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Article

Effectiveness of Anaerobic Soil Disinfestation for Weed and Nematode Management in Organic Sweetpotato Production

by
Simardeep Singh
1,
William Rutter
2,
Phillip A. Wadl
2,
Harrison Tyler Campbell
1,
Churamani Khanal
1 and
Matthew Cutulle
1,*
1
Plant and Environmental Sciences Department, Coastal Research and Education Center, Clemson University, Charleston, SC 29414, USA
2
United States Department of Agriculture, Agricultural Research Service, U.S. Vegetable Laboratory, Charleston, SC 29414, USA
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(9), 1935; https://doi.org/10.3390/agronomy14091935
Submission received: 3 July 2024 / Revised: 21 August 2024 / Accepted: 22 August 2024 / Published: 28 August 2024
(This article belongs to the Special Issue Application of Natural Products for Weed Control in Agricultural Systems)
Browse Figures
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Abstract

:
Weeds and nematodes are particularly problematic in organic sweetpotato production due to a lack of effective pesticides. Anaerobic soil disinfestation (ASD) has the potential to fit into current pest management practices as an alternative to pesticide application. Greenhouse studies were conducted at the Clemson Coastal Research and Education Center (CREC) in Charleston, SC, to investigate the impact of carbon source amendment and a no carbon source treatment, and soil type on cumulative anaerobicity, weed control, nematode population, and sweetpotato vigor. Microcosms were filled with one of three different soil types (Charleston—loamy/native; Blackville—high coarse sand content; and Clemson—high clay content) and were mixed with cottonseed meal (CSM) or no carbon amendment. The pots were then sealed with plastic totally impenetrable film (Tif) for 6 weeks, followed by the transplanting of sweetpotato (cv Bayou Belle) slips. The results suggested that the CSM-treated microcosms spent more time under anaerobic conditions than those treated with the no carbon amendment. The microcosms that experienced a longer duration of anaerobicity had a lower percent weed cover (49%), fewer nematode egg masses, and a lower gall index when compared to microcosms which experienced a shorter duration of anaerobicity. Significantly higher instances of leaf necrosis were observed in the sweetpotato slips sown in the CSM-treated microcosms. The addition of CSM as a carbon source to facilitate ASD resulted in similar above-ground biomasses of the sweetpotato plants compared to the treatments containing no carbon amendment. However, a significantly lower below-ground biomass of the sweetpotato plants was observed in the CSM-treated microcosms.

