Anaerobic Soil Disinfestation Treatments Research Articles

Effects of Anaerobic Soil Disinfestation for Soilborne Disease and Weed Management on Baby Leaf Lettuce Performance in a High Tunnel Organic Production System

The use of high tunnels, which allows growers to extend their season and improve yields, is increasing in the Southeastern U.S., yet growers face challenges related to weed and disease management, particularly in organic systems. On-station experiments were conducted during fall 2021 and spring 2022 in a split-plot design to assess the efficacy of anaerobic soil disinfestation (ASD) in high tunnels for the organic production of direct-seeded baby leaf lettuce. Soil treatments (ASD, Compost, and Control) and lettuce types (romaine and oakleaf cultivars) were included in whole plots and subplots, respectively. The ASD-treated soils received molasses and granular organic fertilizer as carbon and nitrogen sources. The Compost treatment involved the application of yard waste-based compost and the same organic fertilizer, while the Control soils received organic fertilizer only. The ASD treatment period lasted 8 days rather than the typical 21-day period evaluated in Florida. Crop yield, biometrics (leafy dry matter content, specific leaf area, specific leaf weight), and lettuce quality attributes (leaf color, soluble solids content, total titratable acidity, ferric reducing antioxidant power, ascorbic acid content, total phenolics) were assessed following harvest. In both trials, numerical differences in lettuce fresh weight yield between soil treatments were evident, though not statistically significant. Differences in leaf quality attributes were driven by lettuce cultivar rather than soil treatments. Bottom rot incidence caused by Rhizoctonia solani was reduced by 93% and 87% in the ASD-treated plots compared with the Compost- and Control-treated soils during the spring 2022 trial. The ASD-treated soils had a reduced population density of broadleaf weeds in both baby leaf lettuce production trials. The current study presents novel evidence of the potential of integrating ASD into HT organic production systems without reducing the yield and quality attributes of direct-seeded baby leafy greens compared with common grower practices.

Brewer’s Spent Grain with Yeast Amendment Shows Potential for Anaerobic Soil Disinfestation of Weeds and Pythium irregulare

Anaerobic soil disinfestation (ASD) is a promising alternative to chemical fumigation for controlling soilborne plant pathogens and weeds. This study investigated the impact of brewer’s spent grain (BSG), a locally available carbon source, on various weed species and the oomycete pathogen Pythium irregulare in ASD. Two greenhouse studies were conducted using BSG and yeast at full and reduced rates in a completely randomized design with four replicates and two runs per study. In both studies, ASD treatments significantly decreased the seed viability of all weed species and the Pythium irregulare inoculum, while promoting higher cumulative anaerobicity compared to the non-treated control. The addition of yeast had a notable effect when combined with BSG but not with rice bran. When used in reduced carbon rates, yeast supplementation enhanced the efficacy of BSG, providing comparable control to the full rate for most weed species, including redroot pigweed, white clover, and yellow nutsedge. Interestingly, no ASD treatment affected the soil temperature. Furthermore, BSG treatments caused higher concentrations of volatile fatty acids compared to ASD with rice bran and the non-treated control. This finding suggests that the inclusion of yeast in ASD shows potential for reducing the carbon input required for effective soil disinfestation.

Response of Strawberry Fruit Yield, Soil Chemical and Microbial Properties to Anaerobic Soil Disinfestation with Biochar and Rice Bran

Organic materials added to soil create anaerobic conditions that can reduce soil-borne pathogens that reduce the yield and quality of agricultural crops. Anaerobic soil disinfestation (ASD) requires relatively large quantities of readily available, inexpensive organic materials. We evaluated the impact of ASD with rice bran and biochar organic materials on changes to the soil’s physicochemical properties, microbial taxa, and strawberry fruit yield. We found that the organic materials applied at different dose rates significantly increased the control effect of the soil Fusarium spp. and Phytophthora spp. to 69–99% and 63–98%, respectively. In addition, ASD significantly increased soil organic matter and ammonium nitrogen contents. Strawberry yield also increased significantly after ASD treatment with biochar applied at 10 t/ha, which was positively correlated with increased soil nutrients and a significant reduction in pathogens. High-throughput gene sequencing showed that ASD significantly increased the abundance of some beneficial microorganisms such as Bacillus, Pseudomonas, and Mortierella, possibly due to changes in the soil’s physicochemical properties that favored their survival. We found for the first time that biochar applied at 10 t/ha could create anaerobic conditions that effectively reduced soil-borne pathogens and increased crop yield.

