Conversations about the Future of Dicamba: The Science Behind Off-Target Movement

Cocrystal engineering for sustained release of dicamba: Mitigating secondary drift and reducing leaching

The off-target effects of herbicides present significant challenges in agricultural practices, posing serious threats to both ecological systems and human health. Dicamba, one of the most widely used herbicides, is particularly problematic due to its high volatility and water solubility, which can lead to rapid environmental dispersal, non-target toxicity, and groundwater contamination. To mitigate these issues, we synthesized a novel cocrystal of dicamba and phenazine (DCB-PHE cocrystal) through a combination of theoretical prediction and mechanochemical screening. The DCB-PHE cocrystal was characterized using single-crystal and powder X-ray diffraction, Fourier-transform infrared spectroscopy (FT-IR), and thermal analysis. Compared to pure dicamba, the DCB-PHE cocrystal exhibited a substantial reduction in volatility by 59 % and a decrease in equilibrium solubility by up to 5.4 times across various temperatures (15 °C, 25 °C, 35 °C). Additionally, the dissolution rates were significantly lowered by over 94 %. Leaching experiments demonstrated that the DCB-PHE cocrystal reduced total leachate by 4.9 % and delayed percolation. In greenhouse trials, the DCB-PHE cocrystal caused less damage to exposed soy plants and enhanced herbicidal activity against target weeds, with fresh weight reduction of chicory and ryegrass by 32 % and 28 %, respectively, at the highest dosage. Furthermore, safety assays confirmed that the DCB-PHE cocrystal's safety profile was comparable to that of dicamba in terms of its impact on wheat, and it did not exhibit increased genotoxicity to broad beans. These findings suggest that the DCB-PHE cocrystal is a promising candidate for reducing the environmental impacts of dicamba while maintaining its herbicidal efficacy.

Identification of a chlorosalicylic acid decarboxylase (CsaD) involved in decarboxylation of 3,6-DCSA from an anaerobic dicamba-degrading sludge

3,6-Dichlorosalicylic acid (3,6-DCSA) is the demethylation metabolite of herbicide 3,6-dichloro-2-methoxy benzoic acid (dicamba). Previous studies have shown that anaerobic sludge further transformed 3,6-DCSA through decarboxylation and dechlorination. However, the anaerobe, enzyme, and gene involved in the anaerobic degradation of 3,6-DCSA are still unknown. In this study, an anaerobic sludge that efficiently degraded dicamba was enriched, and a 3,6-DCSA decarboxylase, designated chlorosalicylic acid decarboxylase (CsaD), was partially purified and identified from the anaerobic sludge. Metagenomic analysis showed that the csaD gene was located in a gene cluster of metagenome-assembled genome 8 (MAG8). MAG8 belonged to an uncultured order, OPB41, in the class Coriobacteriia of the phylum Actinobacteria, and its abundance increased approximately once during the enrichment process. CsaD was a non-oxidative decarboxylase in the amidohydrolase 2 family catalyzing the decarboxylation of 3,6-DCSA and 6-chlorosalicylic acid (6-CSA). Its affinity and catalytic efficiency for 3,6-DCSA were significantly higher than those for 6-CSA. This study provides new insights into the anaerobic catabolism of herbicide dicamba.IMPORTANCEDicamba, an important hormone herbicide, easily migrates to anoxic habitats such as sediment, ground water, and deep soil. Thus, the anaerobic catabolism of dicamba is of importance. Anaerobic bacteria or sludge demethylated dicamba to 3,6-DCSA, and in a previous study, based on metabolite identification, it was proposed that 3,6-DCSA be further degraded via two pathways: decarboxylation to 2,5-dichlorophenol, then dechlorination to 3-chlorophenol (3-CP); or dechlorination to 6-CSA, then decarboxylation to 3-CP. However, there was no physiological and genetic validation for the pathway. In this study, CsaD catalyzed the decarboxylation of both 3,6-DCSA and 6-CSA, providing enzyme-level evidence for the anaerobic catabolism of 3,6-DCSA through the two pathways. CsaD was located in MAG8, which belonged to an uncultured anaerobic actinomycetes order, OPB41, indicating that anaerobic actinomycetes in OPB41 was involved in the decarboxylation of 3,6-DCSA. This study provides a basis for understanding the anaerobic catabolism of dicamba and the demethylation product, 3,6-DCSA.