1. Introduction

Sweetpotato (Ipomoea batatas L.) is a perennial dicotyledonous crop in the family Convolvulaceae that produces edible storage roots [1,2]. It is cultivated throughout the tropics, subtropics, and warmer temperate regions [3] and is the seventh most important food crop, next to cassava, among the root and tuber crops grown throughout the world [4]. Among the organic vegetable crops in the United States of America (USA), sweetpotato ranks fifth in terms of commodity sales [5]. In 2021, organic sweetpotato was planted on 8252 hectares, resulting in a total monetary value of USD 46.3 million [6]. In comparison to conventional production systems, organic sweetpotato production systems’ market price can be up to 52% higher. Despite the added value, organic sweetpotato production is anticipated to have lower yields than conventional systems due to the management challenges of weeds and nematodes [7].
Weed management in sweetpotato production, more specifically in organic production, is challenging. Weeds that grow above the sweetpotato canopy are the most competitive since sweetpotato vines grow along the soil, and the canopy height of sweetpotato plants is frequently less than 0.5 m [8]. Weed species such as yellow nutsedge (Cyperus esculentus L.) and Palmer amaranth (Amaranthus palmer L.) compete for resources with the sweetpotato crop and reduce the yield and quality of its storage roots [9,10]. In 2015, Meyers and Shankle reported that yellow nutsedge with densities of 5 to 90 shoots m−2 reduced the marketable yield of sweetpotato by 6 to 80%. Similarly, Palmar amaranth interferes with sweetpotato growth and reduces sweetpotato yields by 81% [9]. The management of diverse weed flora in USA sweetpotato crops is more difficult as there is a lack of herbicides registered for sweetpotato production; further, the available herbicide options are unable to provide reasonable weed management in sweetpotato crops [11,12]. Because the use of synthetic herbicides is prohibited in organic agriculture, weed control is more difficult and costly in organic sweetpotato production than in conventional production. For weed management, organic farmers rely on a combination of cultural practices such as cultivation and manual weeding [11]. The most common weed management practice among sweetpotato growers is mechanical weeding, which is usually performed with mowers and wiper applicators. The major limitation of using mechanical weeders is that some weeds escape from the mechanical weeders. These escaped weeds can then grow above the sweetpotato crop canopy, where they compete for light with the sweetpotato crop until the escaped weeds are pulled out manually [12]. In the Southeastern USA, hand weeding of sweetpotato fields costs an estimated USD 1260 per ha [13]. Due to the aggressive nature of weeds that invade sweetpotato fields, there is an urgent need to develop alternative weed management strategies that do not utilize synthetic herbicides.
Nematodes are the most abundant metazoans on the planet [14] and constitute a significant portion of soil microbiota [15]. Plant-parasitic nematodes (PPNs) are a serious hazard to agriculture, causing an estimated USD 157 billion in annual productivity loss worldwide [16]. Root-knot nematodes (RKNs) are the most damaging nematodes for vegetable crops [17]. RKNs are classified as obligate endoparasites, infecting the roots of more than 3000 different plant species [18]. The infection caused by RKNs results in gall development on sweetpotato roots, which results in decreased plant vigor, yield, and marketability [19]. Meloidogyne enterolobii, commonly referred to as the guava root-knot nematode, is highly virulent and an emergent species of RKNs that presents a significant agricultural threat due to its global prevalence and the wide range of its host species [20]. Sweetpotato is a suitable host for M. enterolobii. This relationship has the potential to significantly and negatively impact sweetpotato production as all widely grown cultivars are highly susceptible. Meloidogyne enterolobii infection results in the development of distinct galls and cracks on storage roots. These detrimental effects significantly reduce the overall storage capacity and market value of the harvested storage roots. Meloidogyne enterolobii was initially documented in the United States of America (USA) state of Florida [21], and subsequent reports have confirmed its presence in Louisiana, Georgia, South Carolina, and North Carolina [22,23]. The sweetpotato production regions in the Carolinas have experienced significant negative impacts due to the introduction of M. enterolobii, resulting in a relatively rapid spread of this nematode within the sweetpotato production areas [23]. Synthetic chemicals have been employed for the purpose of nematode control, but these nematicides have potential environmental hazards [24]. Several fumigant nematicides, such as ethylene dibromide (EDB), dibromochloropropane, and methyl bromide, have been removed from the market due to their carcinogenic properties [25]. Moreover, application of synthetic chemical nematicides is not an option in organic sweetpotato production. Due to the lack of effective methods for the management of M. enterolobii, there is an urgent need to develop environmentally sustainable alternatives for controlling M. enterolobii in organic sweetpotato production.
Anaerobic soil disinfestation (ASD) is a promising alternative to soil fumigation for organic production. ASD was developed independently in the Netherlands and Japan [26,27,28,29,30]. Using locally available carbon sources, ASD can be used everywhere [31,32]. To initiate ASD, a carbon source is added to the soil and then covered with an impermeable polyethylene film to prevent the exchange of gases. Finally, the soil is irrigated to saturation beneath the film [27,28,29,30,31,32,33]. The decomposition of the carbon sources during ASD induces changes in soil microbial communities that includes the formation of volatile compounds and organic acids and changes in metal concentrations and soil pH, thereby creating unfavorable conditions for many soil-borne phytopathogens [28,29,30,31,32,33,34].
The majority of ASD research has focused on disease management in tomato crops followed by strawberry, potato, and bell pepper crops [34]. No studies have been conducted to study the impact of ASD on weed and nematode control in sweetpotato production. The main objective of this study was to evaluate the impact of ASD on the management of weeds and guava root-knot nematodes in organic sweetpotato production.

2. Materials and Methods

2.1. Greenhouse Experiments

The experiments were conducted at Clemson University’s Coastal Research and Education Center (CREC), located in Charleston, SC, USA (32.79040° N, 80.06083° W). This experimental study was repeated in space and time with the first experiment being initiated on 9 November 2021 and the second experiment initiated on 23 November 2021. Similar average daily temperatures (day/night) and 12 h of light were maintained in both greenhouse experiments.

2.2. Experimental Setup

To evaluate the response of soil type in regard to ASD utilization, surface soil (0 to 15 cm) from three different locations was examined during these experiments (Table 1). Soil from the Clemson University Student Organic Farm (Clemson soil) (34.67428° N, 82.84598° W), CREC (Charleston soil), and Clemson University’s Edisto Research and Education Center (EREC) (Blackville soil) (33.36543° N, 81.32982° W) were used; they consisted of clay, sandy loam, and Wagram sand in texture, respectively, with organic matter contents of 7, 2, and 0.75 percent, respectively.
Plastic microcosms (15 L) (Plastic Pail; ULINE, Pleasant Prairie, WI, USA) were used as the experimental units in this study. After sieving the soil through a 4 mm sieve, the soil was used to fill the microcosms, and a total of eighteen microcosms were used in each experiment. The experiments used a randomized complete block design with three replications, and the treatments were structured as a factorial of three different types of soil (clay, sandy loam, and Wagram sand) with carbon source amendment [cottonseed meal (CSM)] and no carbon source. A no additional carbon treatment served as the control.