Carbon Sources for Anaerobic Soil Disinfestation in Southern California Strawberry

Anaerobic soil disinfestation (ASD) has been adopted in over 900 ha in California strawberry production as an alternative to chemical fumigation. Rice bran, the predominant carbon source for ASD, has become increasingly expensive. In 2021–22 and the 2022–23 field studies, we evaluated 20–30% lower-priced wheat middlings (Midds) and dried distillers’ grain (DDG) at 21,800 kg ha−1 (in 2021) and 17,000 kg ha−1 (in 2022) as alternative carbon sources to rice bran. The study was placed at Santa Paula, California in September of each season in preparation for strawberry planting in October. Soil and air temperatures were 18–26 °C during that time. After the incorporation of carbon sources into the top 30 cm of bed soil, beds were reshaped, and irrigation drip lines were installed and covered with totally impermeable film (TIF) to prevent gas exchange. Beds were irrigated to saturate the bed soil within 48 h after TIF installation. Anaerobic conditions were measured with soil redox potential (Eh) sensors placed at 15 cm depth in all plots. Both DDG and Midds plots maintained Eh at −180 to 0 mV during the two ASD weeks, while untreated soil was aerobic at 200 to 400 mV. Permeable bags with inocula of Macrophomina phaseolina, a lethal soil-borne pathogen of strawberry, and tubers of a perennial weed Cyperus esculentus were placed 15 cm deep in the soil at ASD initiation and retrieved two weeks later for analyses. Two weeks after that, holes were cut to aerate beds and ‘Victor’ or ‘Fronteras’ bare-root strawberries were transplanted into them. ASD with DDG reduced viable microsclerotia of M. phaseolina by 49% in the first season and 75 to 85% with both carbon sources in the second season. Both ASD treatments reduced tuber germination of C. esculentus 86–90% compared to untreated soil in one of two years. Additionally, Midds and DDG provided greater sufficiency of plant-available nitrogen and phosphorus compared to untreated soil with synthetic pre-plant fertilizer and improved fruit yields by 11–29%. ASD with these carbon sources can suppress soil pathogens and weeds and help sustain organic strawberry production in California.

Integrated Pathogen Management in Stevia Using Anaerobic Soil Disinfestation Combined with Different Fungicide Programs in USA, Mexico, and Paraguay

Stevia is a semi-perennial crop grown to obtain the diterpene glycosides in its leaves, which are processed to manufacture non-caloric sweeteners. Anaerobic soil disinfestation (ASD) and fungicide application were evaluated for the management of stevia stem rot (SSR) and Septoria leaf spot (SLS) in lab and field experiments. In 2019 and 2021, experiments using carbon sources for ASD were carried out in microplots at NCSU (Clayton, NC, USA). In 2020/21 and 2021/22 seasons, field experiments were conducted at CSAEGRO, Mexico (MX) and CEDIT, Paraguay (PY) using a 2 × 3 factorial design with two ASD treatments and three fungicide treatments. ASD treatments included soil amended with cornmeal (MX) or wheat bran (PY) at a rate of 20.2 Mg ha−1, molasses at 10.1 Mg ha−1, and non-amended controls. Fungicide applications included chemical (azoxystrobin), organic (pyroligneous acid, PA), and a non-treated control. ASD was effective in reducing sclerotia viability of Sclerotium rolfsii in laboratory assays (p < 0.0001) and microplot trials (p < 0.0001) in NC. During field trials, the viability of sclerotia was significantly reduced (p < 0.0001) in soils amended with cornmeal + molasses or wheat bran + molasses as carbon sources for ASD. While there was no significant effectiveness of ASD in reducing SLS in 2020 and 2021 or SSR in MX 2020 field trials (p = 0.83), it did exhibit efficacy on SSR in 2021 (p < 0.001). The application of fungicides was significantly effective in reducing SSR (p = 0.01) and SLS (p = 0.001), with azoxystrobin being the most consistent and PA not being statistically different from the control or azoxystrobin. The effects of ASD on fresh yield were inconsistent, exhibiting significant effects in Mexican fields in 2020 but not in 2021. During Paraguayan field trials, ASD only significantly interacted with fungicide applications in the dry yield in 2022. In the 2020/21 MX and 2020 PY field trials, fungicides were significantly effective in enhancing dry leaf yields, with azoxystrobin showing the highest consistency among treatments and PA variable control. In conclusion, utilizing ASD alongside organic fungicides can be a valuable tool for stevia farmers when the use of chemical fungicides is limited. Further research is required to enhance consistency and reduce the costs associated with these treatments under diverse edaphoclimatic conditions.

