Unraveling the Interplay: Soil Biogeochemical Factors Shaping the Efficacy of Anaerobic Soil Disinfestation in Suppressing Fusarium Root Rot of Strawberry.

Nontoxic alternatives to chemical soil fumigants for suppressing soilborne pathogens such as Fusarium oxysporum (Fo), one causative agent of strawberry black root rot complex prevalent in the Southeastern United States, are urgently needed. A promising alternative is anaerobic soil disinfestation, in which soil is amended with labile organic materials, irrigated to field capacity, and tarped to induce anaerobic fermentation for a brief period before planting. Pathogen-suppression mechanisms of anaerobic soil disinfestation include anaerobic conditions and generation of reduced metal cations (Fe2+ and Mn2+) and volatile fatty acids (VFAs; e.g., acetic, n-butyric, isovaleric, and others). However, little is known about how the interaction between VFAs, reduced metals, soil texture, and liming influences suppression of Fo. We investigated Fo suppression by VFAs and reduced metal cations in both aqueous and soil-based incubation trials. Inoculum containing Fo chlamydospores was added to aqueous medium containing either 5 or 10 mmol/liter VFAs and either 0.01 or 0.05% (wt/wt) reduced metals. In soil-based incubations, chlamydospore-containing inoculum was applied to sandy, sandy loam, and silty clay soil saturated by solutions containing 10 or 20 mmol/liter VFAs with or without 0.05% (wt/wt) reduced metals. VFAs, particularly in combination with Fe2+ in aqueous solutions and Mn2+ in soils, significantly reduced Fo viability. At the same time, liming and higher soil clay content reduced the effectiveness of VFAs and reduced metals for suppressing Fo, highlighting the influence of soil pH and soil texture on anaerobic soil disinfestation effectiveness.

Impacts of the Ban on the Soil-Applied Fumigant Methyl Bromide.

The loss of the soil fumigant methyl bromide (MeBr) and adoption of soil fumigant alternatives has been challenging for farmers, particularly for those crops in which pathogens previously controlled by MeBr have emerged as significant problems, but it has resulted in some unanticipated benefits for the scientific community and the environment. Applauded as one of the most effective environmental agreements to date, the universally accepted Montreal Protocol on Ozone Depleting Substances has had a significant impact on the environment, reducing the release of halogenated compounds from anthropogenic sources enough to mitigate global warming by an estimated 1.1°C by 2021. The funding associated with various MeBr transition programs has increased collaboration across scientific disciplines, commodity groups, industry, and regulatory agencies. Chemical alternatives and improved application strategies, including the development of gas-retentive agricultural films, coupled with sound efficacy data and grower ingenuity have resulted in the sustained production of many of the impacted crops; although there has been some loss of acreage and value, particularly for Florida fumigated crops, for some, value has continued to increase, allowing production to continue. The loss of a single, broad-spectrum tool for pest control has led to a deeper understanding of the specific pest complexes impacting these at-risk crops, as well as the development of new, biologically based management tools for their control while increasing our understanding of the role of the soil microbiome in pest control and crop production.

Evaluation of Anaerobic Soil Disinfestation to Reduce Soilborne Diseases in Soilless and Soil-Based Substrates for Specialty Cut Flower Production.

Anaerobic soil disinfestation (ASD) is a nonchemical soil treatment where an easily decomposable carbon source is incorporated into soil, which is then irrigated to saturation and tarped to create anaerobic conditions, which prompts shifts in the soil microbiota from aerobes to anaerobes. ASD has been tested successfully for soilborne disease management in a variety of cropping systems but has not been sufficiently investigated in ornamentals. In this study, ASD was evaluated in soil-based and soilless substrates commonly used in specialty cut flower production using two model pathosystems: Rhizoctonia solani-Zinnia elegans and Phytophthora drechsleri-Gerbera jamesonii. Each substrate was mixed with pathogen-infested vermiculite and amended with either wheat bran, tomato pomace, or soybean meal as the carbon source. Amended substrates were incubated at 25°C for 4 weeks and used as growing substrates for the two crops mentioned above, which were monitored weekly for disease development for up to 5 weeks posttransplant. Additional experiments tested the effect of plant age and inoculum concentration in the substrate on ASD efficacy. Results showed that ASD has the potential to be deployed successfully for the control of Rhizoctonia stem rot in both substrates. Conversely, ASD was not effective at controlling Phytophthora crown rot on gerbera daisy in any of the experiments conducted in this study. More research is needed to understand the influence of carbon amendments, inoculum thresholds, and environmental conditions on ASD efficacy.