2.3. Treatment Setup and ASD Initiation

Individual four-week-old okra seedlings were planted in each experimental unit, followed by inoculation with 10,000 eggs of M. enterolobii. ASD was initiated one month post-inoculation to allow the nematodes to complete one life cycle. CSM was selected as the carbon source to facilitate ASD based on the general local availability for much of the Southeastern USA. CSM was applied at a rate of 10,000 kg ha−1 [35]. Based on the calculated air-filled porosity, all microcosms were irrigated to saturation with tap water, covered with Tif white plastic mulch (TriEst Ag Group, Greenville, NC, USA) to inhibit any exchange of gases with the surrounding environment, and kept secured for six weeks with heavy duty rubber bands (Global Industries, Buford, GA, USA) [35,36,37,38]. Sweetpotato cultivar Bayou Belle was transplanted immediately after ASD was terminated. A single four-node sweetpotato slip was planted in each microcosm in such a way that two nodes were below the soil surface, and two nodes were above the soil surface. Each sweetpotato slip was approximately 30 cm in length. Twenty tubers of yellow nutsedge (Cyperus esculentus L.) and 100 seeds each of carpetweed (Mollugo verticillate L.), large crabgrass (Digitaria sanguinalis L. Scop), and yellow woodsorrel (Oxalis stricta L.) were mixed in the upper 15 cm of the soil before the initiation of ASD [35,36,37,38]. To monitor the anaerobic soil conditions during the ASD period, oxidation–reduction potential sensors (S550C-ORP; Sensorex, Garden Grove, CA, USA) were installed in the center of each experimental unit at a depth of 15 cm. The outputs from these sensors were recorded using a data logger system (CR-1000X with AM 16/32 multiplexers; Campbell Scientific, Logan, UT, USA) [36,37,38,39,40]. Hourly soil redox potential observations were summed across the 6-week ASD period to determine the typical anaerobic conditions.

2.4. Data Collection

Percent weed cover was recorded at 0 and 35 days after the termination of ASD. The weed cover percentage was recorded visually by comparing CSM-treated experimental units with no additional carbon units in each replication on a scale of 0 to 100%, where 0% means no weed cover and 100% represents complete weed cover in each experimental unit [41,42,43]. The individual weed counts of yellow nutsedge (Cyperus esculentus L.), carpetweed (Mollugo verticillate L.), large crabgrass (Digitaria sanguinalis L. Scop), and yellow woodsorrel (Oxalis stricta L.) were also counted at 0 days after the termination of ASD. Necrosis data were recorded at 7, 28, and 35 days after the termination of ASD. The necrosis percentage of the sweetpotato above-ground foliage was visually assessed in each replication with a score from 0 to 100% (where 0% indicates no necrosis, and 100% indicates total plant loss) [44,45]. The experiments were terminated six weeks after the sweetpotato slips were transplanted. Sweetpotato fresh above- and below-ground biomasses were recorded at the time of termination. Second-stage juveniles (J2) were extracted from a sub-sample of 100 cm3 of soil using the centrifugal flotation method [46]. The gall index and number of egg masses per root system of the sweetpotato plants were also observed to evaluate the disease severity and nematode reproduction. At the termination of the experiment, each sweetpotato root was washed and soaked for 5 min in red food coloring to stain nematode egg masses [47]. The stained egg masses were counted under a dissecting microscope. The gall index was visually calculated as the total percentage of the sweetpotato root system with galling [23,48].

2.5. Data Analysis

The data were analyzed using the Mixed Model in JMP ver. 16 (SAS Institute Inc., Cary, NC, USA). Soil type (Blackville, Charleston, and Clemson), carbon source (carbon source amendment and no carbon source), and their interaction effects were considered fixed effects, while replication was considered a random effect. The data were checked for normality using Anderson Darling and Shapiro–Wilk tests. The data were pooled whenever no significant treatment × experiment interactions were indicated (p ≤ 0.05), otherwise, they were presented separately. Redox potential, percent weed cover, individual weed counts, necrosis percentage, J2, and sweetpotato above- and below-ground biomass data from both greenhouse experiments were pooled due to absence of significant experiment × treatment interactions. Whenever necessary, the data were normalized using either square root, logarithmic, or arcsine square root transformation. Weed count (yellow nutsedge and carpet weed), gall percentage, and number of egg masses was transformed and used only for statistical interpretations. Treatment means were separated using Tukey’s HSD test (p ≤ 0.05). The treatment means presented in tables and figures are untransformed values. In figures, the data are presented as mean ± standard error (n = 12) for soil type and as mean ± standard error (n = 18) for carbon source treatments; bars with different letters indicate that the means are significantly different based on Tukey’s HSD test (p < 0.05).

