Introduction
Virginia pepperweed, native to North and Central America, is an herbaceous winter annual weed in the Brassicaceae (mustard) family commonly found throughout the United States. In general, Brassicaceae weed species are difficult to control during the growing season, especially when they are mature or at later growth stages. Brassica species go through two major developmental stages during their life cycles: a basal rosette and bolting.
Virginia pepperweed is unique and considered a naturalized weed in crop production systems and an invasive weed species in Pacific Island ecosystem regions (PIER 2013; Wagner et al. Reference Wagner, Herbst and Sohmer1999). It is found in no-till cropping systems, roadsides, and other undisturbed areas. Virginia pepperweed reproduces mainly by seed and has a high fecundity rate (∼100,000 seeds plant−1), making it challenging to control during the growing season. Owing to its unique, peppery taste, Virginia pepperweed is also known as poor-man’s pepper.
Currently no studies report on yield losses caused by Virginia pepperweed interference in cropping systems. However, reports are available on brassica weed species with similar growth habits, such as wild mustard (Sinapis arvensis L.), field pepperweed [Lepidium campestre (L.) W.T. Aiton], and field pennycress (Thlaspi arvense L.), that are known to reduce wheat (Triticum aestivum L.) yields by 38% to 50% (Alex Reference Alex1970; Burrows and Olson Reference Burrows and Olson1955; Carter et al. Reference Carter, Lefforge and Shenberger1946; Northam et al. Reference Northam, Stahlman and Abd El-Hamid1993). Weed density is a key factor affecting weed interference in various cropping systems (Faria et al. Reference Faria, Barros and Santos2014). A previous survey conducted on 52 weed species in corn (Zea mays L.) fields in southern Wisconsin found that Virginia pepperweed had a density of 0.8 plants m−2 with a competitive index of 1 (Fickett et al. Reference Fickett, Boerboom and Stoltenberg2013).
Several chemical control options are available for controlling Virginia pepperweed. Glyphosate, 2,4-D, dicamba, and glufosinate are some of the herbicides labeled as burndown applications for managing it before planting crops. Smisek et al. (Reference Smisek, Doucet, Jones and Weaver1998) reported resistance to paraquat herbicide (Weed Science Society of America Group 22) in Ontario, Canada. So far, herbicide resistance has not been reported for Virginia pepperweed in the United States. In general, weed size remains one of the key factors for controlling brassica weed species like Virginia pepperweed in different cropping systems (Cahoon Reference Cahoon2016; Culpepper Reference Culpepper2009; DiTomaso et al. Reference DiTomaso, Kyser, Oneto, Wilson, Orloff, Anderson, Wright, Roncoroni, Miller, Prather, Ransom, Beck, Duncan, Wilson and Mann2013; Ferrell et al. Reference Ferrell, Sellers, MacDonald and Devkota2020).
Recently, Virginia pepperweed has emerged as a troublesome and difficult-to-control weed in and around the major row crops of the Mississippi Delta. To date, limited research has been conducted on Virginia pepperweed control in Mississippi Delta cropping systems. The objective of this study was to evaluate the activity of burndown herbicides commonly used to control Virginia pepperweed at three different growth stages. This study focused on Virginia pepperweed established throughout the Mississippi Delta region.
