2013 Volume 38 Issue 4 Pages 220-222
The prevalence of herbicide resistance in row crop agriculture is leading to increased concerns over similar resistance development in ornamental horticulture production systems. It is well established that widespread use of herbicides with similar modes of action will increase selection pressure for herbicide resistant weed biotypes. While there are currently 388 biotypes across 208 weed species that exhibit herbicide resistance, no weed populations have been detected that are resistant to the cellulose biosynthesis inhibitor herbicide group.1) The lack of resistance to cellulose biosynthesis inhibiting herbicides could be due to the complexity of the cellulose synthase gene family associated with the cellulose biosynthesis as well as the ability of herbicides to affect multiple sites within the cellulose biosynthesis pathway.2,3)
Dichlobenil is a preemergence (PRE) benzonitrile herbicide first developed in the 1960’s that inhibits cellulose biosynthesis in susceptible broadleaf, grass, and sedge species.4–7) Dichlobenil prevents incorporation of glucose into cellulose structures within plant tissues, primarily by inhibiting the movement and proper formation of the cellulose synthase protein complex (CESA6) needed for cellulose synthesis.8,9)
Modification of commercially available herbicides through the introduction of heterocyclic motifs (i.e., cyclic ring structures containing atoms other than carbon) has been beneficial in discovering new agents that offer improved weed control and environmental safety. The objective of this research was to design, synthesize, and purchase heterocyclic analogues of dichlobenil and to evaluate these compounds, in comparison to a dichlobenil standard, for weed control and phytotoxicity when applied over the top of a woody ornamental species, Japanese holly (Ilex crenata).
Herbicidal activity of the active ingredient in 2,6-dichlorobenzonitrile (dichlobenil), was compared to the following analogues: 3,5-dichloropyridine-4-carbonitrile (TN1), 2,4-dichloropyridine-3-carbonitrile (TN2) and 4,6-dichloropyrimidine-5-carbonitrile (TN3) (Fig. 1). The TN1 and TN3 compounds were obtained through commercial sources (Sigma-Aldrich Co., St. Louis, MO, USA; Activate Scientific Corp., Prien, Germany). The TN2 compound was synthesized using the Sandmeyer reaction.10–12) Nuclear magnetic resonance (NMR) spectra of synthetic compounds were obtained using a Varian Gemini 300 MHz spectrometer (Agilent Technologies, Santa Clara, CA, USA).
Synthesis of 2,4-dichloropyridine-3-carbonitrile (TN2) was initiated when the starting material, 2,4-dichloropyridin-3-amine (1 g, 6.2 mmol), was dissolved in 5 mL of acetonitrile (Ark Pharm, Inc. Libertyville, IL, USA). Deionized water (20 mL) was then added to the solution, followed by 3 mL concentrated hydrochloric acid. The solution was cooled to 0°C and sodium nitrite (8.6 g, 12.4 mmol) was added. The solution was then allowed to stir for 30 min before being warmed to 20°C and stirred for an additional 2 hr. At this point, a solution of potassium cyanide (1.21 g, 18.6 mmol) and cupric cyanide (0.61 g, 6.8 mmol) dissolved in 20 mL of deionized water was prepared and added drop wise to the reaction. The resulting solution was stirred overnight at 20°C then extracted with ethyl acetate (2×45 mL). The combined ethyl acetate layers were dried with magnesium sulfate, filtered and the solvent was removed by a rotary evaporator under vacuum (Buchi R-114. Buchi Labortechnik AG. Flawil, Switzerland). The resulting crude product was purified using a flash chromatography column with gradient elution of 3–10% methanol/dichloromethane. This provided TN2 as a light brown solid (52 mg, 0.05% yield). 1H NMR (300 MHz, CDCl3) δH, 5.81, J=6 Hz; d, 7.23, J=9 Hz.
Experimental compounds (TN1, TN2, TN3) were then tested for herbicidal activity in greenhouse studies at the University of Tennessee (35.98 N, 83.91 W) during 2011 and 2012 by comparing them to the commercial standard dichlobenil. Compounds were applied to Japanese holly (I. crenata), large crabgrass (Digitaria sanguinalis), and common purslane (Portulaca oleracea) in order to evaluate ornamental tolerance and weed susceptibility to TN1, TN2, TN3, and dichlobenil. Each plant species was established in separate 10.2 cm×10.2 cm plastic pots (Dillen Products/Myers Industries, Inc. Middlefield, OH, USA) filled with Sequatchie loam soil (fine-loamy, siliceous, semiactive, thermic, and humic Hapludult) having a pH of 5.8 and organic matter content of 2.1%. This growing medium was blended with a calcined clay soil conditioner (Turface. Profile Products, LLC. Buffalo Grove, IL, USA) in a 3 : 1 soil : clay ratio. Japanese holly cuttings were transplanted into pots two months prior to treatment. Japanese holly plants were 14 cm at time of application. All weed species were surface seeded just prior to treatment and incorporated into the top 3 cm of soil. During the study plants were kept under natural light conditions and watered daily.