3. Results and Discussion

3.1. Soil Redox Potential

Soil redox potential data from both experiments were pooled as there were no experiment × treatment interactions. The soil type (Blackville, Charleston, and Clemson) did not significantly impact the soil redox potential after 6 weeks of ASD (Figure 1). We had hypothesized that the soil with a higher organic matter content might have greater cumulative anaerobicity. However, our results indicated that the amount of organic matter content in the soil did not influence anaerobicity (Figure 1A). The addition of CSM as the carbon source to facilitate ASD resulted in significantly greater cumulative anaerobicity compared to the no additional carbon source treatment (Figure 1B). The use of CSM resulted in a 74% increase in cumulative anaerobicity compared to the no additional carbon source treatment.
Based on past studies, greater cumulative anaerobic conditions are a critical sign of successful weed and nematode management [31,49]. The occurrence of redox reactions under anaerobic conditions leads to the production of volatile organic compounds (VOCs) and methane, alterations in microbial communities, and a decline in soil pH. These factors collectively exhibit lethality toward weed species and other soil pathogens [36,37,38,39,40,41,42,43,44,45,46,47,48,49,50].

3.2. Impact of ASD on Weeds

3.2.1. Weed Counts

Due to significant experiment × treatment interactions for the weed counts of yellow nutsedge and crabgrass, the data from both experiments are presented separately (Table 2). However, the data on the weed counts of carpetweed and yellow woodsorrel were pooled due to the absence of treatment × experiment interactions.
Different weed species showed different responses to the carbon source treatments. For instance, the yellow nutsedge counts were significantly lower with application of the CSM treatment. The complete control of the yellow nutsedge was observed in the CSM-treated microcosms in experiment 2 only (Table 2). These results indicate the ability of CSM as a carbon source for ASD to control yellow nutsedge in the Charleston soil.
Yellow nutsedge is one of the most challenging weeds to control because it reproduces by both tubers and seeds [51]. Moreover, yellow nutsedge has sharp leaf tips that easily puncture plastic mulch [52], and tubers of yellow nutsedge partially grow in the sweet potato storage roots and degrade their quality [53]. The results of this study indicated that regardless of the soil type, CSM significantly suppressed yellow nutsedge. These results align with those of several other studies that showed that yellow nutsedge is sensitive to ASD. For example, the application of ASD with peanut shells, paper mulch, and rice bran as carbon sources resulted in complete tuber mortality of yellow nutsedge [38]. Whether or not a carbon source was used, a substantial reduction in tuber sprouting of yellow nutsedge was seen in soil that had been treated with ASD compared to the non-ASD controls [54]. Prior research has indicated that crabgrass is not susceptibility to ASD. The results from our study also showed that ASD moderately controlled the shoot count of crabgrass. However, in another study, the crabgrass populations were significantly controlled with ASD using mustard meal as the carbon source [35]. Overall, using CSM as a carbon source for ASD in our study suppressed the total counts of yellow nutsedge. Further research is required to identify the best carbon source for effectively controlling crabgrass populations with ASD in sweetpotato crops.

3.2.2. Percent Weed Cover

The percent weed cover data from both greenhouse experiments were pooled as there were no significant experiment × treatment interactions (Figure 2). The soil type did not significantly influence the percent weed cover at 0 DAT. However, at 35 DAT, the soil type significantly affected the weed cover percentage. At 35 DAT, the percent weed cover was the highest for the Clemson soil (66%) and the lowest for the Charleston soil (25%). The Clemson soil recorded 40 and 62 percent higher weed cover percentages over the Blackville and Charleston soil types, respectively (Figure 2A). From 0 to 35 DAT, an increase of 35, 0.3, and 57 percent weed cover was recorded in the Blackville, Charleston, and Clemson soils, respectively. The rise in percent weed cover over time in all the soil types except the Charleston soil indicated that the effects of ASD decline over time. As a result, weed seeds present in the soil began growing, resulting in a higher weed cover percentage at 35 DAT.
The application of CSM as a carbon source significantly decreased the percent weed cover at 0 and 35 DAT compared to the no additional carbon source treatment (Figure 2B). The weed cover percentage with CSM was 49–50 percent lower at both timepoints compared to the no additional carbon treatment. These findings suggest that CSM can be used for effective weed control. Previous research on ASD has also shown its potential to effectively control weeds. For instance, the use of molasses in combination with mustard meal, chicken manure, corn gluten meal, and mashed sweetpotato resulted in 96%, 89%, 79%, and 75% weed control over the control [35]. In another multiyear and multi-location study, ASD successfully suppressed the weed densities by 85% [55]. The application of goat manure as a carbon source coupled with bio-solarization resulted in a significant reduction in weed density. Specifically, at 45 days after transplanting tomatoes, the weed density decreased by 88% compared to non-covered and non-amended control plots. Similarly, at 165 days after transplanting, the weed density was reduced by 51% in the bio-solarized plots [56].