Assessing the environmental sustainability of different soil disinfestation methods used in solar greenhouse vegetable production systems.

Overuse of fertilizers and irrigation and continuous monocropping is increasingly jeopardizing vegetable production in solar greenhouses as it causes serious soil degradation and the spread of soil-borne diseases. As a countermeasure, the practice of anaerobic soil disinfestation (ASD) has been recently introduced, which is carried out during the summer fallow period. However, ASD may increase N leaching and greenhouse gas (GHG) emissions when large amounts of chicken manure are applied. This study assesses how the use of different amounts of chicken manure (CM) combined with rice shells (RS) or maize straw (MS) affects soil O2 availability, N leaching, and GHG emissions during and following the ASD period. Application of RS or MS alone effectively stimulated long-lasting soil anaerobiosis without major stimulating effects on N2O emissions and N leaching. Seasonal N leaching and N2O emissions were in the ranges of 144-306 and 3-44 kg N ha-1, respectively, and were strongly increasing with increasing rates of manure application. Combining high rates of manure application with the additional incorporation of crop residues further increased N2O emissions by 56 %-90 % as compared to the standard practice of farmers (1200 kg N ha-1 CM). About 56 %-91 % of seasonal N2O emissions occurred during the ASD period, whereas N leaching mainly occurred in the cropping period (75 %-100 %). Our study shows, that for priming ASD incorporation of crop residue is sufficient and that the addition of chicken manure for ASD is not needed and should be reduced or even prohibited as it does not improve yields but stimulates the emission of the strong GHG N2O.

Probiotic Bacteria, Anaerobic Soil Disinfestation, and Mustard Cover Crop Biofumigation Suppress Soilborne Disease and Increase Yield of Strawberry in a Perennial Organic Production System.

Black root rot complex and crown rot of strawberry caused by soilborne fungi limit sustainable strawberry production in the northeastern United States, especially in perennial systems, including matted row and plasticulture. As pathogen populations build up over time in the rhizosphere and infect the root system, feeder roots are pruned, which diminishes nutrient and water uptake and causes stunted plant growth or death. Alternative management options are needed for many organic and small growers who can't use chemical fumigants due to new regulations and potential health hazards. Strawberry plug plants were grown on beneficial microbe-inoculated or uninoculated planting mix followed by transplanting in fruiting field plots that either was biofumigated with mustard cover crop (MCC), anaerobically disinfested (ASD), or left untreated. Different combinations of plug plants and field plot treatments were used to determine the efficacy of individual treatments or synergistic effects from combination treatment. Plug plants were transplanted in pretreated plastic mulched raised beds and grown following a typical organically recommended production system. Plants grown on TerraGrow (TG)-inoculated planting mix showed enhanced plant vigor in the fruiting field compared with untreated plants. Weeds that grew through planting holes were significantly (P ≤ 0.045) suppressed in ASD plots compared with untreated plots in the first year. Plants treated with a combination treatment of TG and ASD had significantly higher fruit yield in both years (2019 and 2020), although the difference was greater in the second year. Plant vigor and survival in treated plots except MCC were also significantly higher in the second year compared with the untreated control. Suppression of pathogenic microbes and plant vigor improvement in treated plots appear to be the factors providing beneficial effects and higher net economic return. Taken together, our results suggest that a combination of beneficial microbes and ASD could be an alternative to synthetic fumigation in a perennial strawberry production system.

Soil Amendment and Storage Effect the Quality of Winter Melons (Benincasa hispida (Thunb) Cogn.) and Their Juice.

