafen pretransplant at 0.28 to 0.84 kg ha−1 alone or followed by S-metolachlor at 1.12 kg ha−1 0 to 7 DAP provided 80 to 100% Palmer amaranth control without reduction of yield and significant (< 13%) injury in Covington and Evangeline sweetpotato. Flumioxazin alone or fb S-metolachlor and flumioxazin fb clomazone fb S-metolachlor provided Palmer amaranth control (≥ 95%) with little injury (≤ 5%) and similar yield to the weed-free check. Clomazone alone did not cause injury, but controlled only 24 to 32% of Palmer amaranth at 50 DAP, which resulted in reduced no. 1, marketable, and total sweetpotato yield. Napropamide provided inconsistent control of Palmer amaranth in both years; therefore jumbo and total sweetpotato yield was reduced as compared to the weed-free check in 2012. Palmer amaranth control, sweetpotato cultivar tolerance, and yield in treatments with fomesafen fb S-metolachlor were similar to flumioxazin fb S-metolachlor. In conclusion, a herbicide program consisting of pretransplant fomesafen (0.28 to 0.42 kg ha−1) fb S-metolachlor (1.12 kg ha−1) is a potential option to control Palmer amaranth without causing significant injury and yield reduction in sweetpotato.

Annual bluegrass is a weed species in turfgrass environments known for exhibiting resistance to multiple herbicide modes of action, including photosystem II (PSII) inhibitors. To evaluate populations of annual bluegrass for susceptibility to PSII inhibitors of varied chemistries, populations were treated with herbicides from triazolinone, triazine, and substituted urea families: amicarbazone, atrazine, and diuron, respectively. Sequencing of the psbA gene confirmed the presence of a Ser264 to Gly amino acid substitution within populations that exhibited resistance to both atrazine and amicarbazone. A single biotype, DR3, which lacked any previously reported psbA gene point mutation, exhibited resistance to diuron, atrazine, and amicarbazone. DR3 had a significantly lower rate of absorption and translocation of atrazine and had enhanced atrazine metabolism when compared with both the Ser264 to Gly resistant mutant and susceptible biotypes. We thus report possible nontarget mechanisms of resistance to PSII-inhibiting herbicides in annual bluegrass.

Aminocyclopyrachlor (AMCP) is a pyrimidine carboxylic acid herbicide that is being evaluated for weed control on highway right-of-ways. The goal of this study was to evaluate weed control capabilities and tolerance of desirable turf to AMCP. The objective of the weed efficacy trial was to determine if AMCP (66, 132, and 263 g ai ha−1) was as effective as aminopyralid (18, 53, and 88 g ai ha−1) for controlling two weed species, mat lippia and Bidens alba. In 2011, AMCP applied at 132 g ai ha−1 resulted in 93% control of mat lippia at 2 mo after treatment (MAT) and 65% control at 3 MAT. In 2012, AMCP at 66 or 132 g ha−1 resulted in 93 to 97% control of mat lippia, respectively, at 3 MAT. Aminopyralid, applied POST, never exceed 10% control of mat lippia in 2011 and 60% control in 2012. AMCP, applied POST, at 66 g ha−1, controlled of B. alba 84%, similar to the 89% control seen with aminopyralid at 88 g ha−1. The level of control from AMCP of the two weed species was equal to or greater than aminopyralid. An additional objective was to determine the tolerance of common bermudagrass and ‘Pensacola' bahiagrass to AMCP (66, 132, and 263 g ai ha−1) and imazapic (35 and 70 g ai ha−1) applied alone and in combination. Applications were made in late spring during seedhead initiation. No additional chlorosis was detected when imazapic was applied in combination with AMCP for either turf species. Averaged across imazapic rates, AMCP at 66 and 132 g ha−1, within the suggested rate range (48.5–132 g ai ha−1), did not cause greater than 25% chlorosis at any time. Chlorosis observed for AMCP applied alone and in combination with imazapic was acceptable and decreased to zero by 8 to 10 wk after treatment.

Weed management in potato and mint rely heavily on two Group 5 photosystem II–inhibiting herbicides, metribuzin and terbacil, respectively. Seed from weed escapes was collected in 2010 from 69 potato and mint production fields in Washington State and tested for resistance to metribuzin and terbacil. Of the seed screened, 26 of 51 Powell amaranth and redroot pigweed accessions and 8 of 23 common lambsquarters accessions tested expressed at least some degree of resistance to one or both herbicides. A Ser264Gly mutation in the chloroplast psbA gene was present in all but three resistant accessions. Herbicides with other mechanisms of action registered in potato and mint controlled all identified photosystem II–resistant accessions when applied at typical labeled rates.