3.3. Necrosis on Sweetpotato Plants

The necrosis data from both experiments were combined because there was no significant experiment × treatment interaction. Planting sweetpotato slips in different soil types did not significantly impact the percent necrosis (Figure 3A). However, the application of CSM as a carbon source for ASD resulted in significantly higher necrosis on the sweetpotato plants at all growth stages compared to the no additional carbon treatment (Figure 3B). The necrosis percent ranged from an average of 14% at 7 DAT to 39% at 35 DAT, whereas the highest necrosis percent was 42% at 28 DAT with the application of CSM. The higher necrosis on the sweetpotato plants during the initial growth stages with the application of CSM may be due to the various anaerobic compounds produced during ASD. Past studies reported that major anaerobic compounds such as high concentrations of ammonia, carbon dioxide, methane, H2, and various volatile organic compounds are produced during ASD [49,50,57]. The sweetpotato slips were planted just one day after the termination of ASD, which might be the reason for the development of necrotic symptoms. A study conducted on ASD using molasses + sweetpotato as the carbon source showed that the sowing of tomato seedlings immediately after the termination of ASD resulted in stunted growth, yellowing of the leaves, and a lower shoot biomass of the tomato plants compared to sowing the tomato seedlings 14 days after the termination of ASD [35]. In two other studies where mustard meal was used as the carbon source for ASD to suppress plant pathogens in tomatoes and strawberries, phytotoxicity symptoms such as stunted growth and a lower shoot biomass were observed in both crops. The authors stated that the release and subsequent breakdown of isothiocyanates and other anaerobic compounds from the mustard meal reduced plant vigor in both crops [54,58]. The application of either compost at a concentration of 15 t/ha or Brassica carrinata pellets at 15 t/ha as carbon sources for ASD resulted in a reduction in the fresh weight of the lettuce that might be attributed to phytotoxicity to lettuce due to the accumulation of various salts and organic acids that are produced during the ASD process [57]. Similarly, a lower shoot biomass of the lettuce crop was observed due to residual phytotoxicity after ASD with grape wine pomace when the crop was planted immediately after the termination of ASD [59]. Based on the findings of our study and previous studies, more research is needed to find the best transplanting window for sweetpotato slips after the termination of ASD and to elucidate the mechanism by which VOCs could potentially be negatively affecting sweetpotato slips that are transplanted too soon after terminating ASD.

3.4. Nematode Reproduction and Gall Index

3.4.1. Nematode Soil Population Density

The nematode reproduction data from both experiments were combined because of the absence of significant experiment × treatment interactions (Figure 4). Soil type (Blackville, Charleston, and Clemson) and carbon source (cotton seed meal and no additional carbon) failed to exert a significant influence on the J2 of M. enterolobii. This lack of statistical significance may simply have resulted from the low population densities observed across all treatments. However, in past studies, a lower nematode population did result from a combination of high temperature [60], low soil pH, and anaerobic conditions during ASD [30,55]. Further, the production of various volatile compounds during the ASD period might be responsible for lowering the nematode populations across all treatments. Previous studies reported that nematode disease severity decreased following ASD treatment [61,62]. Several mechanisms that take place during the breakdown of organic amendments, including the generation of nematicidal substances such as fatty acids and ammonia, the promotion of antagonistic microbes, and modifications to the soil′s physiology, can suppress nematode populations [63]. In another study, the application of ASD resulted in a decrease in nematode populations [30]. This reduction was due to the production of volatile fatty acids [64]. Additional research is required to evaluate the efficacy of different carbon sources against nematodes.

3.4.2. Nematode Egg Production

Due to significant experiment × treatment interactions, the nematode egg data are presented separately (Table 3). In experiment 1, the number of M. enterolobii egg masses were not significantly affected by the soil type and carbon source, and their interaction also failed to exert a significant influence on nematode reproduction. However, in experiment 2, soil type, carbon source, and their interaction significantly affected the egg mass production (Table 3). The greatest numbers of egg masses were observed with the no additional carbon source treatment in Blackville soil (225), followed by Charleston soil (100), and the lowest number of egg masses were found in Clemson soil (5). However, the application of CSM as a carbon source to facilitate ASD resulted in very low to no egg mass production regardless of the soil type. The lower number of egg masses with cottonseed meal indicates the efficacy of CSM in suppressing the reproduction of M. enterolobii. It is also worth noting that 35 days is a comparatively short time interval to allow for M. enterolobii to infect, complete its life cycle, and produce eggs. We hypothesize that extending the time between planting and experimental completion would likely have increased the nematode populations and galling damage seen across all treatments and may have resulted in more statistical power to detect differences between the ASD treatments.

