Abstract
Anaerobic soil disinfestation (ASD) is a preplant pest management technique that involves amending the soil with a labile carbon source, irrigating the soil to stimulate decomposition, and then covering the soil with polyethylene film (polyfilm) to limit gas exchange. During the ASD process, soil microorganisms shift from aerobic to anaerobic metabolism and release phytotoxic byproducts such as organic acids and gases. Although it has been shown that these phytotoxic by-products have a negative impact on weed survival, questions remain about whether commercial-level weed control can be achieved using ASD alone or in combination with other chemicals. Greenhouse and field studies were conducted to evaluate ASD with mustard (Brassica sp.) meal, molasses, and herbicide applications for yellow nutsedge (Cyperus esculentus) control in tomato (Solanum lycopersicum). The treatments in these studies included factorial of two carbon sources [mustard meal + molasses (MMM) or no carbon amendment], three herbicide treatments [halosulfuron applied preemergence (PRE), halosulfuron applied postemergence (POST), and no herbicide] and two polyfilm treatments (polyfilm cover or polyfilm uncover). In field trials two polyfilm cover treatments were punctured and nonpunctured. Soil treatments included molasses at 14,000 L·ha−1 and mustard meal at 2100 kg·ha−1. Halosulfuron was applied at a rate of 1 oz/acre for PRE or POST applications. Greater anaerobic conditions were achieved in polyfilm cover treatments amended with MMM. In greenhouse and field trials, the most effective treatments for reducing yellow nutsedge populations were ASD with MMM or combined with halosulfuron application (PRE- or POST-ASD), which delivered significantly higher weed control than all other treatments tested or controls. In field trials, ASD with MMM caused plant growth stunting 14 d after transplantation (DAT); however, plants recovered, and stunting or injury was often not observed at 42 DAT. These studies demonstrated that ASD using MMM can be an effective strategy for reducing yellow nutsedge populations; however, the more research is needed to ensure crop safety while using ASD technology.
Tomato (Solanum lycopersicum) production in the United States was worth $1.66 billion in 2020, and South Carolina ranks eighth in fresh market tomato production in the nation [U.S. Department of Agriculture (USDA), National Agricultural Statistics Service, 2018, 2020)]. Nearly all tomato production in South Carolina uses polyethylene mulches. Polyethylene mulches are widely used in tomato and other specialty crops as they promote early plant growth, uniform soil moisture, increased yield, and suppress the most of grasses and broadleaf weeds (Lamont, 2005; Zhang et al., 2019). However, polyethylene mulch does not adequately control nutsedge (Cyperus sp.), which compete with tomato plants for both above- and below-ground resources and cause significant yield losses (Chase et al., 1998; Patterson, 1998).
Yellow nutsedge (Cyperus esculentus) is one of most widespread and problematic weed species in vegetable crops in the United States (Van Wychen, 2019; Webster, 2010). Yellow nutsedge is difficult to control in polyethylene mulch production, because its leaf tips and strong midribs allow it to puncture plastic mulch, greatly diminishing the plastic’s durability and longevity, in addition to competing with crop (Adcock et al., 2008; Santos et al., 1997). The resulting loss of integrity of the plastic mulch due to punctures allows other weed species to emerge on raised beds, interfering with crop growth and causing further reduction in the crop yield (Norsworthy et al., 2008). Additionally, production costs rise due to rapid deterioration of plastic mulch, which growers expect to use for multiple cropping seasons. Other characteristics of yellow nutsedge, such as asexual reproduction and underground perennating tubers, give it a competitive advantage over crop plants (Benedixen and Nandihalli, 1987; Stoller and Sweet, 1987; Webster, 2005).
Previously, the use of methyl bromide and other soil fumigants had been widespread for controlling pests in polyethylene mulch production systems (Duniway, 2002; Schneider et al., 2003). The phaseout of methyl bromide due to health and environmental concerns has complicated weed management, especially control of nutsedge species in polyethylene mulch vegetable cultivation, leading to numerous studies to find other effective weed control strategies. (Bangarwa et al., 2012; Guo et al., 2017; Muramoto et al., 2008; Shrestha et al., 2018). In polyethylene-mulched vegetable production, herbicides provide some weed control (Dittmar et al., 2012; Miller and Dittmar, 2014). However, the lack of effective herbicide options in specialty crops and an increase in documented cases of herbicide resistance, the future of reliance on herbicide-based weed management programs is uncertain (Fennimore and Cutulle, 2019). Therefore, there is a need to develop new strategies to effectively provide an integrated weed management (IWM).
ASD is a biologically driven process for controlling soilborne diseases, nematodes, and potentially weeds. ASD is facilitated by adding carbon-rich amendments to the soil, tarping with a polyethylene film (polyfilm) cover and saturating the soil under the film with water. This creates an anaerobic environment that reduces or eliminates many of the aerobic plant pests (Blok et al., 2000; Butler et al., 2014; Momma, 2008; Shrestha et al., 2018; Strauss and Kluepfel, 2015). In ASD, anaerobic conditions are maintained in the soil with tarping, ranging from 3 to 10 weeks. Several studies reported that the evolution of volatile organic compounds, shifts in microbial communities, lowered pH and anaerobic conditions developed during ASD period contribute to pest mortality (Hewavitharana and Mazzola, 2016; Shrestha et al., 2018; Testen and Miller, 2018).