Weed Management by In Situ Cover Crops and Anaerobic Soil Disinfestation in Plasticulture

Weeds negatively affect organic vegetable crop growth and profitability. Weed management is the greatest challenge for vegetable organic growers since control options are limited for organic vegetable production. Anaerobic soil disinfestation (ASD) is a novel non-chemical pest management technique that creates anoxic conditions in the topsoil layer for a limited time. ASD is primarily based on the addition of labile carbon sources to topsoil to promote anaerobic conditions driven by microorganisms in moist soil mulched with polyethylene film (polyfim). Field studies were conducted in the summer–fall of 2020 and 2021 to determine the efficacy of warm season cover crops used as carbon sources for ASD and their role in weed management. The study used a factorial experimental design with four cover crop residue treatments (sorghum-sudangrass, sunn hemp, both, or none) in two soil aeration conditions (aerated or non-aerated). Cover crops were grown for 75 days, incorporated into the soil, and sealed with totally impermeable film (TIF) clear mulch, followed by a 4-week ASD process. All incorporated cover crop treatments in non-aerated conditions generated moderate to higher anaerobic conditions (0–150 mV) and provided significantly higher (p < 0.05) weed control than all the other treatments tested or controls. Tomato plants transplanted in non-aerated, cover crops incorporated plots were more vigorous and produced higher yields than aerated plots. No phytotoxicity was observed on tomato plants following ASD treatment in any of the treatments tested. This study demonstrated that warm season cover crops could potentially serve as a carbon source for ASD in organic tomato production.

Use Optimization of Organic Wastes in Anaerobic Soil Disinfestation against Strawberry Charcoal Rot Root

The imminent removal of agrochemicals in Europe is directly affecting the strawberry sector in Spain, specifically in the Huelva province. Huelva is currently the main producer in the country. The lack of efficient fumigants has led to the rise of soilborne diseases. Macrophomina phaseolina (the causal agent of charcoal rot in strawberries) is generating serious problems with strawberry farmers. This work proposes to optimize the process of anaerobic soil disinfestation (ASD) against M. phaseolina in strawberry crops as a more sustainable alternative than chemical fumigation. Two trials with different inoculum doses were conducted, amending the soil with rice bran and residual strawberry extrudate as the carbon source for the ASD to evaluate the propagule reduction. After the ASD, these soils were used to crop strawberries in a growth chamber under controlled conditions to assay the disease reduction. Strawberry extrudate and rice bran favored disease reduction over the control, equaling the efficacy of Metam Sodium or even improving it in some cases. Trichoderma spp. and microbial activity could influence the suppressive effect of the ASD. All amended ASD treatments reduced the M. phaseolina propagules and charcoal rot severity. Rice bran and residual strawberry extrudate are suitable amendments to be used in ASD against the pathogen. The resulting soil disinfestation against M. phaseolina with residual strawberry extrudate at 16.89 t/ha and 25 days was similar to the most effective treatment with rice bran (20 t/ha and 40 days) based on the disease foliar severity. Additionally, both treatments were equivalent in efficacy to the Metam Sodium treatment.

Impact of anaerobic soil disinfestation on seasonal N2O emissions and N leaching in greenhouse vegetable production system depends on amount and quality of organic matter additions

Greenhouse vegetable production (GVP) systems in China receive excessive amounts of fertilizers (>1500 kg N ha−1 yr−1) and irrigation (>1200 mm yr−1), which results in severe soil degradation. Moreover, soil borne diseases are common as the same crop is planted continuously over years. Anaerobic soil disinfestation (ASD) is a method carried out every 3–4 years during the summer fallow period to combat soil-borne diseases and to improve soil health. The standard ASD practice, which is carried out before the cropping season, involves incorporation of organic matter (i.e. rice shells or straw) into the soil, covering of the soil with plastic films and soil irrigation until saturation. However, many farmers incorporate large amounts of organic nitrogen fertilizer for priming ASD. In this study, we investigated if incorporation of rice shells plus chicken manure (ASD+RM; farmers practice) provokes higher environmental N losses (N2O emissions and N leaching) during the ASD and the following tomato crop growing period as compared to the standard ASD practice (ASD+R: only rice shells) or a Control (fallow, but with incorporation of organic manure, standard in non-ASD years). Results showed that ASD+RM increased seasonal (ASD/fallow period plus tomato crop growing period) soil N2O emissions by a factor of 3 (ASD+RM: 14.1 kg N2O-N ha−1; ASD+R: 4.7 kg N2O-N ha−1), with 2/3 of emissions occurring during the 25 days long ASD period. Across all treatments, nitrate (NO3−) leaching dominated total N leaching (75%), with significantly lower rates observed for ASD+R as compared to ASD+RM. For both ASD treatments, total dissolved organic nitrogen (DON) leaching was a factor of two higher than for the Control. Crop productivity was not affected by ASD. Our findings imply that ASD+RM should be abandoned as the additional supply of manure N results in high environmental N losses without further increasing yields.