All compounds were applied PRE at rates of 1, 5, and 10 kg per hectare (kg/ha). Compounds were dissolved in a mixture of acetone (3 mL) and deionized water (32 mL) and agitated using a sonicator (CL-18, Fisher Scientific International Inc. Hampton, NH, USA) before application. All suspensions were further agitated by hand immediately prior to treatment in an enclosed sprayer chamber (Generation III track sprayer. DeVries Manufacturing, Hollandale, MN) at 215 L/hectare through an 8004 EVS nozzle (TeeJet, Wheaton, IL, USA). All treatments were irrigated immediately after application.
Foliar Japanese holly injury and weed control were assessed 14, 21, and 28 days after treatment (DAT) on a 0 (i.e., no injury) to 100% (i.e., complete plant death) scale relative to a non-treated check. Treatments were arranged in a 4×3 factorial, randomized complete block design with three replications. Factors included four compounds (dichlobenil, TN1, TN2, and TN3) and three application rates (10, 5, and 1 kg/ha). The experiment was conducted from 2 November 2011 to 30 November 2011 and repeated from 20 August 2012 to 17 September 2012. Japanese holly injury and weed control data subjected to analysis of variance using the models of McIntosh.13) Fisher’s protected least significant difference test (α=0.05) was used for mean separation.
No significant treatment-by-experimental run interactions were detected in Japanese holly injury or weed control data; thus, data from each experimental run were combined. Japanese holly injury did not vary due to applied treatments; overall injury ranged from only 0 to 7% by 28 DAT (data not presented).
Large crabgrass control varied due to treatment (Table 1). When applied at 10 kg/ha the TN2 and TN3 analogues controlled large crabgrass 20 and 46% respectively by 28 DAT (Table 1). Large crabgrass control with TN2 and TN3 was similar to the herbicide dichlobenil (46%) on this assessment date. No significant differences in large crabgrass control were detected between the 1 and 5 kg/ha application rates of dichlobenil or the pyrimidine analogue (TN3) although both rates controlled large crabgrass less than 10 kg/ha. No differences were detected with among the three application rates of the two pyridine analogues (TN1 and TN2) as large crabgrass control only ranged from 5 to 20% 28 DAT.
| Compound | Rate (kg/ha) | Weed controla | |
|---|---|---|---|
| Large crabgrass (%) | Common purslane (%) | ||
| Dichlobenil | 1 | 12 | 3 |
| 5 | 17 | 20 | |
| 10 | 46 | 70 | |
| TN1 | 1 | 5 | 1 |
| 5 | 8 | 6 | |
| 10 | 13 | 3 | |
| TN2 | 1 | 10 | 15 |
| 5 | 7 | 7 | |
| 10 | 20 | 7 | |
| TN3 | 1 | 3 | 5 |
| 5 | 23 | 18 | |
| 10 | 46 | 62 | |
| LSD0.05 | — | 27 | 24 |
a Weed control was evaluated using a 0 (i.e., no injury) to 100% (i.e., complete plant death) scale relative to a non-treated check.
The pyrimidine analogue (TN3) controlled common purslane similar to dichlobenil when both were applied at 10 kg/ha. By 28 DAT common purslane control ranged from 62 to 70% with TN3 or dichlobenil. Reducing rates of TN3 and dichlobenil to between 1 to 5 kg/ha resulted in 20% or less control of common purslane. Neither pyridine analogue (TN1, TN2) controlled common purslane greater than 15% regardless of rate (Table 1).
Japanese holly tolerance to dichlobenil in this study supports current labeling for use on holly species.4) However, weed control (e.g., large crabgrass, common purslane) with dichlobenil in this study was lower (<20%) than what would be expected with an application at the labeled rate of 5 kg/ha. Poor weed control with the labeled rate of dichlobenil could be attributed to the fact that the active ingredient was not applied as a commercial formulation in this study; rather, technical grade dichlobenil was agitated in mixture with deionized water and acetone before being applied to plants. Formulation can have a dramatic impact on the efficacy of preemergence herbicides for weed control.14,15) Furthermore, even formulated dichlobenil has been shown to be volatile,16) which could also explain the reduced activity observed with dichlobenil rates as high as 10 kg/ha. Future research should compare the ornamental tolerance and weed control efficacy of the heterocyclic analogues in the current study (TN1, TN2, TN3) to dichlobenil under conditions that reduce the risk of volatilization, such as applications to dry soil16) or incorporating compounds into soil after application.17)
The pyrimidine analogue (TN3) was safe for use on Japanese holly and provided PRE control of large crabgrass and common purslane similar to dichlobenil at all rates evaluated. Additional research should be done to determine if TN3 inhibits cellulose biosynthesis and whether the specific sites of action targeted by TN3 are similar to the commercial standard dichlobenil using the methods described by DeBolt et al.9) Experiments should also be conducted using a broader range of ornamental and weed species comparing ornamental plant tolerance and weed control with TN3 and dichlobenil. Differences in ornamental tolerance and weed susceptibility between these two compounds could provide new options for weed management in ornamental horticulture production systems, especially if TN3 targets novel sites of action within the cellulose biosynthesis pathway.
The authors would like to thank Tyler Campbell, Jacob Watson, and the Tennessee Agricultural Experiment Station for their assistance in this research.