There are relatively few studies that evaluate the effects of ASD on weed control in polyethylene-mulched vegetable crops. Previous studies suggested that supplemental herbicides or other weed control methods used in conjunction with ASD are necessary to provide adequate weed control in polyethylene mulch vegetable production. For instance, ASD facilitated by carbon sources such as composted poultry litter and molasses reported unacceptable level of nutsedge control (Di Gioia et al., 2016), the addition of a halosulfuron enhanced weed control (Guo et al., 2017). Another ASD study found that using wheat (Triticum aestivum) bran as a carbon source reduced yellow nutsedge tuber sprouting and reproduction, but the commercial level of weed control was not achieved, which suggested the need to integrate other IWM tactics (Shrestha et al., 2018). Therefore, due to limited ASD research on weed management and its inability to reach commercial levels of weed control, more study is needed. Overall, in previous ASD studies, variations in carbon sources, temperature, integration process, soil composition, and weed species have all been identified as key factors influencing weed control (Butler et al., 2012; Di Gioia et al., 2016; Guo et al., 2017; Khadka et al., 2020; Shrestha et al., 2018).
The selection of carbon source is the most important component of ASD that may be adjusted or studied to improve its efficacy in controlling yellow nutsedge. Molasses is a biostimulant, commonly used carbon source in combination with composted poultry litter to facilitate ASD in Florida (McCarty et al., 2014). Molasses can be used in combination with other allelopathic carbon amendments to increase ASD effectiveness. Mustard (Brassica sp.) seed meal, a byproduct of the oil extraction process, contains a class of secondary plant metabolites called glucosinolates. Allelopathic compounds such as isothiocyanates (ITC) form by the enzymatic degradation of glucosinolates, which suppress certain weed species in aerobic soil conditions (Petersen et al., 2001). These ITCs are volatile and can be lost from uncovered soil. In anaerobic conditions, polyethylene mulch sealing can help minimize ITC loss, which can result in enhanced weed control efficacy.
Halosulfuron is a systemic sulfonylurea herbicide that inhibits acetolactate synthase, a key enzyme in the biosynthesis of the branch-chained amino acids (Zheng et al., 2008). Although POST applications of halosulfuron exhibit better control of nutsedge species than PRE application, PRE activity has also been documented (Adcock et al., 2008; Dittmar et al., 2012). As an IWM approach, yellow nutsedge control efforts aimed at the soil weed propagule/tuber banks with ASD and subsequent suppressions of new plant and foliage growth with halosulfuron may lead to commercial adoption. Furthermore, for weed control effectiveness, it is interesting to compare the ASD approach, which relies mostly on carbon sources, with herbicide treatment.
The current studies examined the effects of a combination of mixed carbon source and herbicide treatments on yellow nutsedge interference in polyethylene-mulched tomato production. In this research, MMM was used as the carbon source, halosulfuron was used as an herbicide, and the weed control efficacy was compared in aerobic and anaerobic soil conditions in tomato production. The main objective of this research was to identify the optimal carbon source–herbicide combination for yellow nutsedge management in polyethylene mulch tomato production.
Materials and methods
Greenhouse experiments
Greenhouse experiments were conducted at Clemson University Coastal Research and Education Center (CREC), Charleston, SC (lat. 32°79′40.9″N, long. 80°06′83.4″W). Experiments were conducted twice, with trial 1 initiated on 15 Feb 2020 and trial 2 initiated on 18 May 2020. Average daily temperatures of greenhouses were 26 ± 2/19 ± 1 °C (day/night) and 32 ± 2/19 ± 1 °C (day/night) for trials 1 and 2, respectively. The average daylength was 11 h 29 min and 14 h 9 min in trial 1 and 2, respectively.
Experimental setup
The soil used in the study was characterized as Charleston loamy fine sand (thermic Aquultic Hapludalfs) with pH 6.4 and 0.8% soil organic matter. The soil was collected from the surface horizon (0 to 15 cm) at the USDA Organic Crops Unit in Charleston, SC, and passed through a 4-mm sieve. Soil was filled in 5-gal plastic pots with 37-cm height and 30-cm upper diameter (The Home Depot, Atlanta, GA), which were used as the experimental units. The experiments were designed as a randomized complete block design with three replications. The treatments were structured as a factorial with two carbon sources (MMM, no carbon amendment) by three herbicides (halosulfuron PRE, halosulfuron POST, no halosulfuron) and two polyfilm cover (polyfilm covered, not covered). Nontreated, polyfilm covered and nontreated, polyfilm uncovered treatments served as controls.