Materials and Methods
Plant Material and Experimental Setup
Greenhouse experiments were conducted during summer 2024 at the U.S. Department of Agriculture Agricultural Research Service Crop Production Systems Research Unit, Stoneville, MS. Mature Virginia pepperweed plants with inflorescences were collected from a nearby soybean [Glycine max (L.) Merr.] research plot (33.425°N, 90.952°W) (Figure 1 A and B). The plants were brought to the greenhouse and allowed to dry. Seed from five plants was harvested and bulked together as one sample from the aforementioned location. Seed was stratified at 4 C in a cooler refrigerator for 4 wk to break dormancy and germinated in a growth chamber (22 C day/18 C night, 12-h photoperiod). The germinated seedlings were transplanted into individual square pots (10 × 10 cm) prefilled with custom growing mix (Jolly Gardner®, Old Castle Lawn and Garden, Atlanta, GA, USA) and kept in the greenhouse. The pots were regularly watered and fertilized (24:8:16, N:P:K, Miracle Grow, Marysville, OH, USA) at the initial establishment stage until seedlings reached the targeted growth stages from planting: early rosette (28 d), late rosette (36 d), and bolting (65 d). The early and late rosette stages had widths of 8 cm and 12 cm, respectively, while the bolting stage had a width of 12 cm and a height ranging from 10 to 12 cm. Conditions inside the greenhouse were maintained at 28/22 ± 3 C day/night temperature with a 12-h photoperiod supplemented by metal halide lamps (560 µmol m−2 s−1).
Figure 1. The United States showing the location of Stoneville, MS (A), and the field from which seed was collected (B).
The experiment was conducted in a randomized complete block design with five treatments and four replications separately for each growth stage. The treatments (1X) consisted of glyphosate at 1,261 g ai ha−1, glufosinate at 672 g ae ha−1, 2,4-D at 1,065 g ai ha−1, paraquat at 840 g ai ha−1, and a nontreated control. A non-ionic surfactant (0.25% v/v) was added to the paraquat treatment before spraying. Detailed information on herbicides used to control Virginia pepperweed, such as trade name, active ingredient, site of action, and manufacturer, are shown in Table 1. Plants were treated with the 1X rates of herbicides at three different growth stages: early rosette, late rosette, and bolting. All herbicide treatments were applied using a Generation 3 research track sprayer (DeVries Manufacturing, Hollandale, MN, USA) with an 8002E nozzle calibrated to deliver 187 L ha−1 at 280 kPa. After spraying, plants were returned to the greenhouse and watered as needed. The experiment was repeated over two runs under the same greenhouse conditions.
Table 1. Herbicide treatments for Virginia pepperweed control used in this study a .
a Abbreviation: WSSA, Weed Science Society of America.
b Sites of action are as follows: 4, synthetic auxin; 9, inhibition of enolpyruvyl shikimate phosphate synthase; 10, inhibition of glutamine synthetase; 22, Photosystem I electron diverter.
c SCANNER® (non-ionic surfactant; Loveland Products, Loveland, CO, USA) at 0.25% v/v.
Data Collection and Analysis
Visible pepperweed control was evaluated weekly up to 28 d after treatment (DAT). Evaluations were made on a scale of 0 to 100, where 0 represented no Virginia pepperweed control and 100 represented complete Virginia pepperweed control (Frans and Talbert Reference Frans and Talbert1986). After control ratings, aboveground shoot biomass of individual plants was harvested at 28 DAT and placed in a paper bag, dried at 65 C for 72 h, and weighed to obtain the shoot biomass for each treatment. Shoot biomass data were expressed as percentage reduction relative to that of the nontreated check using the following equation:
$$Y = {{D{W_c} - D{W_t}} \over {D{W_c}}} \times 100$$
where Y represents percentage shoot biomass reduction, DW c is the shoot biomass of the nontreated control, and DW t is the shoot biomass of treated plants at 28 DAT.
For statistical analyses, data were converted to percent control and biomass reductions relative to untreated control. A generalized linear mixed (GLM) model with repeated measure analysis of variance (ANOVA) was used to assess response variables measured at four different DATs within each replicate, for the three-factor experimental design. PROC GLM was employed to specify both within-subject and between-subject factors (repeated measures). Data from each growth stage were subjected to ANOVA and analyzed separately across DAT. Means were separated at the 5% level of significance using Fisher’s protected least significant difference test declared significant at P ≤ 0.05. The analysis was performed using SAS (version 9.4; SAS Institute 2023).
Results and Discussion
Weed growth stage is one of the key factors affecting herbicide activity. In this research, herbicide applications at the early rosette stage resulted in maximum Virginia pepperweed control when compared to the late rosette and bolting stages (Figures 2 to 4). The effects of glyphosate and glufosinate were visually noticeable 7 to 10 d after their initial application, whereas 2,4-D and paraquat effects were observed 1 to 2 DAT.