Winter melon fruits were grown in the field using anaerobic soil disinfestation (ASD) and conventional fertilizer alone as the control treatment. Fruits were harvested and stored at 20 °C for 120 d, the juice was processed on day one and day 120, and the effects of soil amendment and 120 d storage on the juice's physical and chemical (sugars, acids, volatile and nutritional compounds) properties were evaluated. Fruit juice extracted from ASD-grown fruit had greater magnitude of zeta potential than the control juice, indicating it was physically more stable than the juice obtained from the control conditions. ASD fruit juice had lower soluble solids content (SSC), and lower volatile compounds that contribute green, grass, and sulfur notes, and negatively influence flavor quality. ASD fruit juice had higher vitamin B5 and cytidine. Juice processed from 120 d stored fruit had less yield due to 12.4-15.6% weight loss. The non-soluble solids content was higher and particle size was larger, and the SSC and individual sugars decreased. However, titratable acidity (TA) increased primarily due to increased citric acid. Out of 16 free amino acids, 6 increased and only 1 decreased. However, three out of five nucleosides decreased; vitamins B1 and B6 increased; vitamins B2, B3 and C decreased. Overall, juice derived from fruit produced using ASD was physically more stable and had less SSC and off-odor volatiles than the control, while the fruit juice of those stored for 120 d had lower SSC and higher TA and nutritional profiles, comparable to freshly harvested fruit.

The effect of circular soil biosolarization treatment on the physiology, metabolomics, and microbiome of tomato plants under certain abiotic stresses.

Soil biosolarization (SBS) is an alternative technique for soil pest control to standard techniques such as soil fumigation and soil solarization (SS). By using both solar heating and fermentation of organic amendments, faster and more effective control of soilborne pathogens can be achieved. A circular economy may be created by using the residues of a given crop as organic amendments to biosolarize fields that produce that crop, which is termed circular soil biosolarization (CSBS). In this study, CSBS was employed by biosolarizing soil with amended tomato pomace (TP) residues and examining its impact on tomato cropping under conditions of abiotic stresses, specifically high salinity and nitrogen deficiency. The results showed that in the absence of abiotic stress, CSBS can benefit plant physiological performance, growth and yield relative to SS. Moreover, CSBS significantly mitigated the impacts of abiotic stress conditions. The results also showed that CSBS impacted the soil microbiome and plant metabolome. Mycoplana and Kaistobacter genera were found to be positively correlated with benefits to tomato plants health under abiotic stress conditions. Conversely, the relative abundance of the orders RB41, MND1, and the family Ellin6075 and were negatively correlated with tomato plants health. Moreover, several metabolites were significantly affected in plants grown in SS- and CSBS-treated soils under abiotic stress conditions. The metabolite xylonic acid isomer was found to be significantly negatively correlated with tomato plants health performance across all treatments. These findings improve understanding of the interactions between CSBS, soil ecology, and crop physiology under abiotic stress conditions.

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.

Anaerobic soil disinfestation with incorporation of straw and manure significantly increases greenhouse gases emission and reduces nitrate leaching while increasing leaching of dissolved organic N.

Greenhouse vegetable production in China mostly involves excessive N fertilization and flood irrigation. This causes serious soil degradation and spreading of soil borne diseases. As a countermeasure against soil borne diseases anaerobic soil disinfestation (ASD) is applied during the summer fallow period. Current practices involve the incorporation of organic C sources, covering of the soil with plastic film and flood irrigation. However, farmers not only apply straw but also organic manure in ASD which may result in significant greenhouse gas emissions and N leaching. A field experiment was conducted in a greenhouse during the summer fallow period to test the impact of three ASD practices on soil GHG (N2O, CO2 and CH4) emissions and N leaching: 1) control (CK), bare soil, no ASD; 2) ASD without straw incorporation (ASD-S); 3) ASD plus straw incorporation (ASD+S) and 4) ASD plus straw and chicken manure incorporation (ASD+SM). Applying any form of ASD resulted in an increase in N2O emissions from approximately 1 kg N ha-1 month-1 to 10.7 (ASD)-47.0 (ASD+SM) kg N ha-1 month-1. Furthermore, N leaching from treatments of ASD ranged from 24.1-54.2 kg N ha-1 month-1, with highest values in ASD-S. However, while N leaching in ASD-S was solely in the form of NO3-, DON leaching was with approximately 12-20% a significant component of total N leaching in ASD+S and ASD+SM. Overall, ASD+SM showed the highest environmental N losses, which were dominated by N2O emissions. This highlights the need to advise farmers and policy makers to ban the incorporation of chicken manure instead of straw only during the ASD period and to optimize irrigation schemes instead of flood irrigation to reduce environmental N losses. Putting in more environmental sound ASD practices will certainly help to improve the sustainability of greenhouse vegetable production.