Treatment applications, oxidation-reduction potential sensors installation, and ASD initiation
Mustard meal (Pescadero Gold Mustard Seed Meal; Farm Fuel Inc., Watsonville, CA) was mixed in the upper 20 cm of soil in the pots at the rate of 2100 kg·ha−1 and liquid molasses (Unsulfured Blackstrap Molasses; North Georgia Still Co., Dahlonega, GA) at the rate of 14,000 L·ha−1. Before application, liquid molasses was diluted with water (1:1 by volume) to ease application and poured onto soils. Rates of carbon amendments were based on the results of preliminary studies optimizing carbon sources for weed control in ASD environment (Singh et al., 2020). In each of the experimental units, 20 yellow nutsedge tubers were placed within the first 15 cm of soil depth.
Oxidation-reduction potential sensors (S550C-ORP; Sensorex, Garden Grove, CA) were installed in the center of all pots at a 15-cm depth to monitor anaerobic soil conditions. A data logging system (CR-1000X with AM 16/32 multiplexers; Campbell Scientific, Logan, UT) was used to record the outputs from the sensors which monitored readings every 30 s and averaged on an hourly basis.
The PRE herbicide treatment, halosulfuron (Sandea; Gowan Company, Yuma, AZ) was applied at a rate of 1 oz per acre with a backpack sprayer (Bellspray Inc., Opelousa, LA) calibrated to deliver 200 L·ha−1 water carrier volume. The boom was equipped with equipped with generic description nozzles (8002VS; TeeJet Technologies, Wheaton, IL) pressurized to 40 psi, spaced 20 inches apart. All treatments containing halosulfuron were applied with a nonionic surfactant at a 0.25% (v/v) ratio. All pots were irrigated to saturation with tap water based on calculated air-filled pore space, covered with totally impermeable film (TIF) black 1.25-mil polyfilm cover (TriEst Ag Group, Greenville, NC) and sealed using heavy-duty rubber bands (Global Industries, Buford, GA). Polyfilm cover was used to limit the supply of oxygen. Pots were arranged in a completely randomized design on greenhouse benches and left undisturbed for the 4-week ASD treatment period. The measured weight of all saturated soil in pots was ≈33 kg. The ASD treatment ended on 15 Mar 2020 in trial 1 and on 18 June 2020 in trial 2, with the removal of the polyfilm cover from pots. Subsequently, POST application treatments of halosulfuron were applied. At the time of POST application, the trial had been going on for 4 weeks, and the yellow nutsedge plants in the control pots were 55 ± 5 cm in height. For POST herbicide treatment, halosulfuron was applied at a same rate as PRE application, immediately after the polyfilm cover was removed from pots.
Data collection
After 4-week period, ASD was ended by removing the polyfilm covers from the pots. In this study, weed infestation was evaluated twice. The first evaluation was conducted immediately after the ASD period, when the polyfilm covers were removed from pots and the second 12 d after POST halosulfuron application. Weed ratings consisted of percent yellow nutsedge control and shoot count per experimental unit. Percent yellow nutsedge was estimated by visual observations on a scale of 0% to 100%, by comparing weed infestation between untreated and treated experimental units in each replication, where 0% weed control refers to untreated treatments and 100% refers to complete weed mortality in a pot.
Field experiments
Field preparation and ASD setup
The field trials were conducted at CREC (lat. 32°79′00.9″N, long. 80°05′96.2″W). The field soil was characterized as Charleston loamy fine sand (thermic Aquultic Hapludalfs) with pH 6.4 and 0.8% soil organic matter. Field trials were conducted twice on adjacent field plots, with trial 1 initiated on 19 June 2020, and trial 2 initiated on 9 June 2021. To prepare raised beds, the field was mechanically disked to break down weeds, improve soil granulation and surface uniformity. Raised beds were prepared using tractor-mounted bed former. Each plot was 15 ft long and divided with a 10-ft buffer zone.
The treatments were structured as a factorial with two carbon sources (MMM, no carbon amendment) by three herbicides (halosulfuron PRE, halosulfuron POST, no halosulfuron) and two polyfilm cover (sealed, unsealed). Nontreated polyfilm sealed and nontreated polyfilm unsealed treatments served as controls. Soil carbon treatments were added to the plots manually and mixed with a tractor-mounted peanut hoe. Then, a tractor mounted plastic bedder and drip tape implement were used to rebed the field plots and covered with a TIF black polyfilm cover (1.25 mil). An initial 5 cm of irrigation was applied to facilitate ASD in the soil. Assigned sealed plots were covered and completely sealed with polyfilm cover and unsealed plots were covered but had poked holes on both sides of beds at 2-ft spacing.
The study included poked holes to compare the effects of ASD vs. non-ASD plots because poking holes in the polyfilm cover allows gas exchange to occur in carbon source–treated plots and atmosphere, preventing the formation of anaerobic conditions. The holes were made with 2-cm-diameter circular wooden sticks. The holes were spaced every 2 ft along the sides and top of the beds. The side holes were the same length on both sides, but the top hole was in the center of four side holes. The top holes were used to transplant tomato after a 4-week ASD period.