Amendment Properties Affect Crop Performance, Leaf Tissue Nitrogen, and Soil Nitrogen Availability Following Soil Treatment by Anaerobic Soil Disinfestation

Efficacy of anaerobic soil disinfestation (ASD) for soilborne plant pathogen suppression is strongly influenced by soil environment and organic amendment attributes. At the same time, these factors influence soil nutrient availability, crop nutrition, and crop performance, but published information on ASD amendment property effects, including carbon to nitrogen (C:N) ratio and C substrate bioavailability, on crop performance and soil nutrient availability is limited. We evaluated ASD amendment effects on soil N availability, crop N status, and solanaceous crop performance in a series of trials: (1) greenhouse/growth chamber study of amendments (primarily molasses/soybean hulls and wheat bran) formulated at 10:1, 20:1, 30:1 and 40:1 C:N ratios (4 mg C g−1 soil), (2) field study with molasses/soybean hull-based amendments at equivalent C:N ratios/C rates (3) on-farm study with molasses/soybean hull-based amendments (4 mg C g−1 soil) compared to grower-standard control, and (4) field study of labile to recalcitrant amendment substrates at 30:1 C:N ratio (~3.4 mg C g−1 soil). ASD amendment C:N ratio strongly influenced soil inorganic N and the lowest (10:1) ratio was associated with highest soil inorganic N at ASD treatment termination in both trials 1 and 2, which often persisted into the cropping phase. Accordingly, the lowest amendment C:N ratio was also associated with the highest biomass (trail 1), leaf tissue N (trial 2), and crop yield (trials 1, 2) among treatments, even with application of recommended fertigation rates to all treatments in the field study. In trial 3, ASD treatment induced higher soil inorganic N and crop yield than the control, but no differences were observed in plant tissue N. In trial 4, more decomposable ASD substrates reduced soil inorganic N at ASD treatment termination, with the highest soil inorganic N associated with the most recalcitrant amendment, but there was no effect on crop yield. ASD amendment C:N ratio, and to a lesser extent, amendment decomposability, exert a strong influence soil inorganic N and crop performance. Optimization of ASD treatments for disease management will require simultaneous optimization of crop nutrition practices to facilitate more holistic, less confounded assessment of crop performance and to facilitate recommendations for grower adoption.

Anaerobic Soil Disinfestation Efficacy Against Fusarium oxysporum Is Affected by Soil Temperature, Amendment Type, Rate, and C:N Ratio.

A meta-analysis of anaerobic soil disinfestation (ASD) efficacy against Fusarium oxysporum and F. oxysporum f. sp. lycopersici was conducted emphasizing effects of environment and organic amendment characteristics and pot and field studies conducted on ASD amendment C:N ratio and soil temperature effects on F. oxysporum f. sp. lycopersici inoculum survival. In a pot study, two organic amendments, dry molasses-based or wheat bran-based, applied at 4 mg of C/g of soil, with 40:1, 30:1, 20:1, and 10:1 C:N ratios, were evaluated against F. oxysporum f. sp. lycopersici at 15 to 25°C. This study was followed by a pot study with temperature regimes of 15 to 25°C and 25 to 35°C and two C:N ratios (20:1 and 40:1), and a field study at 40:1, 30:1, 20:1, and 10:1 C:N ratios, a 30:1 C:N ratio at a lower C rate (2 mg of C/g of soil), and an anaerobic control. Soil temperature >25°C and more labile amendments increased ASD suppression of F. oxysporum/F. oxysporum f. sp. lycopersici in the meta-analysis. In pot studies, F. oxysporum f. sp. lycopersici survival was reduced for molasses-based mixtures at 20:1 and 30:1 C:N ratios compared with wheat bran-based mixtures but not compared with the anaerobic control. At 25 to 35°C, all ASD treatments suppressed F. oxysporum f. sp. lycopersici relative to controls. In the field, all ASD treatments reduced F. oxysporum f. sp. lycopersici survival compared with the anaerobic control, and 4 mg of C/g of soil amendment rates induced higher anaerobic conditions and higher F. oxysporum f. sp. lycopersici mortality compared with the 2 mg of C/g of soil rate. Although amendment C:N ratios from 10 to 40:1 were similarly suppressive of F. oxysporum, lower temperatures reduced ASD effectiveness against F. oxysporum/F. oxysporum f. sp. lycopersici and further work is warranted to enhance suppression at soil temperatures <25°C.

