David M. Butler

Exploring warm-season cover crops as carbon sources for anaerobic soil disinfestation (ASD)

Background and aimsAnaerobic soil disinfestation (ASD) has been shown to be an effective strategy for controlling soilborne plant pathogens and plant-parasitic nematodes in vegetable and other specialty crop production systems. Anaerobic soil disinfestation is based upon supplying labile carbon (C) to stimulate microbially-driven anaerobic soil conditions in moist soils covered with polyethylene mulch. To test the effectiveness of warm-season cover crops as C sources for ASD, a greenhouse study was conducted using a sandy field soil in which several warm-season legumes and grasses were grown and incorporated and compared to molasses-amended and no C source controls.MethodsGreenhouse pots were irrigated to fill soil porosity and covered with a transparent polyethylene mulch to initiate a 3-week ASD treatment prior to planting tomatoes. Soilborne plant pathogen inoculum packets, yellow nutsedge (Cyperus esculentus L.) tubers, and Southern root-knot nematode (Meloidogyne incognita (Kofoid & White) Chitwood; M.i.) eggs and juveniles were introduced at cover crop incorporation.ResultsIn nearly all cases, ASD treatment utilizing cover crops as a C source resulted in soil anaerobicity values that were equal to the molasses-amended fallow control and greater than the no C source fallow control. In trial 1, Fusarium oxysporum Schlechtend.:Fr. (F.o.) survival was reduced by more than 97% in all C source treatments compared to the no C source control but there was no effect of C source in Trial 2. Carbon source treatments were inconsistent in their effects on survival of Sclerotium rolfsii Sacc. (S.r). In general, the number of M.i. extracted from tomato root tissue and root gall ratings were low in all treatments with cover crop C source, molasses C source, or composted poultry litter. Germination of yellow nutsedge tubers was highest in the no C source control (76%), lowest in the molasses control (31%), and intermediate from cover crop treatments (49% to 61%).ConclusionsWarm-season cover crops have potential to serve as a C source for ASD in vegetable and other crop production systems, but more work is needed to improve consistency and further elucidate mechanisms of control of soilborne plant pathogens and weeds during ASD treatment utilizing cover crops.

Anaerobic Soil Disinfestation and Soilborne Pest Management

Anaerobic soil disinfestation is a biologically based, preplant soil treatment that provides an alternative to chemical fumigation for soilborne pest and disease management. The method involves the incorporation of organic amendments that contain a labile carbon source, covering soil with clear or gas-impermeable polyethylene tarp, and irrigating until saturation. Populations of facultative anaerobes utilize the carbon source and deplete the soil of oxygen for a short period of time. The approach has been tested for use in multiple crops, soil types, and regions and inputs can be modified to target specific pathogens. It is a particularly valuable tool for use in organic production, for use in fumigant buffer zones, and for use in regions where soil solarization is not feasible due to environmental constraints. In addition to oxygen depletion, the mechanisms involved in disease suppression include biological control and the production of organic acids. Significant shifts in the composition of the soil microbial community occur during the treatment, and the effects of ASD can be seen beyond the first cropping system and have the potential to contribute to the development of a disease-suppressive soil.

A new model for dung decomposition and phosphorus transformations and loss in runoff.

Non point-source pollution of fresh waters by agricultural phosphorus (P) can accelerate eutrophication of surface waters and limit their use for drinking, recreation, and industry. An important pathway of agricultural P transport is surface runoff, to which unincorporated dung from grazing cattle can be a significant contributor. Computer models commonly used to identify agricultural areas with a high potential for P export do not adequately simulate dung application to the soil surface, dung disappearance, and dung P loss to runoff. We developed a new model to simulate these processes for grazing cattle dung. The model simulates dung organic matter decomposition and assimilation into soil by bioturbation as a function of air temperature and dung moisture. We validated that the model can accurately predict rates of dung disappearance, using data from 12 published studies. The model also simulates four pools of inorganic and organic P, P mineralisation to water-extractable P, leaching of dung water-extractable P into soil by rain, and loss of dissolved inorganic P in runoff. We validated the ability of the model to reliably simulate these P processes, using data from six published dung P transformation studies and six runoff studies. Overall, the model represents a novel approach for assessing the environmental impact of grazing dairy and beef cattle. Research should investigate the impact of dung deposition rate as a function of time and animal diet and type, where deposition occurs relative to runoff movement, weather conditions, and the ability of dung pad crusting to reduce P release to runoff.