Can Anaerobic Soil Disinfestation (ASD) be a Game Changer in Tropical Agriculture?

Anaerobic soil disinfection (ASD) has been identified as an alternative soil-borne pathogen control strategy to chemical fumigation. ASD involves the application of an easily liable carbon source followed by irrigation to field capacity and maintenance of an anaerobic condition for a certain period. A literature search undertaken on ASD found that more than 50 comprehensive research projects have been conducted since its first discovery in 2000. Most of these studies were conducted in the USA and in the Netherlands. Though the exact mechanism of ASD in pathogen control is unknown, promising results have been reported against a wide range of pathogens such as fungi, nematodes, protists, and oomycetes. However, it is interesting to note that, except for a few studies, ASD research in the developing world and in the tropical countries has lagged behind. Nevertheless, with soil quality depletion, reduction in arable lands, and exponential population growth, a drastic change to the current agricultural practices should be adapted since yield gain has reached a plateau for major staple crops. Under such circumstances, we identified the gaps and the potentials of ASD in tropical agricultural systems and proposed promising biodegradable materials.

Organic Amendments for Pathogen and Nematode Control.

The loss of methyl bromide as a soil fumigant and minimal advances in the development and registration of new chemical fumigants has resulted in a resurgence of interest in the application of organic amendments (OAs) for soilborne plant pathogen and plant-parasitic nematode management. Significant progress has been made in the characterization of OAs, application of strategies for their use, and elucidation of mechanisms by which they suppress soilborne pests. Nonetheless, their utility is limited by the variability of disease control, expense, and the logistics of introducing them into crop production systems. Recent advances in molecular techniques have led to significant progress in the elucidation of the role of bacteria and fungi and their metabolic products on disease suppression with the addition of OAs. Biosolarization and anaerobic soil disinfestation, developed to manipulate systems and favor beneficial microorganisms to maximize their impact on plant pathogens, are built on a strong historical research foundation in OAs and the physical, chemical, and biological characteristics of disease-suppressive soils. This review focuses on recent applications of OAs and their potential for the management of soilborne plant pathogens and plant-parasitic nematodes, with emphasis primarily on annual fruit and vegetable production systems.

Dissipation of fomesafen in fumigated, anaerobic soil disinfestation-treated, and organic-amended soil in Florida tomato production systems.

BACKGROUND: Fumigated, anaerobic soil disinfestation-treated (ASD), and organic-amended soil management strategies have been investigated as potential methyl bromide (MBr) alternatives for controlling diseases, nematodes, and weeds in soil. Nutsedge and broadleaf weed control using fomesafen has been reported to be comparable to MBr in normal cropping systems. There is no information on the fate of fomesafen used in combination with alternative practices. In this study, the fate of fomesafen in these alternative systems was measured by liquid chromatography-tandem mass spectrometry (LC/MS-MS) following extraction using a modified Quick Easy Cheap Effective Safe (QuEChERS) method. RESULTS: The reported half-life (DT50 ) values for fomesafen in the top 15 cm of soil were from 62.9 to 107.3 days. The DT50 values in organic-amended soil were higher than in ASD-treated soil in the top 15 cm. For all treatments, reductions in concentrations were positively correlated with lower redox potentials and organic matter content. Some leaching of fomesafen into the 16-30 cm zone was observed in all treatments. CONCLUSIONS: The DT50 values in this study were generally higher than those reported in previous studies performed at different locations. Due to increased losses of the herbicide and subsequent reduction in weed control, fomesafen is likely not to be suitable for effective weed control in systems using ASD techniques employing composted poultry litter and molasses. Integration of fomesafen using composted yard waste 1 (CYW1) and Soil Symphony Amendment (SSA) may result in acceptable weed control. Given that the soil was very sandy and the pH was higher than the pKa, fomesafen might leach deeper than 30 cm, particularly with the use of chemical soil fumigants (CSFs). © 2019 Society of Chemical Industry.

