Herbicide Resistance of Centaurea cyanus L. in Poland in the Context of Its Management

Cornflower (Centaurea cyanus L.), one of the main weeds found among winter crops in Poland, has developed herbicide resistance (HR) to acetolactate synthase (ALS) herbicides, a finding first reported in 2006, and in recent years, farmers have been complaining about inadequate chemical control of this weed. This study aimed to characterise the current state of cornflower HR to ALS inhibitors and synthetic auxins in Poland and the agricultural practices in fields with herbicide-resistant populations. From 2017 to 2020, 159 seed samples together with the field history were collected across the country and biological tests performed in glasshouses. This revealed that 47 populations of C. cyanus were cross-resistant to both tribenuron and florasulam, 28 and 8 populations were single resistant to tribenuron and florasulam, respectively, and 3 populations had developed multiple resistance to both ALS inhibitors and synthetic auxins, i.e., 2,4-D and dicamba. Resistant populations were found mostly frequent in northern Poland, but also in the eastern and western parts of the country. Based on a survey of farmers, the resistant populations were found in winter crops regardless of the tillage system (77% of fields with HR cornflower were mouldboard ploughed). Based on the proposed population treatment (PT) index showing the frequency of herbicide use during three consecutive seasons on farms with HR cornflower, the average PT for all the surveyed farms was 5.4. The highest PT of 7.4 was found in the province of Warmia-Masuria in northern Poland.

First Report of Amaranthus palmeri S. Wats. in Cotton, Maize and Sorghum in Greece and Problems with Its Management

Amaranthus palmeri S. Wats., or commonly Palmer amaranth, is an invasive plant species that has invaded many countries worldwide and causes significant yield losses to annual spring crops. Palmer amaranth biotypes were detected in maize and sorghum fields in Western Greece and cotton fields in Central Greece in 2020. The infestations were recorded both inside the crops and in the margins of the fields, indicating that Palmer amaranth could be characterized as an invasive weed for these regions and, thus, must be urgently managed. Four biotypes were screened for resistance to 2,4-D and nicosulfuron by using NDVI, canopy cover, photochemical efficiency, plant height and fresh weight values. All biotypes showed resistance to nicosulfuron. Resistance cases to important herbicides (i.e., ALS and synthetic auxins) in spring crops, such as maize and cotton, indicate that management could become even more challenging if farmers do not adopt integrated strategies and tools to manage this weed. The eradication of Palmer amaranth plants is imperative, since it has yet to be dispersed in several agricultural areas of Greece. Palmer amaranth is recommended to be enlisted as quarantine pest for regulation in Greece.

Baseline survey of herbicide resistance in Russian thistle (Salsola tragus L.) finds no resistance in Manitoba

Recent confirmations of glyphosate-resistant Russian thistle (<i>Salsola tragus</i> L.) in Montana, Washington, and Oregon, warrant greater surveillance of herbicide-resistant Russian thistle in western Canada. A randomized-stratified survey of 315 sites in Manitoba was conducted in 2018 to determine the incidence of herbicide resistance in Russian thistle and other weeds sampled post-harvest. Russian thistle populations were collected from 14 of the 315 sites surveyed. None of these populations exhibited resistance to acetolactate synthase inhibitors (tribenuron/thifensulfuron), synthetic auxins (2,4-D ester or fluroxypyr), or glyphosate. This Manitoba survey of herbicide-resistant Russian thistle serves as a baseline for future surveillance efforts.

An in-frame deletion mutation in the degron tail of auxin co-receptor IAA2 confers resistance to the herbicide 2,4-D in Sisymbrium orientale