3.4.3. Root Gall Index

Root gall index is a measure of disease severity due to root-knot nematode infection. Due to significant experiment × treatment interactions, the data on the gall index are presented separately (Table 3). In experiment 1, soil type and carbon source did not significantly influence root galling, and their interaction also remained non-significant. However, in experiment 2, soil type, carbon source, and their interaction significantly affected root galling (Table 3). Regardless of soil type, no galling was observed under the CSM treatment on sweetpotato roots at harvest. Significantly higher sweetpotato galling was found in the Blackville soil with no additional carbon source, followed by the Charleston and Clemson soil types with no additional carbon source. These results indicate that cotton seed meal reduces the severity of the disease caused by M. enterolobii on sweetpotato roots. A study conducted in Florida on bell pepper and eggplant showed that the combined application of solarization with composted poultry manure + molasses resulted in lower galling on the bell pepper and eggplant plants [33]. In another study, using organic amendments combined with solarization resulted in a lower gall incidence on tomato plants [62].

3.5. Sweetpotato Biomass

3.5.1. Sweetpotato Above-Ground Biomass

The sweetpotato above-ground biomass data from both experiments were pooled as there were no significant experiment × treatment interactions. Scrutiny of the data revealed that the above-ground biomass of the sweetpotato slips was not significantly influenced by soil type or carbon source (Figure 5). Transplanting the sweetpotato slips immediately into CSM-treated microcosms resulted in necrotic symptoms on their leaves (Figure 3) due to the phytotoxic anaerobic compounds released during the ASD. These results align with those of Gilardi et al. [57], where Brassica carinata and compost were used as carbon sources for ASD in lettuce crops. A lower fresh weight of the lettuce was observed after ASD in the first crop cycle compared to the control (no ASD). The use of high-nitrogen and carbon amendments as sources for liable carbon in ASD results in the production of ammonia, which is toxic to various pathogens, weeds, and nematodes [65,66,67], but also has a significant negative impact on plant health [68]. In our study, the possible reason for the higher necrosis (Figure 3) and lower above-ground biomass might be linked to the accumulation of salts and organic acids during the ASD treatments. Nevertheless, over time, the sweetpotato plants recovered from the necrosis. This was indicated by the lower percent necrosis observed at 35 DAT (42%) compared to 28 DAT (39%) (Figure 3), and similar above-ground biomasses. Previous studies also showed less vigorous plants under ASD treatments compared to the control treatment [35,43].

3.5.2. Sweetpotato Root Biomass

The impact of ASD on sweetpotato root biomass is presented in Figure 6 The data from both experiments were pooled as there were no significant experiment × treatment interactions. The soil type did not exert a significant influence on the root biomass of the sweetpotato plants (Figure 6A). However, the carbon source treatments significantly influenced the root biomass of the sweetpotato plants. The application of CSM resulted in 59% less root biomass than the no additional carbon treatment (Figure 6B). The higher root biomass observed with the no additional carbon treatment might be due to the significantly lower leaf necrosis observed with this treatment (Figure 3B). The higher necrosis with the CSM treatments might have hindered the accumulation of photosynthates in the roots compared to the no additional carbon treatment. Planting sweetpotato slips immediately after the termination of ASD might be the reason for the higher necrosis. The production of various phytotoxic compounds, lower soil pH, and other weed inhibitory conditions after ASD might be responsible for the lower root biomass of the sweetpotato slips. Similarly, sowing of lettuce immediately after termination of a bio-solarization treatment resulted in low germination, a lower plant biomass, and a stunted root length compared to planting the lettuce after four or eight days after termination of the bio-solarization [59]. In another study, a strawberry yield reduction was observed due to early plant phytotoxicity after ASD with mustard meal as the carbon source [57]. Significant reductions in seedling emergence of beet and lettuce were also seen when these crops were sown after treatment with brassica meals for weed management under organic conditions [69]. Thus, the findings of this study suggest that there is a need to conduct more research on the selection of carbon sources and safe planting times for sweetpotato slips after ASD to avoid any potential phytotoxicity.

4. Conclusions

This research showcased the promising results of ASD for weed and guava root-knot nematode management for organic growers in South Carolina, while highlighting the need for further research examining transplant timing in regard to sweetpotato crop health in ASD settings, and to screen and identify any sweetpotato cultivars with greater post-ASD tolerance. The different soils used in this study resulted in similar cumulative anaerobicity and highlights the ability to create anaerobic soil conditions in a wide range of soil types. ASD with cottonseed meal resulted in reasonable control of weeds and nematode disease suppression. However, significantly higher phytotoxicity was observed after sowing the sweetpotato slips in ASD-treated microcosms. This increase in phytotoxicity in ASD-treated microcosms resulted in a lower below-ground biomass of the sweetpotato crops. Based on the findings of this study, further research is required to identify the safe planting time after ASD treatment for sweetpotato to overcome phytotoxicity symptoms. Research is also required to find the best locally available carbon source that is economically affordable and provides reasonably good control of guava root-knot nematodes so that growers can utilize ASD as an additional pest management strategy in complex organic production systems.

