Mangroves are a critical component of many coastal ecosystems in Florida. Woody species, including Brazilian peppertree (Schinus terebinthifolia Raddi), have invaded thousands of hectares of mangrove habitat. The difficulty associated with ground-based management of invasive plants in mangrove communities has warranted a need to identify selective herbicides that can be applied aerially. Recent work suggests that Florida mangrove species are extremely sensitive to synthetic auxin herbicides; however, other herbicides have yet to be tested for selectivity. Greenhouse studies in 2018 and 2019 evaluated broadcast foliar applications of the acetolactate synthase inhibitor imazamox and protoporphyrinogen oxidase inhibitor carfentrazone-ethyl, both as individual treatments and in combinations, for control of S. terebinthifolia and injury to four non-target mangrove species. Across all posttreatment sample dates and species tested, there were no significant interactions between imazamox applied at 0.28 or 0.56 kg ai ha−1 in combination with carfentrazone-ethyl applied at 0 or 0.1 kg ha−1. Main effects of imazamox applied at 0.56 kg ai ha−1 and carfentrazone-ethyl applied at 0.1 kg ha−1 resulted in 99% and 97% defoliation, respectively, of Schinus terebinthifolia at 180 DAT. However, S. terebinthifolia percent survival was 56% and 44% for the same treatments. Both herbicides severely injured all four mangroves by 90 DAT and resulted in 58% to 100% defoliation across species. At 180 DAT, significant increases in percent cambium kill were also observed for all four species. Across species, mangrove survival varied, but red mangrove (Rhizophora mangle L.) survival was reduced to 6% when imazamox was applied at 0.56 kg ha−1. These results indicate both imazamox and carfentrazone-ethyl exhibit activity on S. terebinthifolia but also injure all four mangroves enough to preclude their use as selective treatments.

This report updates the incidence of herbicide-resistant (HR) weeds across western Canada from the last report covering 2007 to 2011. This third round of preharvest surveys was conducted in Saskatchewan in 2014 and 2015, Manitoba in 2016, and Alberta in 2017, totaling 798 randomly selected cropped fields across 28 million ha. In addition, we screened 1,108 weed seed samples submitted by prairie growers or industry between 2012 and 2016. Of 578 fields where wild oat seed was collected, 398 (69%) had an HR biotype: 62% acetyl-CoA carboxylase inhibitor (WSSA Group 1) resistant, 34% acetolactate synthase inhibitor (Group 2) resistant, and 27% Group 1+2 resistant (vs. 41%, 12%, and 8%, respectively, in the previous second-round surveys from 2007 to 2009). The sharp increase in Group 2 resistance is the result of reliance on this site of action to manage Group 1 resistance and the resultant increased selection pressure. There are no POST options to control Group 1+2–HR wild oat in wheat or barley. The rise of Group 2 resistance in green foxtail (11% of sampled fields) and yellow foxtail (17% of Manitoba fields), which was not detected in the previous survey round, parallels the results for wild oat resistance. Various Group 2–HR populations of broadleaf weeds were confirmed, with cleavers and field pennycress being most abundant. Results of submission-sample testing reflected survey results. Although not included in this study, a postharvest survey in Alberta in 2017 indicated widespread Groups 2, 4 (dicamba), and 9 (glyphosate) resistance in kochia and Group 2 resistance in Russian thistle. These surveys bring greater awareness of HR weeds to growers and land managers at local and regional levels, and highlight the urgency to preserve herbicide susceptibility in our key economic weed species.