Anaerobic soil disinfestation was performed for a 4-week period and then the polyfilm cover sealed beds were punctured with holes. Plots punctured immediately after irrigation were referred to as the unsealed treatment condition, and plots punctured after the 4-week period were referred to as the sealed treatment condition. The sealed plots with applied carbon sources were assumed to have longer durations of anaerobic soil conditions than the unsealed carbon source-augmented plots (Donahoo et al., 2021).
Crop transplantation and management
One week after the ASD conditions were ended, tomato plants were transplanted. Plots with 7-inch in-row spacing had 10 plants per plot. The cultivar transplanted was Red Bounty (determinate) and was selected based on tolerance to southeast U.S. environmental abiotic and biotic stresses. The transplants were irrigated and fertilized daily through the drip tape connected to the centrally controlled irrigation system according to recommendations from the 2020 Southeastern Vegetable Growers Handbook (Kemble et al., 2020). After 2 weeks, staking was done by inserting 6-ft wooden stakes and tying the plants with strings using the Florida weave stacking method (Kelley and Boyhan, 2017). Plants were checked periodically and tied to the stakes depending on the growth. Tomato plants were also pruned (suckered) once per week in the growth period to reduce the number of branches, by removing the suckers grows between the main stem and leafy branch. Due to the determinate nature of the tomato cultivar used, only suckers below the first flower cluster were pruned.
Crop harvesting and data collection
Anaerobic soil disinfestation effects on weed control were recorded by counting yellow nutsedge shoots that pierced the TIF mulch 0, 30, 60, and 90 d after ASD treatment. Weed counts were taken in 4-square-foot quadrats that were randomly placed five times on plots and then the data were averaged. Plant vigor was estimated at four different time intervals of 14, 28, 42, and 56 d after transplantation. Plant vigor was visually accessed with a score of 1 to 10 (where 1 is the least vigorous plot and 10 is the most vigorous plot, which were determined based on plant height). The plots were harvested weekly or biweekly according to the crop harvest conditions between 15 Oct and 30 Nov for both years’ trials. Crop yield was graded and sorted following USDA guidelines (USDA, 1997). Unmarketable fruit were discarded, and the remaining fruit were graded based on diameter as follows: extralarge (2 3/4 to 3 inches), large (2 1/2 to 2 3/4 inches), medium (2 1/4 to 2 1/2 inches), and small (2 to 2 1/4 inches).
Data analysis
Data were analyzed separately for greenhouse and field experiments. All data were subjected to analysis of variance using mixed model methodology (JMP ver. 14; SAS Institute Inc., Cary, NC). Carbon source, herbicide treatment, polyfilm cover, trial, and all interactions between these effects were considered fixed while replication was considered random. In greenhouse and field experiments, data were pooled for both trials when there was no treatment by trial interaction. All data sets were examined for normal distribution with the Shapiro-Wilk and Anderson Darling tests. When necessary, either square root, log, or arcsine square root transformation was used to normalize the data. The weed control, vigor and tomato yield data were transformed. The transformed data were used for statistical interpretation, but the back-transformed data were presented. Means were separated using Tukey-Kramer’ honestly significant difference test.
Results and discussion
Greenhouse experiments
Percent yellow nutsedge control and yellow nutsedge shoot counts were different between trial runs; therefore, data are presented separately. We illustrate the effects and P values of data obtained 12 d after polyfilm cover removal or POST halosulfuron application (Table 1). Redox potential data were pooled across both runs because there was no treatment by trial interaction (Table 2). The complete data sets are provided to show how the weed population looks 0 and 12 d after the polyfilm cover is removed (Tables 3 and 4).
Carbon source, herbicide, and polyethylene film (polyfilm) cover effects on redox potential, yellow nutsedge control, and shoot counts in two greenhouse experiments conducted at Clemson University Coastal Research and Education Center, Charleston, SC. Carbon source treatments were mustard meal + molasses and no carbon source; herbicide treatments were halosulfuron preemergence, halosulfuron post-emergence, and no halosulfuron; polyfilm cover treatments were polyfilm covered and not covered. Data were collected 12 d following a 4-week anaerobic soil disinfestation period. Pot surface area was 0.79 ft2 (0.0734 m2).
Effects of polyethylene film (polyfilm) cover, carbon source, and herbicide treatments on average soil redox potential recorded over a 4-week period of anaerobic soil disinfestation (ASD) in two greenhouse experiments conducted at the Clemson University Coastal Research and Education Center, Charleston, SC. Carbon source treatments were mustard meal + molasses (MMM) and no carbon source; herbicide treatments were halosulfuron preemergence (PRE), halosulfuron post-emergence (POST), and no halosulfuron; polyfilm cover treatments were polyfilm covered and not covered.