Temporal Responses of Microbial Communities to Anaerobic Soil Disinfestation.

Anaerobic soil disinfestation (ASD) is an organic amendment-based management tool for controlling soil-borne plant diseases and is increasingly used in a variety of crops. ASD results in a marked decrease in soil redox potential and other physicochemical changes, and a turnover in the composition of the soil microbiome. Mechanisms of ASD-mediated pathogen control are not fully understood, but appear to depend on the carbon source used to initiate the process and involve a combination of biological (i.e., release of volatile organic compounds) and abiotic (i.e., lowered pH, release of metal ions) factors. In this study, we examined how the soil microbiome changes over time in response to ASD initiated with rice bran, tomato pomace, or red grape pomace as amendments using growth chamber mesocosms that replicate ASD-induced field soil redox conditions. Within 2 days, the soil microbiome rapidly shifted from a diverse assemblage of taxa to being dominated by members of the Firmicutes for all ASD treatments, whereas control mesocosms maintained diverse and more evenly distributed communities. Rice bran and tomato pomace amendments resulted in microbial communities with similar compositions and trajectories that were different from red grape pomace communities. Quantitative PCR showed nitrogenase gene abundances were higher in ASD communities and tended to increase over time, suggesting the potential for altering soil nitrogen availability. These results highlight the need for temporal and functional studies to understand how pathogen suppressive microbial communities assemble and function in ASD-treated soils.

Almond processing residues as a source of organic acid biopesticides during biosolarization.

Biosolarization utilizes organic amendments to produce biopesticide compounds in soil that can work in tandem with other stresses to inactivate agricultural pests. The prospect of using by-products from industrial almond processing as amendments for biosolarization was assessed. Soil mesocosms were used to simulate biosolarization using various almond by-products, application rates, and incubation times. Several potentially biopesticidal organic acids were identified and quantified in the soil, and the toxicity of soil extracts was evaluated for the root lesion nematode (Pratylenchus vulnus). It was determined that both almond hulls and a mixture of hulls and shells harbored several acids, the concentration of which was enhanced 1-7 fold via fermentation by native soil microbes. Organic acid concentration in the soil showed a significant linear relationship with the quantity of waste biomass amended. Extracts from soils containing at least 2.5% incorporated biomass by dry weight showed a 84-100% mortality of nematodes, which corresponded to acid concentrations 0.75 mg/g (2.0 g/L) or greater. This study showed that almond processing by-products - hulls and a hull and shell mixture - were suitable amendments for control of P. vulnus and potentially other soil agricultural pests in the context of biosolarization.

Temporal Dynamics of the Soil Metabolome and Microbiome During Simulated Anaerobic Soil Disinfestation.

Significant interest exists in engineering the soil microbiome to attain suppression of soil-borne plant diseases. Anaerobic soil disinfestation (ASD) has potential as a biologically regulated disease control method; however, the role of specific metabolites and microbial community dynamics contributing to ASD mediated disease control is mostly uncharacterized. Understanding the trajectory of co-evolutionary processes leading to syntrophic generation of functional metabolites during ASD is a necessary prelude to the predictive utilization of this disease management approach. Consequently, metabolic and microbial community profiling were used to generate highly dimensional datasets and network analysis to identify sequential transformations through aerobic, facultatively anaerobic, and anaerobic soil phases of the ASD process and distinct groups of metabolites and microorganisms linked with those stages. Transient alterations in abundance of specific microbial groups, not consistently accounted for in previous studies of the ASD process, were documented in this time-course study. Such events initially were associated with increases and subsequent diminution in highly labile metabolites conferred by the carbon input. Proliferation and dynamic compositional changes in the Firmicutes community continued throughout the anaerobic phase and was linked to temporal changes in metabolite abundance including accumulation of small chain organic acids, methyl sulfide compounds, hydrocarbons, and p-cresol with antimicrobial properties. Novel potential modes of disease control during ASD were identified and the importance of the amendment and "community metabolism" for temporally supplying specific classes of labile compounds were revealed.