Palmer amaranth is one of the most problematic weeds in cropping systems of North America, especially in midsouthern United States, because of its competitive ability and propensity to evolve resistance to several herbicide sites of action. Previously, we confirmed and characterized the first case of nontarget site resistance (NTSR) to fomesafen in a Palmer amaranth accession from Randolph County, AR (RCA). The primary basis of the present study was to evaluate the cross- and multiple-resistance profile of the RCA accession. The fomesafen dose-response assay in the presence of malathion revealed a lower level of RCA resistance when compared with fomesafen alone. The resistance index of the RCA accession, based on 50% biomass reduction, ranged from 63-fold (fomesafen alone) to 22-fold (malathion plus fomesafen), when compared with a 2007 susceptible, and 476-fold and 167-fold, respectively, relative to a 1986 susceptible check. The RCA accession was resistant to other protoporphyrinogen oxidase (PPO) inhibitors (i.e., flumioxazin, acifluorfen, saflufenacil) as well as the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor tembotrione and acetolactate synthase (ALS) inhibitor pyrithiobac sodium. Sequencing of the ALS gene revealed no point mutations, indicating that a target-site mechanism is not involved in conferring ALS-inhibitor resistance in the RCA accession. Of the three PPO-inhibiting herbicides tested in combination with the malathion, saflufenacil resulted in the greatest biomass reduction (80%; P < 0.05) and lowest survival rate (23%; P < 0.05) relative to nontreated plants. The application of cytochrome P450 or glutathione S-transferase inhibitors with fomesafen did not lead to any adverse effects on soybean, suggesting a possible role for these compounds for management of NTSR under field conditions. These results shed light on the relative unpredictability of NTSR in conferring herbicide cross- and multiple resistance in Palmer amaranth.

MKH 6562 is a new acetolactate synthase inhibitor herbicide that would provide an alternative to control wild oat. Three experiments were conducted at Scott, SK, Canada, from 1996 to 1999 to evaluate MKH 6562 rates (20 and 30 g ai/ha), carrier volumes (30, 50, and 100 L/ha), time of applications (two- to three-leaf and three- to four-leaf stages of wild oat), and broadleaf weed herbicide tank mixtures. Reduced wild oat control (lower visual rating of percent control and higher fresh weight) often occurred when MKH 6562 was applied at a rate of 20 vs. 30 g/ha, with carrier volumes of 30 vs. 50 and 100 L/ha, and at the three- to four-leaf vs. two- to three-leaf stages of wild oat. Wild oat control generally was lower for MKH 6562 tank mixed with dicamba + mecoprop + MCPA 1:1:4.4 and bromoxynil + MCPA 1:1 compared with MKH 6562 applied alone or tank mixed with other broadleaf herbicides. MKH 6562 tank mixed with MCPA, and to a lesser extent 2,4-D, resulted in decreased wild oat control when applied at the three- to four-leaf stage of wild oat, but not at the two- to three-leaf stage. Wheat grain yield usually was not affected by MKH 6562 rate and carrier volume. Yield was 7% lower when MKH 6562 was tank mixed with dicamba + mecoprop + MCPA or fluroxypyr + 2,4-D and was 8% lower when MKH 6562 was applied in a mixture with 2,4-D formulations or bromoxynil + MCPA at the three- to four-leaf stage compared with the two- to three-leaf stage. Similar yields were achieved when MKH 6562 was applied alone at both leaf stages of wild oat. Wild oat control and wheat yield with MKH 6562 were as good as or better than with ICIA 0604 and imazamethabenz and were as good as or poorer than with CGA 184927. MKH 6562 provides adequate control of wild oat in wheat when applied early with the recommended carrier volume. Satisfactory control may be achieved with reduced rates if wild oat infestations are light.

Sulfonylurea (SU)-resistant monochoria has recently been found in rice paddies in Korea. A quick and accurate means of confirming herbicide resistance is necessary to take timely management decisions. This article describes a rapid and reliable assay to detect SU-resistant biotype of monochoria. The techniques tested include seed germination, in vivo and in vitro acetolactate synthase (ALS; EC 4.1.3.18) activity, leaf, and whole-plant bioassays. In the whole-plant bioassay, shoot dry weight of the resistant (R) biotype was 3,200-fold less affected by imazosulfuron and sevenfold less affected by pyrazosulfuron-ethyl than the susceptible (S) biotype. Although the whole-plant bioassay is reliable, it is expensive, requires a lot of infrastructure, and takes a few months to complete. The germination rate of the R biotype in petri dish bioassays was > 200-fold less inhibited by imazosulfuron and 100-fold less inhibited by pyrazosulfuron-ethyl than that of the S biotype. Seed germination bioassays in petri dishes do not require as much infrastructure as whole-plant bioassays do and can be completed in a shorter time. Leaf bioassays showed that leaf color of the R biotype was > 1,600- and 300-fold less affected by imazosulfuron and pyrazosulfuron-ethyl, respectively, compared with that of the S biotype. This assay takes about 6 d to complete. In vivo ALS assays showed lower levels of resistance to ALS herbicides than did in vitro ALS assays, where the R biotype was about 200- and 30-fold less sensitive to imazosulfuron and pyrazosulfuron-ethyl, respectively, than the S biotype. All assays successfully distinguished the R from the S biotype, but in vitro ALS assays are the simplest and the quickest. The in vitro ALS assay was chosen as the standard procedure for future confirmation of resistance in monochoria populations. Caution is needed because the in vitro assay is not appropriate in cases wherein the resistance mechanism is increased metabolism of the herbicide or overexpression of the target enzyme. Results should be interpreted in relation to field history and field observations. Follow-up studies also are needed to verify that other resistance mechanisms do not confound the in vitro assay.