Yellow nutsedge control 0 and 12 d after polyethylene film (polyfilm) removal (DAPR) from pots in trials 1 and 2 in the greenhouse conditions at Clemson University Coastal Research and Education Center, Charleston, SC. Carbon source treatments were mustard meal + molasses (MMM) and no carbon source; herbicide treatments were halosulfuron preemergence (PRE), halosulfuron post-emergence (POST), and no halosulfuron; polyfilm cover treatments were polyfilm covered and not covered.
Yellow nutsedge shoot counts 0 and 12 d after polyethylene film (polyfilm) removal (DAPR) from pots in trials 1 and 2 in the greenhouse conditions at Clemson University Coastal Research and Education Center, Charleston, SC. Carbon source treatments were mustard meal + molasses (MMM) and no carbon source; herbicide treatments were halosulfuron preemergence (PRE), halosulfuron post-emergence (POST), and no halosulfuron; polyfilm cover treatments were polyfilm covered and not covered.
Redox potential measurements (anaerobic conditions)
Redox potential readings were recorded on an hourly basis throughout the 4-week ASD period and averaged to quantify typical anaerobic conditions. The redox potential value (<200 mV) was selected as the level below which the soil was considered anaerobic (Butler et al., 2014; Fiedler et al., 2007). The reduction in the redox potential value (<200 mV) implied the consumption of oxygen and formation of anaerobic conditions in the soil. The redox potential in the MMM-amended, polyfilm-covered pots remained below 200 mV for the 4-week ASD period and decreased to an average of –92 mV (Table 2). Similar to MMM-amended, polyfilm-covered pots, the decreased average redox potential observed in the MMM + halosulfuron PRE/POST, polyfilm-covered pots (Table 2). The average redox potential value was >200 mV, increased to 252 mV in the nontreated, polyfilm-covered pots, and 295 mV in the nontreated, polyfilm-uncovered pots (Table 2).
In both trials, the average redox potential in the MMM-amended, polyfilm-covered pots reduced to negative values, indicating the development of severely reduced or strong anaerobic soil conditions. Our results are consistent with previous studies, which reported that addition of carbon source in ASD rapidly decreased redox potential values and reached levels indicative of anaerobic soil conditions (Blok et al., 2000; Momma, 2008). Accumulated anaerobic conditions are reported as a key indicator for successful weed control in ASD (Guo et al., 2017). Our findings suggests that higher anaerobic soil conditions were achieved in treatments containing MMM and polyfilm cover, compared with all other treatments, and the addition of halosulfuron PRE to MMM and polyfilm cover did not impact anaerobic conditions during ASD.
Weed control
Yellow nutsedge percent control
Carbon source, polyfilm cover, herbicide, and all their interactions had a significant effect on percent yellow nutsedge control in both trials (P < 0.001; Table 1). In both trials, MMM with polyfilm combined with PRE or POST halosulfuron resulted in at least 98% control of yellow nutsedge (Table 3). Similar yellow nutsedge control was observed in the MMM polyfilm-covered treatment. PRE halosulfuron application, along with MMM and polyfilm cover, improved yellow nutsedge control (98% and 100% in trials 1 and 2; Table 3) compared with only PRE halosulfuron application and polyfilm cover treatment (77% and 82% in trials 1 and 2, respectively; Table 3).
Polyfilm cover significantly influenced weed control in contrast to non–polyfilm cover treatments in both trials; however, the effect was more profound in trial 2, which were subjected to higher temperatures (Table 3). Additionally, the increased efficacy of polyfilm cover alone and with MMM in trial 2 may also be associated with daylength in the greenhouse and subsequent temperature increase (Guo et al., 2017). Yellow nutsedge control decreased in nonamended polyfilm covered treatments from 0 to 12 d after polyfilm cover removal, while the MMM-amended treatments remained constant with no further shoot emergence, which might be a possible indication that ASD is providing permanent tuber mortality (Table 3). However, the studies were not designed to recover tubers from the soil after ASD, which is one of the main limitations. In both trials, halosulfuron did not improve weed control for MMM polyfilm cover treatments. MMM + halosulfuron PRE or POST polyfilm-cover treatments were similarly effective as MMM polyfilm-cover treatments, which indicates that carbon source was more important for yellow nutsedge control in this experiment, likely because of the promotion of anaerobic conditions in polyfilm-cover treatments. Thus, MMM polyfilm-cover treatments may be used in place of halosulfuron, which may be more advantageous in addressing the growing concerns regarding herbicide resistance in weed management.
Yellow nutsedge shoot counts
In trial 2, more vegetative growth/shoot counts of yellow nutsedge were observed in the non–polyfilm-cover treatments, which can be explained as the effects of increased daylength. Daylength is reported as the main factor that stimulates growth of yellow nutsedge; short photoperiods stimulate reproductive growth, and long photoperiods stimulate vegetative growth (Jansen, 1971). In trial 1, plants were grown with photoperiods of 9 to 12 h, and therefore, less vegetative growth was observed; in trial 2, photoperiods were longer than 12 h, and therefore, more vegetative growth was observed.