Author Contributions

Conceptualization, M.C.; Methodology, C.K.; Formal analysis, S.S.; Investigation, H.T.C. and M.C.; Resources, W.R., P.A.W. and M.C.; Data curation, M.C.; Writing—original draft, S.S.; Writing—review & editing, W.R., P.A.W., C.K. and M.C.; Supervision, W.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Average cumulative anaerobicity over a 6-week anerobic soil disinfestation period as affected by different soil types (A) and carbon source (B) treatments. The data from both experiments were pooled because there was no experiment*soil type*carbon source interaction; p-values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
Figure 1. Average cumulative anaerobicity over a 6-week anerobic soil disinfestation period as affected by different soil types (A) and carbon source (B) treatments. The data from both experiments were pooled because there was no experiment*soil type*carbon source interaction; p-values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
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Figure 2. Impact of soil types (A) and carbon source (B) treatments on percent weed cover at 0 and 35 days after treatment (DAT) with anerobic soil disinfestation. The data from both experiments were pooled because there was no significant experiment*soil type*carbon source interaction; p-values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
Figure 2. Impact of soil types (A) and carbon source (B) treatments on percent weed cover at 0 and 35 days after treatment (DAT) with anerobic soil disinfestation. The data from both experiments were pooled because there was no significant experiment*soil type*carbon source interaction; p-values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
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Figure 3. Impact of soil types (A) and carbon source (B) treatments on leaf necrosis percent of sweetpotato (Ipomoea batatas) plants at 7, 28, and 35 days after treatment (DAT) with ASD. The data from both experiments were pooled because there was no experiment by soil type × carbon source interaction; p-values (p ≤ 0.05) indicate a significant effect. The lowercase letters in subgraph indicates the significance.
Figure 3. Impact of soil types (A) and carbon source (B) treatments on leaf necrosis percent of sweetpotato (Ipomoea batatas) plants at 7, 28, and 35 days after treatment (DAT) with ASD. The data from both experiments were pooled because there was no experiment by soil type × carbon source interaction; p-values (p ≤ 0.05) indicate a significant effect. The lowercase letters in subgraph indicates the significance.
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Figure 4. Impact of soil types (A) and carbon source (B) treatments on number of J2 per 100 cm3 of soil after anerobic soil disinfestation. The data from both experiments were pooled because there was no experiment*soil type*carbon source interaction; p values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
Figure 4. Impact of soil types (A) and carbon source (B) treatments on number of J2 per 100 cm3 of soil after anerobic soil disinfestation. The data from both experiments were pooled because there was no experiment*soil type*carbon source interaction; p values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
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Figure 5. Impact of soil types (A) and carbon source (B) treatments on sweetpotato (Ipomoea batatas) fresh above-ground biomass (g/microcosm). The data from both experiments were pooled because there was no experiment*soil type*carbon source interaction; p values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
Figure 5. Impact of soil types (A) and carbon source (B) treatments on sweetpotato (Ipomoea batatas) fresh above-ground biomass (g/microcosm). The data from both experiments were pooled because there was no experiment*soil type*carbon source interaction; p values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
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Figure 6. Impact of soil types (A) and carbon source (B) treatments on sweetpotato (Ipomoea batatas) fresh root biomass (g/microcosm). The data from both experiments were pooled because there was no experiment*soil type*carbon source interaction; p values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
Figure 6. Impact of soil types (A) and carbon source (B) treatments on sweetpotato (Ipomoea batatas) fresh root biomass (g/microcosm). The data from both experiments were pooled because there was no experiment*soil type*carbon source interaction; p values (p ≤ 0.05) indicate significant effects. The lowercase letters in subgraph indicates the significance.
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Table 1. Soil properties of three different types of soil used in greenhouse experiments conducted at the Clemson University Coastal Research and Education Center, Charleston, SC, USA.