Figure 2. Virginia pepperweed control 28 d after treatment (DAT) with herbicides sprayed at 1X rates (glyphosate at 1,261 g ai ha−1, glufosinate at 672 g ae ha−1, 2,4-D at 1,065 g ai ha−1, and paraquat at 840 g ai ha−1). Herbicides were applied at early rosette stage (inset) in the greenhouse. A non-ionic surfactant (SCANNER® 0.25% v/v, Loveland Products, Loveland, CO, USA) to paraquat treatment was added before spraying.
Figure 3. Virginia pepperweed control 28 DAT with herbicides sprayed at 1X rates (glyphosate at 1,261 g ai ha−1, glufosinate at 672 g ae ha−1, 2,4-D at 1,065 g ai ha−1, and paraquat at 840 g ai ha−1). Herbicides were applied at late rosette stage (inset) in the greenhouse. A non-ionic surfactant (SCANNER® 0.25% v/v) to paraquat treatment was added before spraying.
Figure 4. Virginia pepperweed control 28 DAT with herbicides sprayed at 1X rates (glyphosate at 1,261 g ai ha−1, glufosinate at 672 g ae ha−1, 2,4-D at 1,065 g ai ha−1, and paraquat at 840 g ai ha−1). Herbicides were applied at bolting stage (inset) in the greenhouse. A non-ionic surfactant (SCANNER® 0.25% v/v) to paraquat treatment was added before spraying. Note that four out of five pots (replications) were used for analysis; the fifth pot shown in the figure was included solely for representation.
Percent Virginia Pepperweed Control and Biomass Reduction
Of the four herbicides evaluated in this study (Table 1), 2,4-D application resulted in maximum 95% to 100% Virginia pepperweed control at all three growth stages 28 DAT (Figures 2 to 5). During the 4-wk evaluation period, Virginia pepperweed control with 2,4-D ranged from 25% at 7 DAT to 100% by 28 DAT (Figure 5). Ferrell et al. Reference Ferrell, Sellers, MacDonald and Devkota2020) reported greater than 90% control with 2,4-D in wild radish (Raphanus raphanistrum L.), a brassica weed species similar in growth habit to Virginia pepperweed. Control with 2,4-D declined to 70% when applied to 30-cm wild radish and declined further to 40% when applied at the bolting stage (Ferrell et al. 2015). Similar results were reported by Culpepper (Reference Culpepper2009) in wild radish after 2,4-D application. In corn and soybean, 2,4-D, either alone or applied as a tank mix with glyphosate or paraquat, was found to be effective in controlling Brassicaceae weeds, such as wild mustard and wild radish (GROW 2025). Application of 2,4-D was also found to be effective at mature growth stages of wild mustard and wild radish (Cahoon Reference Cahoon2016). Similarly, in our current study, there was 95% to 100% Virginia pepperweed control with 2,4-D at the late rosette and bolting stages 28 DAT (Figure 5 B and C).
Figure 5. Virginia pepperweed control at 7, 14, 21, and 28 DAT with glyphosate, glufosinate, 2,4-D, and paraquat applied at early rosette (A), late rosette (B), and bolting (C) stages in the greenhouse. Treatments associated with the same letter are not significantly different (P ≤ 0.05). Vertical bars represent ± standard error of the mean for each treatment and time assessment.
Glyphosate application resulted in 40% to 50% Virginia pepperweed control at the three growth stages 28 DAT (Figure 5). Glyphosate application alone is not an effective strategy for controlling weed species like wild mustard and wild radish. Tank mixing glyphosate with 2,4-D or sequential application of glyphosate followed by paraquat is recommended for achieving complete control of Brassicaceae weed species, especially at mature growth stages. In a cover crop termination study, glyphosate + 2,4-D was the most effective (96%) in controlling 12-cm-tall rapeseed (Brassica napus L.) 28 d after early termination (Askew et al. Reference Askew, Cahoon, Flessner, VanGessel, Langston and Ferebee2019).