Evaluation of Agricultural Byproducts and Cover Crops as Anaerobic Soil Disinfestation Carbon Sources for Managing a Soilborne Disease Complex in High Tunnel Tomatoes

Anaerobic soil disinfestation (ASD) is a viable option for disease management in tomato production and reduces damage due to a soilborne disease complex consisting of Pyrenochaeta lycopersici, Colletotrichum coccodes, Verticillium dahliae, and Meloidogyne spp. There are plentiful options for ASD carbon sources using byproducts of Midwestern United States agriculture or cover crops, yet these carbon sources have not been evaluated for use in Midwestern settings. Low (10.1 Mg/ha) and high (20.2 Mg/ha) rates of corn gluten meal, distillers dried grains, soybean meal, wheat bran, and dry sweet whey were evaluated as ASD carbon sources in growth chamber and greenhouse bioassays. Cover crops including buckwheat, cowpea, crimson clover, mustard, oilseed radish, sorghum-sudangrass, white clover, and winter rye were evaluated in similar bioassays with one amendment rate (20.2 Mg/ha). Reducing conditions developed in soils regardless of carbon source or rate. Use of high rates of corn gluten meal, distillers dried grains, soybean meal, and wheat bran led to the lowest levels of root rot severity compared to non-treated controls. The higher rate of any byproduct carbon source was always more effective than the lower rate in reducing root rot severity. Use of both rates of soybean meal or corn gluten meal and the high rate of distillers dried grains or dry sweet whey led to significant increases in dry root and shoot biomass compared to controls. For cover crops, ASD with crimson clover, sorghum-sudangrass, white clover, or winter rye amendments reduced root rot severity relative to the aerobic control, but not relative to the anaerobic control. Use of cover crops did not significantly impact plant biomass. A subset of three ASD carbon sources [distillers dried grains, soybean meal, and wheat middlings (midds), all 20.2 Mg/ha] were evaluated in five on-farm ASD trials in high tunnels. Soil temperatures were low during the application period, limiting treatment efficacy. Reducing conditions developed in all soils during ASD treatment, and a moderate but significant reduction in root rot severity was observed following ASD with the soybean meal or wheat midds compared to ASD with distillers dried grains. Tomato yield was not significantly affected by ASD treatment.

Synergy of Anaerobic Soil Disinfestation and Trichoderma spp. in Rhizoctonia Root Rot Suppression

Potential synergy between anaerobic soil disinfestation (ASD) and Trichoderma spp. in suppression of Rhizoctonia root rot in radish was evaluated. A split-plot design with three replications was used; main plots were Trichoderma harzianum T22, Trichoderma asperellum NT25 and a non-Trichoderma control. Subplots were ASD carbon sources wheat bran, molasses, chicken manure, and mustard greens and two non-amended controls: anaerobic (covered and flooded) and aerobic (not covered or flooded). Carbon sources and Rhizoctonia solani inoculant were mixed with soil, placed in pots, and flooded, followed by drenching Trichoderma spore suspensions and sealing the pots in zip-lock bags. After 3 weeks, bags were removed, soil was aired for 1 week and radish “SSR-RR-27” was seeded. Rhizoctonia root rot severity and incidence were lowest in radish plants grown in ASD-treated soil amended with wheat bran, molasses, or mustard greens across all Trichoderma treatments. Disease severity was lower in radish plants treated with NT25 than with T22 or the non-Trichoderma control across all ASD treatments, and in radish grown in ASD-treated soil amended with wheat bran plus NT25 compared to ASD-wheat bran or NT25 alone. Rhizoctonia solani populations were significantly reduced by ASD treatment regardless of carbon source, while Trichoderma populations were not affected by ASD treatment with the exception of ASD-mustard greens. The interactions of either Trichoderma isolate and ASD with most carbon sources were additive, while T22 with ASD-molasses and NT25 with ASD–wheat bran interactions were synergistic in reducing disease severity. One interaction, T22 with ASD-chicken manure was antagonistic. Enhancement of ASD efficacy in suppressing soilborne diseases such as Rhizoctonia root rot by additional soil amendment with Trichoderma spp. during the process appears to be dependent on both Trichoderma isolate and ASD carbon source.