Fabrication of ionic liquid self-assemblies based on dicamba with improved herbicidal activity and reduced environmental risks

The off-target loss of pesticide formulations caused by volatilization and leaching has reduced effective utilization and increased risks to the ecological environment and human health. Self-assembly of pesticides has been widely concerned due to the improved bioactivity and environmental compatibility. Herbicidal ionic liquids (HILs) could effectively decrease off-target loss and increase efficacy and environmental safety by improving the physicochemical properties of herbicides. Herein, HILs were prepared by pairing dicamba with quaternary ammonium salts containing different alkyl chain lengths and aromatic groups and subsequently self-assembled into spherical nanoparticles (HIL NPs) via electrostatic interaction and hydrophobic effect. Compared with dicamba, the obtained HIL NPs with an average particle size of 6-55 nm exhibited improved physicochemical properties, including high zeta potential values (+20.3 to +27.8 mV), low volatilization rate (2.4-3.9 %) and surface tension (22.83-33.07 mN m-1), decreased contact angle (32.25-41.55°) and leaching potential (76.2-86.5 %), and high soil adsorption (12.1-23.8 %), suggesting low risks to the environment. The control efficacy against Amaranthus retroflexus of HIL3 NPs pairing dicamba with octadecyl-trimethyl ammonium chloride was better than that of dicamba sodium salt at different concentrations. Therefore, the ionic liquid self-assembly developed by a facile and green preparation approach to reduce the volatility and leaching of pesticides would have enormous potential in sustainable agriculture.

Elucidating Factors Contributing to Dicamba Volatilization by Characterizing Chemical Speciation in Dried Dicamba-Amine Residues

Dicamba is a semivolatile herbicide that has caused widespread unintentional damage to vegetation due to its volatilization from genetically engineered dicamba-tolerant crops. Strategies to reduce dicamba volatilization rely on the use of formulations containing amines, which deprotonate dicamba to generate a nonvolatile anion in aqueous solution. Dicamba volatilization in the field is also expected to occur after aqueous spray droplets dry to produce a residue; however, dicamba speciation in this phase is poorly understood. We applied Fourier transform infrared (FTIR) spectroscopy to evaluate dicamba protonation state in dried dicamba-amine residues. We first demonstrated that commercially relevant amines such as diglycolamine (DGA) and n,n-bis(3-aminopropyl)methylamine (BAPMA) fully deprotonated dicamba when applied at an equimolar molar ratio, while dimethylamine (DMA) allowed neutral dicamba to remain detectable, which corresponded to greater dicamba volatilization. Expanding the amines tested, we determined that dicamba speciation in the residues was unrelated to solution-phase amine pKa, but instead was affected by other amine characteristics (i.e., number of hydrogen bonding sites) that also correlated with greater dicamba volatilization. Finally, we characterized dicamba-amine residues containing an additional component (i.e., the herbicide S-metolachlor registered for use alongside dicamba) to investigate dicamba speciation in a more complex chemical environment encountered in field applications.

Synthesis and Antifungal Activity of Coumarin Derivatives Containing Hydrazone Moiety

Plant disease control mainly relies on pesticides. In this study, a series of coumarin derivatives containing hydrazone moiety were designed and synthesized. The synthesized compounds were characterized and used to evaluate the antifungal activity against four pathogens, Botrytis cinerea, Alternaria solani, Fusarium oxysporum, and Alternaria alternata. The results showed that the inhibition rate of some compounds at 100 μg/mL in 96 hours reached around 70 % against A. alternata, higher than that of the positive control. The corresponding EC50 values were found at around 30 μg/mL. Finally, the compound 3 b was screened out with the lowest EC50 value (19.49 μg/mL). The analysis of SEM and TEM confirmed that the compound 3 b can obviously damage the morphological structure of hyphae, resulting in the depletion of the cells by the destruction of morphological matrix and leakage of contents. RNA sequencing showed that compounds 3 b mainly affected the pentose phosphate pathway, which caused to destroy the layer of mitochondrial structure. Molecular docking showed that compounds 3 b fitted the binding pocket of yeast transketolase and interacted with lysine at the hydrazone structure. Our results suggested that the introduction of hydrazone was an effective strategy for the design of novel bioactive compounds.

Ecotoxicity studies reveal that organic cations in dicamba-derived ionic liquids can pose a greater environmental risk than the herbicide itself

The following research provides novel and relevant insights into potential environmental consequences of combination of various organic cations with commercial systemic herbicide (dicamba), in accordance with a 'herbicidal ionic liquids' (HILs) strategy. Toxicity assays of five dicamba-based HILs comprising different hydrophobic and hydrophilic cations, namely choline [CHOL][DIC], ethyl betainate [BETC2][DIC], decyl betainate [BETC10][DIC], hexadecyl betainate [BETC16][DIC] and didecyldimethylammonium [DDA][DIC]), have been tested towards bacteria (Pseudomonas putida, Escherichia coli, Bacillus subtilis), algae (Chlorella vulgaris), fresh and marine water crustaceans (Daphnia magna, Artemia franciscana). The structure of respective substituents in the cation emerged as a decisive determinant of toxicity in the case of tested species. In consequence, small ions of natural origin ([CHOL] and [BETC2]) demonstrated toxicity numerous orders of magnitude lower compared to fully synthetic [DDA]. These results emphasize the role of cations' hydrophobicity, as well as origin, in the observed acute toxic effect. Time-dependent toxicity assays also indicated that betaine-type cations comprising an ester bond can rapidly transform into less harmful substances, which can generally result in a reduction in toxicity by even several orders of magnitude. Nonetheless, these findings challenge the concept of ionic liquids with herbicidal activity and give apparent parallels to adjuvant-dependent toxicity issues recently noted in typical herbicidal formulations.