Similar trends to percent control were observed in yellow nutsedge shoot count assessments. Carbon source, polyfilm cover, herbicide, and all interactions between them had affected yellow nutsedge shoot counts in both trials (Table 1). In trial 1, yellow nutsedge shoot counts were reduced with MMM and polyfilm covering with or without halosulfuron compared with controls (nontreated, polyfilm-covered and nontreated, polyfilm-uncovered; Table 4). Interestingly, in trial 2, no nutsedge plants were observed in the MMM and polyfilm covering with or without halosulfuron. POST halosulfuron application following polyfilm cover removal significantly suppressed yellow nutsedge shoot emergence relative to the controls.
PRE halosulfuron polyfilm-covered treatment resulted in similar (trial 1) or increased (trial 2) yellow nutsedge shoot emergence compared with MMM polyfilm covered treatment (Table 4). These trends might be explained by the higher anaerobic conditions generated in both trials with the MMM polyfilm covered treatments (Table 2). Soil temperature (Butler et al., 2014; Momma et al., 2013; Muramoto et al., 2016) plays a synergetic role in achieving ASD-mediated pest management, leading to increased weed control, as was observed in trial 2 (Table 4). The temperature difference between the two greenhouse trials posed a constraint in this study. Due to the COVID-19 lockdown, access to greenhouse operations was restricted, which compelled us to relocate the study to another available greenhouse with a malfunctioning temperature control, resulting in an average 4 °C temperature difference between the two trials.
Field experiment
Weed control
Treatment by year interaction was significant (P < 0.05), so the results from 2 years are presented separately. Both year studies were conducted on field plots heavily infested with yellow nutsedge. Weed control is estimated based on the number of yellow nutsedge plants that emerged from puncturing polyfilm cover and tomato planting holes. Yellow nutsedge plants emerged by puncturing the polyfilm cover and from planting holes and were counted with 4-square-foot quadrats at 0, 30, 60, and 90 d after treatment (DAT).
In the 2020 field trial, at 90 DAT, sealed polyfilm-covered plots amended with MMM had an average of three yellow nutsedge shoots, whereas unsealed polyfilm-covered nonamended plots had an average of 44 yellow nutsedge shoots, indicating a higher level of weed control in ASD (Table 5). Similarly, in the 2021 field trial, at 90 DAT, sealed polyfilm-covered plots amended with MMM had an average of four yellow nutsedge shoots, whereas unsealed polyfilm-covered nonamended plots had an average of 52 yellow nutsedge shoots (Table 6). These findings imply that the ASD with MMM may provide the acceptable or commercial level of weed control in polyethylene-mulched tomato production. ASD with MMM can be seen as a potential organic weed-management technique. Previous research has advised organic growers to avoid fields infested with nutsedge (Bangarwa et al., 2008). However, the findings of this study suggest that ASD with MMM may be used to control heavy yellow nutsedge infested farms before attempting organic production in polyethylene mulched vegetable production.
Yellow nutsedge shoot counts 0, 30, 60, and 90 d after treatment (DAT) of a 4-week anaerobic soil disinfestation (ASD) during field trial at Clemson University Coastal Research and Education Center, Charleston, SC, in 2020. Carbon source treatments were mustard meal + molasses (MMM) and no carbon source; herbicide treatments were halosulfuron preemergence (PRE), halosulfuron postemergence (POST), and no halosulfuron; polyethylene film (polyfilm) cover treatments were polyfilm sealed and unsealed. Field plot dimensions were 15 × 3 ft (4.57 × 0.91 m).
Yellow nutsedge shoot counts 0, 30, 60, and 90 d after treatment (DAT) of a 4-week anaerobic soil disinfestation (ASD) during field trial at Clemson University Coastal Research and Education Center, Charleston, SC, in 2021. Carbon source treatments were mustard meal + molasses (MMM) and no carbon source; herbicide treatments were halosulfuron preemergence (PRE), halosulfuron postemergence (POST), and no halosulfuron; polyethylene film (polyfilm) cover treatments were polyfilm sealed and unsealed. Field plot dimensions were 15 × 3 ft (4.57 × 0.91 m).
In both years, MMM + halosulfuron PRE or POST sealed polyfilm cover treatments were similarly effective as MMM sealed polyfilm cover treatment and greatly reduced yellow nutsedge populations than all other treatments tested in the study (Tables 5 and 6). Under the conditions of this study, these results suggest that the use of halosulfuron with MMM in ASD may not be beneficial because MMM alone is reported to be similarly effective to reduce yellow nutsedge interference. However, we optimized halosulfuron at two time intervals, either PRE-ASD or POST-ASD. Additional research with herbicides or other weed-management techniques, followed by an ASD program late in the season may be effective to control yellow nutsedge.