Production of ß-1,3-glucanase and chitosanase from clostridial strains isolated from the soil subjected to biological disinfestation.

Biological soil disinfestation (BSD) or anaerobic (reductive) soil disinfestation (ASD/RSD) is a bioremediation method used to eliminate soil-borne plant pathogens by exploiting the activities of anaerobic bacteria in soil. In this study, two obligate anaerobic bacterial strains isolated from BSD-treated soil and identified as Clostridium beijerinckii were examined for their abilities to suppress the spinach wilt disease pathogen (Fusarium oxysporum f. sp. spinaciae) as a representative soil-borne fungal plant pathogen. Both strains degraded ß-1,3-glucan and chitosan, two major polysaccharide components of ascomycetes fungal cell wall, supplemented in the medium. ß-1,3-Glucanase was detected in the supernatants of cultures supplemented with different types of glucan. Similarly, chitosanase was detected in cultures supplemented with chitosan. Both the enzyme activities were also detected in cultures containing glucose as a substrate. Live cells of F. oxysporum f. sp. spinaciae that were co-incubated with each anaerobic strain under anaerobic conditions using glucose as a substrate died during incubation. Freeze-dried dead fungal biomass of the pathogen, when added to the culture, supported good growth of both anaerobes and production of both enzymes. Severe and nearly complete degradation of both live and dead fungal cells during incubation with anaerobic bacteria was observed by fluorescence microscopy. When individual anaerobic bacterial strain was co-incubated with live pathogenic fungal cells in wheat bran, a popular material for BSD-treatment, both the strains grew well and killed the fungal pathogen promptly by producing both enzymes. These results indicate that both the bacterial strains attack the fungal cells by releasing extracellular fungal cell wall-degrading enzymes, thereby eliminating the pathogen.

The initial soil microbiota impacts the potential for lignocellulose degradation during soil solarization.

AIMS: Soil biosolarization (SBS) is a pest control technology that includes the incorporation of organic matter into soil prior to solarization. The objective of this study was to measure the impact of the initial soil microbiome on the temporal evolution of genes encoding lignocellulose-degrading enzymes during SBS. METHODS AND RESULTS: Soil biosolarization field experiments were completed using green waste (GW) as a soil amendment and in the presence and absence of compost activating inoculum. Samples were collected over time and at two different soil depths for measurement of the microbial community and the predicted lignocellulosic-degrading microbiome. Compost inoculum had a significant positive effect on several predicted genes encoding enzymes involved in cellulose, hemicellulose and lignin degradation. These included beta-glucosidase, endo-1,3(4)-beta-glucanase, alpha-galactosidase and laccase. CONCLUSION: Amendment of micro-organisms found in compost to soil prior to SBS enhanced the degradation potential of cellulose, hemicellulose and lignin found in GW. SIGNIFICANCE AND IMPACT OF THE STUDY: The type of organic matter amended and its biotransformation by soil micro-organisms impact the efficacy of SBS. The results suggest that co-amending highly recalcitrant biomass with micro-organisms found in compost improves biomass conversion during SBS.

Role of anaerobic bacteria in biological soil disinfestation for elimination of soil-borne plant pathogens in agriculture.