The natural auxin indole-3-acetic acid (IAA) is a key regulator of many aspects of plant growth and development. Synthetic auxin herbicides mimic the effects of IAA by inducing strong auxinic signaling responses in plants. Synthetic auxins are crucial herbicides in agriculture, made more important by the recent introduction of transgenic synthetic auxin resistant soybean and cotton. Currently, 41 weed species have evolved resistance to synthetic auxin herbicides and, in all but one case, the molecular basis of these resistance mechanisms is unknown. To determine the mechanism of 2,4-D resistance in a Sisymbrium orientale (Indian hedge mustard) weed population, we performed a transcriptome analysis of 2,4-D-resistant (R) and -susceptible (S) genotypes that revealed an in-frame 27-nucleotide deletion removing 9 amino acids in the degron tail (DT) of the auxin co-receptor Aux/IAA2 (SoIAA2). The deletion allele co-segregated with 2,4-D resistance in recombinant inbred lines. Further, this deletion was also detected in several 2,4-D resistant field populations of this species. Arabidopsis transgenic lines expressing the SoIAA2 mutant allele were resistant to 2,4-D and dicamba. The IAA2-DT deletion reduced binding to TIR1 in vitro with both natural and synthetic auxins, causing reduced association and increased dissociation rates. This novel mechanism of synthetic auxin herbicide resistance assigns a new in planta function to the DT region of this Aux/IAA co-receptor for its role in synthetic auxin binding kinetics and reveals a potential biotechnological approach to produce synthetic auxin resistant crops using gene editing.

Physiological responses of Pea plants to treatment with synthetic auxins and auxin-type herbicide

The effect of exogenously applied 2,4-D (2,4-dichlorophenoxyacetic acid) on growth and antioxidant defence of pea plants, preliminary treated with two synthetic auxin compounds 1-[2-chloroethoxycarbonyl-methyl]-4-naphthalenesulfonic acid calcium salt (TA-12) and 1-[2-dimethylaminoethoxycarbonylmethyl]naphthalene chlormethylate (TA-14) was examined. All chemicals were applied by foliar spraying. Applied alone, TA-12 and TA-14 had no significant effects, but they modulated the 2,4-D induced changes on most investigated biochemical parameters. The shoot fresh weight reduction caused by 2,4-D was partially overcome by the use of TAs. The use of TAs partially overcame the shoot fresh weight reduction induced by 2,4-D. Apart from this, no significant changes were observed in the other biometric parameters. Treatment with 2,4-D did not enhance lipid peroxidation, and hydrogen peroxide content was slightly increased. These data indicate that treatment with 2,4-D did not cause severe oxidative stress, which is also confirmed by the results of the antioxidant defence system. The application of 2,4-D provoked mild accumulation of thiol-containing compounds, free proline and phenolic compounds and increased the antioxidant enzyme activities (GST, SOD, CAT, POD and GR) to a moderate degree. Pretreatment with TAs noticeably decreased the non-enzymatic antioxidants (free proline, total phenolics and total low-molecular thiols) compared to plants treated with 2,4-D only. Except for GR, TAs pretreatment returned the enzyme activities to levels close to the controls. Based on the results obtained, we suggest that the application of both synthetic auxins could modulate 2,4-D herbicide effects.

Digitaria insularis control by using herbicide mixtures application in soybean pre-emergence

Sourgrass (Digitaria insularis) is one of the main weeds in the soybean crop. In order to control its growth, an increase of herbicide rates is required to simplify its management as it a plant with high vegetative capacity and seed production. It implies to select the herbicide-resistent Digitaria insularis biotypes. Nevertheless, some information is still contrasting the antagonist of synthetic auxinic herbicides, associated with glyphosate and ACCase inhibitors mixtures, for the control of weeds resistant or tolerant to herbicides. This study aimed to evaluate the D. insularis control, with a mixture of herbicides applied in soybean pre-emergence, with sequential application in soybean post-emergence, and to check possible antagonism between ACCase inhibitors herbicides with synthetic auxins and other latifolicides. The experiment was conducted in Palotina, Paraná (Brazil) and Corpus Christi, Canindeyú, (Paraguay.) The treatments consisted of associations of glyphosate, ACCase inhibitors (clethodim, haloxyfop), and latifolicides (2,4-D, triclopyr, dicamba, carfentrazone, saflufenacil, chlorimuron). A randomized block design was used. Only in Palotina, the weed control was satisfactory after sequential application in post-emergence. An antagonism for all synthetic auxins was observed with glyphosate+clethodim or haloxyfop mixtures, in both locations. As a result, in Palotina an efficacious control of perennial D. insularis was found in pre-emergence burndown for some mixtures such as glyphosate+ACCase inhibitor added to carfentrazone, saflufenacil, or chlorimuron. Antagonism was observed for all synthetic auxins, in both locations. In Corpus Christi, the herbicide associations were not effective, even with the postemergence application in soybean of glyphosate+clethodim. With ineffective control for treatments composed with synthetic auxins, the post-emergence application in soybean increased the weed control with satisfactory final controls for all treatments.