In both years’ field trials, there were no differences in yellow nutsedge counts observed between halosulfuron PRE- and POST-emergent treated plots when the polyfilm cover was sealed or unsealed (Tables 5 and 6). In contrast, significantly lower yellow nutsedge counts were observed in MMM sealed polyfilm cover plots than in unsealed polyfilm cover plots, highlighting the importance of MMM as a carbon source in ASD in providing effective weed control. The carbon source in combination with sealed polyfilm cover used in this study, appears to target the weed propagule bank by delivering PRE-emergent herbicidal effects for season-long yellow nutsedge control. These results suggests that ASD with MMM may provide permanent tuber mortality to yellow nutsedge; however, the studies were not designed to recover tubers from the soil after ASD, which is one of the major limitations. Additional studies are required to determine whether ASD with MMM permanently kills weed propagules/tubers or induces dormant weed propagule conditions in the soil.
Our results are consistent with numerous other studies, which have demonstrated that nutsedge tubers are highly susceptible to ASD. For instance, in studies conducted in Virginia, ASD with paper mulch, peanut (Arachis hypogaea) shells, and rice (Oryza sativa) bran as carbon sources resulted in high nutsedge tuber mortality (Liu et al., 2020). Similarly, regardless of the carbon source type used, ASD-treated soil sprouted significantly fewer tubers than untreated soil (Muramoto et al., 2008; Shrestha et al., 2018). Similar to the scenario presented in this study, another study conducted in Tennessee found that combining ASD with a mustard/arugula (Eruca sativa) cover crop, with or without molasses, significantly reduced the number of monocot weeds compared with untreated control (McCarty et al., 2014); the authors concluded that the increased control was due to the chemical properties of the amendments—specifically, the release of isothiocyanates from the mustard/arugula cover crops.
Most studies on the use of mustard meal or other similar soil amendments in ASD have been conducted under the assumption that chemical or physical mechanisms of action are the key determinants of pest control outcome (Blok et al., 2000; Muramoto et al., 2016). Our ASD study with MMM provides confirmation, by either implying or explicitly demonstrating, the involvement of chemical and physical factors in the process. The mechanism involved in weed suppression may vary depending on the target weed and may also vary in time (Mazzola, 2007; Momma et al., 2013) and sequential or combination treatments may result in the intentional modification of the action or elimination of biological entities involved in pest control. Therefore, further research is needed to understand how ASD with mustard meal or other similar amendments, alone or in combination with herbicides, works against the wide range of weeds found in specialty production systems. Detailed research is required to ensure the commercially required levels of weed control previously attained through preplanting soil fumigation with methyl bromide.
Tomato vigor, growth, and yield
The plant vigor ratings indicated that the tomato plants in the MMM unsealed polyfilm cover treatments with or without halosulfuron in both year field trials were more vigorous than those in the other sealed polyfilm cover treatments amended with MMM or halosulfuron (Figs. 1 and 2). The rating done 42 and 56 d after transplantation indicated tomato plants in all sealed polyfilm cover treatments were recovered (Figs. 1 and 2). The MMM sealed polyfilm cover treatments with or without halosulfuron were generally less vigorous 14 and 28 d after ASD; however, later ratings at 42 and 56 d after ASD demonstrated plants recovering and achieving similar tomato vigor.
Average tomato plant vigor estimated over time followed by 4-week anaerobic soil disinfestation (ASD) in 2020 field trial at the Clemson University Coastal Research and Education Center, Charleston, SC. Treatment combinations included factorial of two carbon source [mustard meal + molasses (MMM), no carbon source], three herbicide [halosulfuron preemergence (PRE), halosulfuron postemergence (POST), no halosulfuron] and two polyethylene film (polyfilm) cover (sealed, unsealed). Polyfilm cover was 1.25-mil (0.0318 mm) black color totally impermeable film; MMM was a mixture of mustard meal used at rate of 2100 kg·ha−1 (1873.6 lb/acre) and molasses used at rate of 14,000 L·ha−1 (1496.7 gal/acre); halosulfuron applied PRE-ASD and halosulfuron applied POST-ASD used at rate of 1 oz/acre (70.1 g·ha−1). Plant vigor was visually accessed with a score of 1–10 (where 1 is the least vigorous plot and 10 is the most vigorous plot). Unsealed plots were punctured after ASD treatment and sealed plots 5 weeks later. Data are means of three replications and vertical error bars on data points represent SE.
Citation: HortTechnology 32, 5; 10.21273/HORTTECH05040-22
Average tomato plant vigor estimated over time followed by 4-week anaerobic soil disinfestation (ASD) in 2021 field trial at the Clemson University Coastal Research and Education Center, Charleston, SC. Treatment combinations included factorial of two carbon source [mustard meal + molasses (MMM), no carbon source], three herbicide [halosulfuron preemergence (PRE), halosulfuron postemergence (POST), no halosulfuron], and two polyethylene film (polyfilm) cover (sealed, unsealed). Polyfilm cover was 1.25-mil (0.0318 mm) black color totally impermeable film; MMM was a mixture of mustard meal used at rate of 2100 kg·ha−1 (1873.6 lb/acre) and molasses used at rate of 14,000 L·ha−1 (1496.7 gal/acre); halosulfuron applied PRE-ASD and halosulfuron applied POST-ASD used at rate of 1 oz/acre (70.1 g·ha−1). Plant vigor was visually accessed with a score of 1 to 10 (where 1 is the least vigorous plot and 10 is the most vigorous plot). Unsealed plots were punctured after ASD treatment and sealed plots 5 weeks later. Data are means of three replications and vertical error bars on data points represent standard error.