Biological soil disinfestation (BSD) or reductive soil disinfestation (RSD) is an environmental biotechnology to eliminate soil-borne plant pathogens based on functions of indigenous microbes. BSD treatments using different types of organic materials have been reported to effectively control a wide range of plant pathogens. Various studies have shown that development of reducing or anoxic conditions in soil is the most important aspect for effective BSD treatments. Substances such as organic acids, FeS, or phenolic compounds generated in the treated soil have been suggested to contribute to inactivation of pathogens. Additionally, anaerobic bacteria grown in the BSD-treated soil may produce and release enzymes with anti-pathogenic activities in soil. Clone library analyses as well as a next-generation sequence analysis based on 16S rRNA genes have revealed prosperity of obligate anaerobic bacteria from the class Clostridia in differently treated BSD soils. Two anaerobic bacterial strains isolated from BSD-treated soil samples and identified as Clostridium beijerinckii were found to decompose major cell wall polysaccharides of ascomycetous fungi, chitosan and ß-1,3-glucan. C. beijerinckii is a species most frequently detected in the clone library analyses for various BSD-treated soils as a closely related species. The two anaerobic isolates severely degraded mycelial cells of the Fusarium pathogen of spinach wilt disease during anaerobic co-incubation of each isolate and the Fusarium pathogen. These reports suggest that antifungal enzymes produced by predominant anaerobic bacteria grown in the BSD-treated soil play important roles to control soil-borne fungal pathogens. Further studies using different bacterial isolates from BSD-treated soils are expected to know their anti-pathogenic abilities.

Assessment of tomato and wine processing solid wastes as soil amendments for biosolarization.

Pomaces from tomato paste and wine production are the most abundant fruit processing residues in California. These residues were examined as soil amendments for solarization to promote conditions conducive to soil disinfestation (biosolarization). Simulated biosolarization studies were performed in both aerobic and anaerobic soil environments and soil temperature elevation, pH, and evolution of CO2, H2 and CH4 gases were measured as metrics of soil microbial activity. Tomato pomace amendment induced conditions associated with soil pest inactivation, including elevation of soil temperature by up to 2°C for a duration of 4days under aerobic conditions and a reduction of soil pH from 6.5 to 4.68 under anaerobic conditions. White wine grape pomace amendment showed similar trends but to a lesser extent. Red wine grape pomace was generally less suitable for biosolarization due to significantly lower soil temperature elevations, reduced acidification relative to the other pomaces and induction of methanogenesis in the soil.

Evaluation of Cover Crops with Potential for Use in Anaerobic Soil Disinfestation (ASD) for Susceptibility to Three Species of Meloidogyne.

Several cover crops with potential for use in tropical and subtropical regions were assessed for susceptibility to three common species of root-knot nematode, Meloidogyne arenaria, M. incognita, and M. javanica. Crops were selected based on potential use as organic amendments in anaerobic soil disinfestation (ASD) applications. Nematode juvenile (J2) numbers in soil and roots, egg production, and host plant root galling were evaluated on arugula (Eruca sativa, cv. Nemat), cowpea (Vigna unguiculata, cv. Iron & Clay), jack bean (Canavalia ensiformis, cv. Comum), two commercial mixtures of Indian mustard and white mustard (Brassica juncea & Sinapis alba, mixtures Caliente 61 and Caliente 99), pearl millet (Pennisetum glaucum, cv. Tifleaf III), sorghum-sudangrass hybrid (Sorghum bicolor × S. bicolor var. sudanense, cv. Sugar Grazer II), and three cultivars of sunflower (Helianthus annuus, cvs. 545A, Nusun 660CL, and Nusun 5672). Tomato (Solanum lycopersicum, cv. Rutgers) was included in all trials as a susceptible host to all three nematode species. The majority of cover crops tested were less susceptible than tomato to M. arenaria, with the exception of jack bean. Sunflower cv. Nusun 5672 had fewer M. arenaria J2 isolated from roots than the other sunflower cultivars, less galling than tomato, and fewer eggs than tomato and sunflower cv. 545A. Several cover crops did not support high populations of M. incognita in roots or exhibit significant galling, although high numbers of M. incognita J2 were isolated from the soil. Arugula, cowpea, and mustard mixture Caliente 99 did not support M. incognita in soil or roots. Jack bean and all three cultivars of sunflower were highly susceptible to M. javanica, and all sunflower cultivars had high numbers of eggs isolated from roots. Sunflower, jack bean, and both mustard mixtures exhibited significant galling in response to M. javanica. Arugula, cowpea, and sorghum-sudangrass consistently had low numbers of all three Meloidogyne species associated with roots and are good selections for use in ASD for root-knot nematode control. The remainder of crops tested had significant levels of galling, J2, and eggs associated with roots, which varied among the Meloidogyne species tested.