Table 1. Soil properties of three different types of soil used in greenhouse experiments conducted at the Clemson University Coastal Research and Education Center, Charleston, SC, USA.
Soil Source OriginSoil TextureOrganic Matter (%)Soil pHP
lbs/A		K
lbs/A		Ca
lbs/A		Mg
lbs/A		Zn
lbs/A		Mn
lbs/A		Cu
lbs/A		B
lbs/A		Na
lbs/A
Blackville, SCWagram sand0.756.196896981015.6120.90.65
Charleston, SCSandy loam26.721813810901775272.80.311
Clemson, SCClay73.02613314902224.9491.82.713
Table 2. Effect of soil type and carbon source treatment on weed population after anaerobic soil disinfestation (ASD) in greenhouse experiments conducted at the Coastal Research and Education Center, Clemson University, Charleston, SC, USA. Data on weed counts of yellow nutsedge and crabgrass were not pooled due to presence of a significant experiment*soil type*carbon source interaction. However, data on weed counts of carpetweed and yellow woodsorrel were pooled due to the absence of experiment*soil type*carbon source interactions.
Table 2. Effect of soil type and carbon source treatment on weed population after anaerobic soil disinfestation (ASD) in greenhouse experiments conducted at the Coastal Research and Education Center, Clemson University, Charleston, SC, USA. Data on weed counts of yellow nutsedge and crabgrass were not pooled due to presence of a significant experiment*soil type*carbon source interaction. However, data on weed counts of carpetweed and yellow woodsorrel were pooled due to the absence of experiment*soil type*carbon source interactions.
Soil TypeCarbon SourceWeed Population/Microcosm
Yellow NutsedgeCrabgrassCarpet WeedYellow Woodsorrel
Exp. 1Exp. 2Exp. 1Exp. 2
BlackvilleCotton seed meal0.33 a0.0 b1.0 b0.7 a12.5 a3.3 a
No additional carbon3.33 a2.0 b3.7 ab2.7 a21.0 a2.8 a
CharlestonCotton seed meal0.66 a0.0 b1.7 ab1.0 a1.5 a0.8 a
No additional carbon3.33 a20.0 a1.7 ab5.0 a9.5 a8.5 a
ClemsonCotton seed meal0.66 a0.0 b6.7 a12.0 a8.3 a10.0 a
No additional carbon3.66 a2.3 b3.0 ab6.3 a16.1 a6.5 a
p-value
Soil type0.94620.0037 *0.0273 *0.0262 *0.07210.0992
Carbon source0.0041 *0.0014 *0.70760.95800.0440 *0.5322
Soil type × carbon source0.98170.0037 *0.0353 *0.17300.99730.0662
Means within the same column followed by the same letter are not significantly different based on Tukey’s HSD test (p < 0.05); * on text indicates an interaction and on p values, it indicates significant effects.
Table 3. Effect of soil type and carbon source treatment on gall percentage of sweetpotato (Ipomoea batatas) plants and Meloidogyne enterolobii egg mass after anaerobic soil disinfestation in greenhouse experiments conducted at the Coastal Research and Education Center, Clemson University, Charleston, SC, USA. Data on gall index and egg masses were not pooled due to the presence of significant experiment*soil type*carbon source interactions.
Table 3. Effect of soil type and carbon source treatment on gall percentage of sweetpotato (Ipomoea batatas) plants and Meloidogyne enterolobii egg mass after anaerobic soil disinfestation in greenhouse experiments conducted at the Coastal Research and Education Center, Clemson University, Charleston, SC, USA. Data on gall index and egg masses were not pooled due to the presence of significant experiment*soil type*carbon source interactions.
Soil TypeCarbon SourceGall Index/Root SystemEgg Mass/Root System
Exp. 1Exp. 2Exp. 1Exp. 2
BlackvilleCotton seed meal36.6 a0.0 b70.3 a0.0 c
No additional carbon13.3 a80.0 a20.4 a225.0 a
CharlestonCotton seed meal13.3 a0.0 b66.6 a0.0 c
No additional carbon23.3 a30.0 b21.0 a100.3 b
ClemsonCotton seed meal0.0 a0.0 b0.0 a0.3 c
No additional carbon5.0 a10.0 b16.6 a5.3 c
p-value
Soil type0.36830.0028 *0.52290.0013 *
Carbon source0.8311<0001 *0.9377<0001 *
Soil type × carbon source0.53380.0028 *0.59260.0005 *
Means within the same column followed by the same letter are not significantly different based on Tukey′s HSD test (p < 0.05); * on text indicates an interaction and on p values, it indicates significant effects.
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Singh, S.; Rutter, W.; Wadl, P.A.; Campbell, H.T.; Khanal, C.; Cutulle, M. Effectiveness of Anaerobic Soil Disinfestation for Weed and Nematode Management in Organic Sweetpotato Production. Agronomy 2024, 14, 1935. https://doi.org/10.3390/agronomy14091935

AMA Style

Singh S, Rutter W, Wadl PA, Campbell HT, Khanal C, Cutulle M. Effectiveness of Anaerobic Soil Disinfestation for Weed and Nematode Management in Organic Sweetpotato Production. Agronomy. 2024; 14(9):1935. https://doi.org/10.3390/agronomy14091935

Chicago/Turabian Style

Singh, Simardeep, William Rutter, Phillip A. Wadl, Harrison Tyler Campbell, Churamani Khanal, and Matthew Cutulle. 2024. "Effectiveness of Anaerobic Soil Disinfestation for Weed and Nematode Management in Organic Sweetpotato Production" Agronomy 14, no. 9: 1935. https://doi.org/10.3390/agronomy14091935

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