Red rice is a major weed problem in rice production of the southern United States and other rice-producing countries. One hundred thirty red rice accessions from 26 rice-growing counties in Arkansas were tested for tolerance to imazethapyr in seed- and whole-plant response bioassays. The red rice accessions were compared with imazethapyr-resistant (ClearfieldTM) rice cultivars (‘CL121’, ‘CL161’, and ‘CL-XL8’) and conventional rice cultivars (‘Bengal’, ‘Dongjin’, ‘Drew’, and ‘Wells’). Red rice accessions 79, 84, and 118 showed 17-, 48-, and 37-fold more tolerance to imazethapyr, respectively, than the standard susceptible red rice accession (82) in whole-plant bioassays. The imazethapyr-resistant rice cultivars, CL121, CL161, and CL-XL8 were 41-, >177-, and 48-fold more resistant to imazethapyr, respectively than the susceptible standard. The imazethapyr-tolerant red rice and ClearfieldTM cultivars were generally cross tolerant to other acetolactate synthase (ALS; EC 4.1.3.18) inhibiting herbicides such as imazapyr, imazaquin, imazamox, and pyrithiobac. The tolerance level of red rice or rice to imidazolinone herbicides was highest with imazaquin and lowest with imazapyr. The imazethapyr-tolerant red rice accessions and ClearfieldTM rice were susceptible to glufosinate and glyphosate. The ALS enzyme of tolerant red rice accessions was less sensitive to imazethapyr than the susceptible standard, but tolerance at the enzyme level was less than at the whole-plant level. Therefore, tolerance of red rice to imazethapyr may involve other mechanisms besides an insensitive target site. We learned that a few imazethapyr-tolerant red rice populations existed probably before ClearfieldTM rice was introduced, supporting the hypothesis that evolution of herbicide-resistant red rice populations can happen with intensive herbicide selection pressure.

Glyphosate-resistant common waterhemp is a difficult-to-control annual broadleaf weed that has become a serious management challenge for growers in Nebraska and other states in the United States. The objectives of this study were to confirm glyphosate-resistant common waterhemp in Nebraska by quantifying level of resistance in a dose-response study, and to determine the sensitivity and efficacy of POST soybean herbicides for controlling suspected glyphosate-resistant common waterhemp biotypes. Seeds of suspected glyphosate-resistant common waterhemp biotypes were collected from seven eastern Nebraska counties. Greenhouse dose-response experiments were conducted to evaluate the response of common waterhemp biotypes to nine rates of glyphosate (0 to 16×). Common waterhemp biotypes were 3- to 39-fold resistant to glyphosate depending on the biotype being investigated and the susceptible biotype used for comparison. Results of the POST soybean herbicides efficacy experiment suggested that glyphosate-resistant biotypes, except a biotype from Pawnee County, had reduced sensitivity to acetolactate synthase (ALS)–inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Glufosinate and protoporphyrinogen oxidase (PPO)–inhibiting herbicides (acifluorfen, fluthiacet-methyl, fomesafen, and lactofen) provided ≥ 80% control of glyphosate-resistant common waterhemp at 21 d after treatment (DAT). This study confirmed the first occurrence of glyphosate-resistant common waterhemp in Nebraska, and also revealed reduced sensitivity to ALS-inhibiting herbicides in most of the biotypes tested in this study.