The potential for Fusarium oxysporum f. sp. fragariae, cause of fusarium wilt of strawberry, to colonize organic matter in soil and persist through anaerobic soil disinfestation

AbstractFusarium wilt of strawberry, caused by Fusarium oxysporum f. sp. fragariae, is a disease of primary concern for strawberry production in many countries. Crop rotation and anaerobic soil disinfestation (ASD) have gained recent interest for their potential to contribute to management of this disease. Both techniques involve incorporation of organic matter into soil, which may be utilized by strains of Fusarium that are competitive saprophytes. We show that F. oxysporum f. sp. fragariae can colonize strawberry, lettuce, raspberry, and broccoli leaf tissues, which are sources of organic matter generated during crop rotation. This pathogen increased in soil population density during ASD treatments that did not become anaerobic, possibly as a result of growth on the organic amendment. However, significant population decreases were observed after ASD treatment when at least 100,000 cumulative reduced mV hours occurred in a 14‐day experiment. Post‐ASD abundance of F. oxysporum f. sp. fragariae in soil was negatively correlated with cumulative reduced mV hours. The only treatment that consistently caused disinfestation was exposed to a maximum temperature of 22 °C, which indicates there is potential for developing effective ASD treatments in the cool climates where strawberries are grown. Awareness that F. oxysporum f. sp. fragariae can act as a competitive soil saprophyte should be further investigated for its potential to alter disease outcomes where organic amendments are applied.

Weed Control Assessment of Various Carbon Sources for Anaerobic Soil Disinfestation

ABSTRACT Greenhouse trials were conducted to evaluate the effect of several locally available carbon (C) sources on weed suppression using anaerobic soil disinfestation (ASD). Carbon sources included rice bran, sorghum-sudangrass, cowpea, buckwheat, paper mulch, brewer`s spent grain, waste coffee grounds, and peanut shells applied at 4 mg of C/g of soil. All trials were conducted in containers of 0.2-m height and 0.15-m diameter. The germination of common chickweed, redroot pigweed, white clover, and yellow nutsedge was reduced similarly with all C sources used for ASD. The addition of distiller’s yeast at 10 kg/ha to C sources at 4 mg of C/g of soil provided similar or better weed control than ASD treatments with C sources alone. ASD treatments in all trials reduced weed viability from 50% to 100% compared to the non-treated control. Redox potential in all ASD treatments during the 3-week treatment was lower (more anaerobic) than the non-treated control.

Soil prokaryotes are associated with decreasing Fusarium oxysporum density during anaerobic soil disinfestation in the tomato field

Anaerobic soil disinfestation (ASD) is a chemical-independent method that can reduce pathogens, such as Fusarium oxysporum. Although soil microbes play essential roles in ASD, the relationship between the microbial community structure and disinfestation efficiency remains unclear. Thus for improving our understanding of this relationship, we investigated the changes in the microbial community and pathogen density during an ASD period for 14 days in a greenhouse using three different substrates, as wheat bran, sugar-contained diatoms, and dried molasses. Soil samples were collected at 0, 3, 7, and 14 days after ASD treatment. The pathogen densities were analyzed by real-time polymerase chain reactions. Furthermore, prokaryotic community analysis was conducted by using an Illumina Miseq sequencer, and the factors related to pathogen density were statistically analyzed. The pathogen density rapidly decreased by >90% at 3 days after treatment and then slowly decreased until day 14; however the rate of decrease differed among the substrates. On the other hand, the microbial communities were altered after 3 days but recovered to their original state on day 14. The iron reduction level, microbial diversity, richness, and community structure did not correlate with pathogen density. The operational taxonomic units that drastically negatively correlated with pathogen density were Clostridia and Bacilli, both belonging to Firmicutes. This study showed that the changes in the prokaryotic community as a whole did not correlate to ASD efficiency, whereas changes in the abundance of specific microbes in the community were likely related to disinfestation efficiency.