Anaerobic Degradation of Dicamba via a Novel Catabolic Pathway by a Consortium Enriched from Deep Paddy Soil

Dicamba is widely used in the paddy field to control broadleaf weeds. Dicamba easily migrates to deep soil, which is anoxic; however, the anaerobic catabolism of dicamba in paddy soil is still unknown. In this study, an anaerobic dicamba-degrading consortium was enriched from deep paddy soil. The consortium completely degraded 0.83 mM dicamba within 7 days. Five metabolites were identified, one of which is a new metabolite, 2,5-dichlorophenol, and a novel anaerobic dicamba degradation pathway was proposed. 2.5 mM dicamba, 1.5-2.0% NaCl, and 20 mM electron acceptors Na2SO4, NaNO3, and FeCl3, and 0.5 mM or more of metabolites 3-CP and 2,5-DCP strongly inhibited the degradation efficiency. During enrichment, the microbial community of the consortium was significantly changed with OTU numbers, and diversity decreased. The study is valuable to elucidate the catabolism and ecotoxicology studies of dicamba in paddy soil and to facilitate the engineering application of anaerobic technology to treat dicamba-manufacturing wastewater.

Microbiology and Biochemistry of Pesticides Biodegradation

Pesticides are chemicals used in agriculture, forestry, and, to some extent, public health. As effective as they can be, due to the limited biodegradability and toxicity of some of them, they can also have negative environmental and health impacts. Pesticide biodegradation is important because it can help mitigate the negative effects of pesticides. Many types of microorganisms, including bacteria, fungi, and algae, can degrade pesticides; microorganisms are able to bioremediate pesticides using diverse metabolic pathways where enzymatic degradation plays a crucial role in achieving chemical transformation of the pesticides. The growing concern about the environmental and health impacts of pesticides is pushing the industry of these products to develop more sustainable alternatives, such as high biodegradable chemicals. The degradative properties of microorganisms could be fully exploited using the advances in genetic engineering and biotechnology, paving the way for more effective bioremediation strategies, new technologies, and novel applications. The purpose of the current review is to discuss the microorganisms that have demonstrated their capacity to degrade pesticides and those categorized by the World Health Organization as important for the impact they may have on human health. A comprehensive list of microorganisms is presented, and some metabolic pathways and enzymes for pesticide degradation and the genetics behind this process are discussed. Due to the high number of microorganisms known to be capable of degrading pesticides and the low number of metabolic pathways that are fully described for this purpose, more research must be conducted in this field, and more enzymes and genes are yet to be discovered with the possibility of finding more efficient metabolic pathways for pesticide biodegradation.

The consequences of synthetic auxin herbicide on plant-herbivore interactions

Although herbicide drift is a common side effect of herbicide application in agroecosystems, its effects on the ecology and evolution of natural communities are rarely studied. A recent shift to dicamba, a synthetic auxin herbicide known for 'drifting' to nontarget areas, necessitates the examination of drift effects on the plant-insect interactions that drive eco-evo dynamics in weed communities. We review current knowledge of direct effects of synthetic auxin herbicides on plant-insect interactions, focusing on plant herbivory, and discuss potential indirect effects, which are cascading effects on organisms that interact with herbicide-exposed plants. We end by developing a framework for the study of plant-insect interactions given drift, highlighting potential changes to plant developmental timing, resource quantity, quality, and cues.

Impact of volatility reduction agents on dicamba and glyphosate spray solution pH, droplet dynamics, and weed control

BACKGROUND

Regulations in 2021 required the addition of a volatility reduction agent (VRA) to dicamba spray mixtures for postemergence weed control. Understanding the impact of VRAs on weed control, droplet dynamics, and spray pH is essential.

RESULTS

Adding glyphosate to dicamba decreased the solution pH by 0.63 to 1.85 units. Across locations, potassium carbonate increased the tank-mixture pH by 0.85 to 1.65 units while potassium acetate raised the pH by 0.46 to 0.53 units. Glyphosate and dicamba in tank-mixture reduced Palmer amaranth control by 14 percentage points compared to dicamba alone and decreased barnyardgrass control by 12 percentage points compared to glyphosate alone 4 weeks after application (WAA). VRAs resulted in a 5-percentage point reduction in barnyardgrass control 4 WAA. Common ragweed, common lambsquarters, and giant ragweed control were unaffected by herbicide solution 4 WAA. Dicamba alone produced a larger average droplet size and had the fewest driftable fines (% volume < 200 μm). Potassium acetate produced a larger droplet size than potassium carbonate for D_v0.1 and D_v0.5 . The addition of glyphosate to dicamba decreased droplet size from the entire spray droplet spectrum (D_v0.1 , D_v0.5 , D_v0.9 ).

CONCLUSION

A reduction in spray pH, droplet size, and weed control was observed from mixing dicamba and glyphosate. It may be advisable to avoid tank-mixtures of these herbicides and instead, apply them sequentially to maximize effectiveness. VRAs differed in their impacts on spray solution pH and droplet dynamics, but resulted in a minimal negative to no impact on weed control. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.