In our study, greater than 70% control was observed with paraquat at 28 DAT, especially when applied at the early rosette stage (Figures 2 and 5 A). At the late rosette and bolting stages, paraquat activity in controlling Virginia pepperweed significantly decreased, from 30% to 0% (Figures 3, 4, and 5 B and C). Plants recovered completely from initial injury when paraquat was applied at the late rosette and bolting stages 28 DAT (Figures 3, 4, and 5 B and C). To our knowledge, data on paraquat usage alone on brassica weeds is limited. It is generally recommended for burndown use as tank-mix partner with herbicides like atrazine or metribuzin in corn and soybean cropping systems (GROW 2025). Virginia pepperweed resistance to paraquat was reported in southern Ontario, Canada, and found to be 10-fold resistant compared to the susceptible population (Smisek et al. Reference Smisek, Doucet, Jones and Weaver1998). The Virginia pepperweed population in our study is likely resistant to paraquat and other herbicides. Further research is needed to confirm herbicide resistance in the Virginia pepperweed population of Mississippi.
Peak glufosinate control greater than 60% was observed 14 to 21 DAT at the early and late rosette stages (Figure 5 A and B). However, plants recovered from the initial glufosinate injury by 28 DAT (Figures 2 to 5 A). Virginia pepperweed control was <50% when glufosinate was applied at the bolting stage, indicating that glufosinate application is more effective at earlier rather than at later growth stages (Figures 4 and 5 C). Glufosinate application should be followed by a different mode of action herbicide for achieving complete Virginia pepperweed control. Glufosinate is recommended primarily for suppression of wild mustard, wild radish, and other brassica weed species at earlier rosette growth stages (GROW 2025).
Aboveground dry biomass reduction was highest when herbicides were applied at the early rosette stage and lowest when applied at the bolting stage of Virginia pepperweed (Figure 6). The highest biomass reduction (>80%) was observed with 2,4-D treatment at the early rosette stage (Figure 6 C). The high biomass reduction corresponds to the maximum Virginia pepperweed control observed with 2,4-D applications at all three growth stages (Figures 2 to 5). At the early rosette stage, biomass reductions with 2,4-D, paraquat, and glyphosate were 25% to 30% greater than those observed with glufosinate treatment. Following glufosinate application at the early rosette stage, plants exhibited rapid recovery and increased regrowth (Figure 2). Overall, there was a significant interaction between the various growth stages and the herbicide active ingredients examined in the study (Figure 6). Significant interaction was observed at all three growth stages for glyphosate and paraquat treatments, while no significant interaction occurred between the two rosette stages for glufosinate and 2,4-D treatments.
Figure 6. Percent biomass reduction in Virginia pepperweed with glyphosate (A), glufosinate (B), 2,4-D (C), and paraquat (D) applied at early rosette, late rosette, and bolting stages in the greenhouse. Treatments associated with the same letter are not significantly different (P ≤ 0.05). Vertical bars represent ± standard error of the mean for each time assessment.
Practical Implications
Application of 2,4-D at the early rosette stage showed maximum activity in controlling Virginia pepperweed. Herbicide application timing is critical when controlling Brassicaceae weeds like Virginia pepperweed. For achieving maximum herbicide activity and weed control, it is recommended that Virginia pepperweed plants be treated at the early rosette stage.
While greenhouse studies provide key advantages, including a controlled environment, minimized environmental variability, and precise parameter control, they also have limitations. Because this study was conducted under optimal conditions, the results may vary in a field setting. Crop competition in the field is a significant factor to consider, and the data, particularly biomass measurements, may vary under such conditions.
Acknowledgments
We thank Earl Gordon, Efron Ford, and Russell Coleman for technical assistance in greenhouse work. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
Funding statement
This research received no specific grant from any funding agency or the commercial or not-for-profit sector.
Competing interests
The authors declare no competing interests.
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