Influence of rootstock genotype on efficacy of anaerobic soil disinfestation for control of apple nursery replant disease

Apple replant disease (ARD) in production nurseries can negatively impact commercial viability by diminishing tree quality and potentially serving as a source of pathogen inoculum. The current study was carried out to determine the potential for plant genotype to influence anaerobic soil disinfestation (ASD) disease control efficacy. M.9, G.41, and G.935 apple rootstock genotypes were employed. ASD was conducted using orchard grass as the carbon input (10 t ha−1 or 20 t ha −1). Rootstock growth in ASD-treated soils was comparable to that attained in response to soil pasteurization or fumigation with 1,3-dichloropropene/chloropicrin (FUM) in both greenhouse (GT) and nursery field trials (NFT). In GT, Rhizoctonia solani AG-5 root infection and growth performance varied with rootstock genotype and soil treatment. ASD reduced pathogen DNA quantity in roots and improved rootstock growth. Genotype, but not soil treatment, influenced root infestation by Pythium ultimum and R. solani in the NFT. ASD with grass input at 20 t ha −1 improved soil nutrient levels, especially NO3− N, and provided significant weed control in the NFT. Treatments significantly altered composition of the bulk soil and rhizosphere microbiome in GT and these effects were prolonged in ASD-treated soils. In NFT, ASD conducted with orchard grass was uniformly as effective as FUM in the control of ARD and increase in trunk diameter increment, the primary determinant of apple rootstock value. This ASD treatment can be suggested as a potential method for effective control of nursery replant disease across rootstock genotypes varying in disease tolerance.

Evaluation of different carbon sources for anaerobic soil disinfestation against Rhizoctonia solani on lettuce in controlled production systems

Effects of anaerobic soil disinfestation (ASD) on Rhizoctonia solani basal rot of lettuce were assessed considering: two soil types; different C-sources; different temperature regimes; two treatment durations; and two lettuce crop cycles, in the presence of a high disease incidence from artificial infestation with the pathogen. C-source, temperature, and incubation period, and their interaction, affected the efficacy of the ASD treatment for the lettuce–R. solani pathosystem, with differences depending on the soil type. Brassica carinata pellets, used as a C-source, reduced incidence of Rhizoctonia basal rot in the first crop cycle by 50 to 69% in a peat soil after 3 weeks of treatment at 21°C, and by 52 to 60% after 3 weeks of treatment at 26 or 31°C, compared to the inoculated and untreated experimental controls without anaerobic conditions. The best disease reduction was provided by B. carinata pellets applied, under anaerobic conditions, to peat soil (79% efficacy) and a sandy loam soil (100% efficacy) kept at 31°C for 6 weeks. Generally, ASD based on B. carinata pellets provided greater disease reduction in the first crop cycle than the second, with the only exception being for results achieved in both soils incubated for 6 weeks at 21, 26 and 31°C. Wild rocket used as a C-source provided the greatest disease reduction (78–83%) on plants grown in peat soil at the first crop cycle after 6 weeks of the ASD treatment at 31°C, while wild rocket provided disease reduction of 29 and 50% when mixed with the sandy-loam soil under the same conditions for 6 weeks. The efficacy of the ASD treatment with compost was improved in the second crop cycle, compared to the first, resulting in the greatest disease reduction (52 and 66% efficacy) reached in the sandy-loam soil incubated for 3 weeks at 21°C, and 63% efficacy in peat soil previously treated for 6 weeks at 26 and 31°C. However, slight reductions in lettuce plant development was generally evident in the first crop cycle, which could be due to differences in efficacy of the tested ASD treatments and to phytotoxicity. The greatest yield from sandy loam soil was obtained for the B. carinata pellets and wild rocket at 26 and 31°C in the second cultivation cycle. Partial efficacy of ASD does not justify the adoption of this method against R. solani on lettuce under intensive crop systems. However, ASD based on Brassicaceae and compost as carbon source applied in a sandy-loam soil may be valuable for reducing R. solani incidence, at lower temperatures than those required for soil solarisation or biosolarisation.

Application sequence modulates microbiome composition, plant growth and apple replant disease control efficiency upon integration of anaerobic soil disinfestation and mustard seed meal amendment