Selectivity and efficacy of herbicide GF-3488, containing synthetic auxins halauxifen-methyl and clopyralid, in winter oilseed rape crops in Ukraine

Halauxifen-methyl is a new synthetic auxin, which in combination with another auxin-like herbicide clopyralid is the active ingredient of complex herbicide GF-3488, that is used for dicotyledonous weeds control in winter oilseed rape crops. The reason of this study was a necessity to test crop selectivity and efficacy of GF-3488 to control dicotyledonous weeds in winter oilseed rape crops in Ukraine and also the possibility of GF-3488 application in the tank mixture with graminicide or/and insecticide to control monocotyledonous weeds and insects. A randomized block experiment was conducted in 2015—2017 in 4 locations. It was found that under spring applying, the GF-3488 is not inferior to another widely used herbicide Galera Super on selectivity to the crop. Herbicide GF-3488 at the rate 1.0 l/ha was significantly better than Galera Super at the rate 0.3 l/ha in controlling of annual dicotyledonous weeds Papaver rhoeas L., Galium aparine L., Consolida regalis S.F. Gray and was effective to control of certain types of cruciferous weeds, which are resistant to Galera Super. Adding graminicide Fusilade Forte and insecticide Nurelle D did not affect the GF-3488 selectivity to the crop and had no negative impact on dicotyledonous weeds control. Efficacy of monocotyledonous weeds control by graminicide did not change in the mixture with GF-3488. It was concluded that herbicide GF-3488 is effective on winter oilseed rape against dicotyledonous weed species. Therefore for the simultaneous control of dicotyledonous and monocotyledonous weeds in winter oilseed rape in the spring after the renewal of the crop vegetation herbicide GF-3488 is advisable to be used in a tank mixture with graminicides, and for simultaneous protection against weeds and pests it can be mixed with insecticides.

Sequential Applications of Synthetic Auxins and Glufosinate for Escaped Palmer Amaranth Control

Field and greenhouse studies were conducted to investigate the influence of sequence and timing of synthetic auxins and glufosinate on large Palmer amaranth (Amaranthus palmeri) control. Field studies were performed in Henry County, AL where treatments were applied to Palmer amaranth with average heights of 37 and 59 cm in 2018 and 2019, respectively. Sequential applications of 2,4-D/dicamba + glyphosate followed by (fb) glufosinate at labeled rates 3 or 7 days after initial treatment (DAIT) were used in addition to the reverse sequence with a 7-day interval. Time intervals of 3 or 7 days between applications did not influence Palmer amaranth control. Palmer amaranth was controlled 100% by dicamba + glyphosate fb glufosinate and 2,4-D + glufosinate fb glufosinate 7 DAIT in 2018. However, herbicide performance was reduced due to delayed application and taller plants in 2019 with up to 23% less visual injury. To further investigate Palmer amaranth response to dicamba and glufosinate applied sequentially, a greenhouse study was conducted in 2019 where physiological measurements were recorded over a 35-day period. Treatments were applied to Palmer amaranth averaging 38 cm tall and included dicamba + glyphosate fb glufosinate 7 DAIT, the reverse sequence, and a single application of dicamba + glufosinate + glyphosate. Glufosinate severely inhibited mid-day photosynthesis compared to dicamba with up to 90% reductions in CO2 assimilation 1 DAIT. In general, Palmer amaranth respiration and stomatal conductance were not affected by herbicides in this study. Applications of dicamba + glyphosate fb glufosinate 7 DAIT was the only treatment hindered Palmer amaranth regrowth with 52% reduction in leaf biomass compared to nontreated control. These data suggest Palmer amaranth infested fields are more likely to be rescued with sequential applications of synthetic auxins and glufosinate, but consistent control of large Palmer is not probable.