Citation: HortTechnology 32, 5; 10.21273/HORTTECH05040-22
Tables 7 and 8 provide the yield of different categories (tonnes per hectare) for 2020 and 2021, respectively. Marketable yield in 2020 was generally higher than the 2021 trial. Total marketable yield, including extralarge and large fruit, was significantly influenced by factor carbon source (P < 0.001) for both years. Only total marketable yield was impacted by carbon source × herbicide interaction (P < 0.05) for both years, and the main factors, herbicide and polyfilm cover × herbicide interactions, were significant (P < 0.05) in 2021 trial. However, the total marketable yield including large or extralarge grade tomato fruit yield were unaffected by any other main factor or interactions for both years. In 2020, extralarge fruit yields were significantly greater in polyfilm cover sealed treatments amended with MMM alone or with halosulfuron POST application than nontreated polyfilm cover unsealed or control treatment. In 2021, total and extralarge fruit tomato yields of tomato in were significantly higher in MMM alone or with halosulfuron POST application treatments than control. Similar yield was observed in polyfilm-covered unsealed treatment with MMM + halosulfuron POST application.
Tomato yield of different fruit size categories, including extralarge (2 3/4–3 inches), large (2 1/2 to 2 3/4 inches), medium (2 1/4 to 2 1/2 inches), and small (2–2 1/4 inches) grown in field plots after 4-week anaerobic soil disinfestation (ASD) at Clemson University Coastal Research and Education Center, Charleston, SC, in 2020. Carbon source treatments were mustard meal + molasses (MMM) and no carbon source; herbicide treatments were halosulfuron preemergence (PRE), halosulfuron postemergence (POST), and no halosulfuron; polyethylene film (polyfilm) cover treatments were polyfilm sealed and unsealed. Field plot dimensions were 15 × 3 ft.i
Tomato yield of different fruit size categories, including extralarge (2 3/4–3 inches), large (2 1/2 to 2 3/4 inches), medium (2 1/4 to 2 1/2 inches), and small (2–2 1/4 inches) grown in field plots after 4-week anaerobic soil disinfestation (ASD) at Clemson University Coastal Research and Education Center, Charleston, SC, in 2021. Carbon source treatments were mustard meal + molasses (MMM) and no carbon source; herbicide treatments were halosulfuron preemergence (PRE), halosulfuron post-emergence (POST), and no halosulfuron; polyethylene film (polyfilm) cover treatments were polyfilm sealed and unsealed. Field plot dimensions were 15 × 3 ft.i
The potential of ASD to inhibit weed germination appears to be facilitated by phytotoxic volatiles produced by microbial activity in anaerobic soil conditions (Achmon et al., 2017; Liu et al., 2020). On the basis of previous research, it can be speculated that phytotoxicity in our ASD study may be the result of the combined effects of ITC produced by mustard meal breakdown, lowered pH, and anaerobic conditions (Achmon et al., 2017). The production of these phytotoxic volatiles and other weed inhibitory conditions can inhibit crop plant growth, which is a matter of concern for growers. Tomato plants were transplanted into the plots 7 d after ASD termination. Previous studies suggested that aerobic remediation time of at least 2 weeks can eliminate phytotoxins from soil (Achmon et al., 2017); however, it can vary based on the chemical properties of carbon sources.
In both years field trials, unsealed treatment combinations resulted in greater plant vigor and yield than sealed polyfilm cover treatments. The vigor and yield findings from these studies suggests that more time may be required between the end of ASD and crop transplantation, which may allow the soil in the sealed plots to fully recover from phytotoxins and anaerobic conditions (Donahoo et al., 2021). Other approaches, such as air delivery via drip tapes (Wang et al., 2022), may also be optimized if growers are short on crop cycle time for alleviating oxygen level in irrigated or anaerobic soils.
Conclusions
Tubers play an important role in the propagation of yellow nutsedge; therefore, effective control could be achieved by killing all viable tubers. ASD is a preplant soil-based pest-management technique. After integration of ASD, weed populations are expected to decline in subsequent crop-growing seasons, potentially reducing the need for multiple herbicide applications and enhancing the longevity of polyethylene mulches for multiple cropping seasons. The current greenhouse and field studies suggest that ASD with MMM provides improved weed control; however, the residual impacts on crop plant growth is matter of concern for commercial production. Additional research is required to determine the effects of reduced MMM rates, increasing the time interval between ASD termination and plant transplantation, and the safety of other herbicides in tomato when used in combined with ASD.
Units
Literature cited
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