Anaerobic soil disinfestation (ASD) and mustard seed meal (MSM) amendments can provide effective control of soil-borne diseases including apple replant disease. These measures rely on both chemical and biological modes of action to yield effective disease control and their integration may prove beneficial or, alternatively, deleterious to overall treatment efficiency when applied in concert. Potential outcomes of integrating ASD with MSM amendments were assessed by determining the effect of treatment application sequence and ASD carbon source on generation of allyl isothiocyanate (AITC) derived from Brassica juncea seed meal, structure of the rhizosphere and soil microbiome, control of apple replant pathogens, and plant growth. In bioassays conducted using ‘Gala’ apple seedlings, application of ASD or MSM treatments independently was as effective or superior to all integrated treatments for the control of replant pathogens. Application of ASD prior to MSM amendment diminished the yield of AITC attained in response to soil incorporation of the Brassica juncea:Sinapis alba seed meal. Treatment application sequence had significant effect on structure of the bulk soil fungal and bacterial community. Correspondingly, treatment application sequence significantly altered plant growth performance when orchard grass was utilized as the ASD carbon input. At harvest, rhizosphere fungal but not bacterial community composition was significantly altered in treated soil relative to the control, and sequence of treatment application had significant effect on rhizosphere fungal community structure. Failure of integrated treatments to enhance overall replant disease control may have resulted from many factors including reduced generation of active metabolites, diminished activity of mechanisms functional in pathogen suppression, or the elevated accumulation and retention of phytotoxic chemistries, the latter which would require extended plant back periods to circumvent. The findings indicate that under the experimental conditions employed, integration of ASD with MSM amendment is unlikely to yield additive or synergistic effects on apple replant disease control.

Economic Analysis of Anaerobic Soil Disinfestation for Open-field Fresh-market Tomato Production in Southwest and North Florida

With the phase-out of methyl bromide due to its impact on ozone depletion, research has focused on developing alternative chemical and biologically based soil disinfestation methods. Anaerobic soil disinfestation (ASD) was developed to control plant-parasitic nematodes, weeds, and soilborne pathogens. However, whether farmers will adopt ASD methods on a large scale is unknown. This study evaluates the economic viability of using ASD in open-field, fresh-market tomato (Solanum lycopersicum) production, drawing on data from field experiments conducted in 2015 in Immokalee, FL, and Citra, FL. The experiment included three treatments: chemical soil fumigation (CSF), ASD1 [the standard ASD treatment with 1482 gal/acre molasses and 9 tons/acre composted poultry litter (CPL)], and ASD0.5 (the reduced rate ASD treatment with 741 gal/acre molasses and 4.5 tons/acre CPL). Results from the economic analysis show that ASD treatments require higher labor costs than CSF regarding land preparation and treatment application. However, yields from ASD treatments are higher than those resulting from CSF, and the improvement in yield was enough to offset the increased labor costs. Relative to CSF, ASD0.5, and ASD1 achieved additional net returns of $630.38/acre and $2770.13/acre, respectively, in Immokalee, FL. However, due to unexpected conditions unrelated to soil treatments, the net return of ASD1 was lower than that of CSF in Citra, FL. Breakeven analysis indicates that ASD treatments would remain favorable even with an increase in the molasses price. However, when the tomato price is low, ASD could potentially lose its advantage over CSF.

Soil inoculation with Trichoderma asperellum, T. harzianum or Streptomyces griseoviridis prior to anaerobic soil disinfestation (ASD) does not increase ASD efficacy against Sclerotium rolfsii germination

Studies on anaerobic soil disinfestation (ASD), a non-chemical alternative to soil fumigants for controlling plant diseases caused by soilborne pathogens, have shown that ASD increases populations of beneficial microorganisms with activity against plant pathogens, including species of Trichoderma and Streptomyces, which are known to parasitize sclerotia of Sclerotium rolfsii. However, studies on ASD effectiveness paired with inoculation of Trichoderma and Streptomyces are lacking. This study compared the effects of ASD, soil incorporation of antagonists [Trichoderma asperellum, T. harzianum (RootShield®), Streptomyces griseoviridis (Mycostop®) and T. harzianum + S. griseoviridis], and ASD + soil incorporation of antagonists at the initiation of ASD treatment, on sclerotial germination and colonization. The effect of ASD and soil incorporation of antagonists on abundance of Trichoderma spp., actinomycetes, and Bacillus spp. in soil were assessed. In contrast to the negative effect of ASD on sclerotial germination, we observed positive or no effect of ASD alone on populations of beneficial microorganisms evaluated. ASD treatment that included addition of T. asperellum or T. harzianum did not increase colonization of sclerotia by Trichoderma spp. compared to ASD alone, whereas ASD treatment with inoculation of S. griseoviridis did increase sclerotia colonization by actinomycetes. Indigenous soil actinomycete populations were suppressed by incorporation of T. asperellum and T. harzianum during ASD treatment. Soil abundance of T. harzianum was increased by ASD treatment, and further increased by T. harzianum inoculation at ASD treatment initiation. Inoculation of T. asperellum, T. harzianum, or S. griseoviridis at ASD treatment did not further reduce germination of sclerotia of S. rolfsii, indicating that addition of these microorganisms to soil prior to ASD treatment is unlikely to improve ASD treatment efficacy against pathogens with resistant sclerotial survival structures.