Assessment of the Georgia Phosphorus Index on farm at the field scale for grassland management.

In order to better manage agricultural phosphorus (P), most states in the United States have adopted a “P indexing” approach that ranks fields according to potential P losses. In Georgia, the Georgia P Index was developed to estimate the risk of bioavailable P loss from agricultural land to surface waters, considering sources of P, transport mechanisms, and management practices. Nine farm fields, managed as pasture or hay systems, were outfitted with 28 small in-field runoff collectors. Runoff P, soil P, and field management practices were monitored from 2004 to 2007. Fields varied from those rich in P (broiler litter or dairy slurry) to those without P amendments (inorganic nitrogen [N] or no amendments). Data relating to nutrient applications, soil properties, soil P, and management were used as input values to determine a Georgia P Index value estimating the risk of P export from each field. Results indicated that the Georgia P Index underrated the risk in only 2% of the cases when considering loads or mass losses of P, partly due to the influence of small annual runoff volumes and thus greater flow-weighted concentrations from some fields. While measured P export was generally low to moderate (<7.5 kg P ha−1 y−1 [<6.7 lb ac−1 yr−1]) from fields rated as a low or medium risk of P export, findings from this study indicated that the Georgia P Index, at times, overestimated the risk of P losses for hay systems and underestimated the risk of P losses for pastures when no amendments were applied.

A Meta-Analysis of the Impact of Anaerobic Soil Disinfestation on Pest Suppression and Yield of Horticultural Crops

Anaerobic soil disinfestation (ASD) is a proven but relatively new strategy to control soil borne pests of horticultural crops through anaerobic decomposition of organic soil amendments. The ASD technique has primarily been used to control soil borne pathogens; however, this technique has also shown potential to control plant parasitic nematodes and weeds. ASD can utilize a broad range of carbon (C) amendments and optimization may improve efficacy across environments. In this context, a meta-analysis using a random-effects model was conducted to determine effect sizes of the ASD effect on soil borne pathogens (533 studies), plant parasitic nematodes (91 studies), and weeds (88 studies) compared with unamended controls. Yield response to ASD was evaluated (123 studies) compared to unamended and fumigated controls. We also examined moderator variables for environmental conditions and amendments to explore the impact of these moderators on ASD effectiveness on pests and yield. Across all pathogen types with the exception of Sclerotinia spp., ASD studies show suppression of bacterial, oomycete and fungal pathogens (59 to 94%). Pathogen suppression was effective under all environmental conditions (50 to 94%) and amendment types (53 to 97%), except when amendments were applied at rates less than 0.3 kg m-2. The ASD effect ranged from 15 to 56% for nematode suppression and 32 to 81% for weed suppression, but these differences were not significant. Significant nematode moderators included study type, soil type, sampling depth, incubation period, and use of mixed amendments. Weed suppression due to ASD showed significant heterogeneity for all environmental conditions, confirming that these studies do not share a common effect size. Total crop yield was not reduced by ASD when compared to a fumigant control and yield was significantly higher (30%) compared to an unamended control, suggesting ASD as a feasible option to maintain yield without chemical soil fumigants. We conclude ASD is effective against soil borne pathogens and while not conclusive due to a limited number of studies, we expect the same for nematodes and weeds given observed effect sizes. Findings should assist researchers in exploring ASD efficacy in particular environmental conditions and allow for development of standard treatment protocols.

Anaerobic soil disinfestation is an alternative to soil fumigation for control of some soilborne pathogens in strawberry production

Alternatives to soil fumigation are needed for soil-borne disease control. Our goal was to test anaerobic soil disinfestation (ASD) as an alternative to soil fumigation for control of critical soil-borne pathogens in California strawberry production. Controlled environment experiments were conducted at 25°C and 15°C testing different materials as carbon sources for ASD using soil inoculated with Verticillium dahliae. Field trials were conducted in three locations comparing ASD with 20Mg ha−1 rice bran (RB) against fumigated and untreated controls, steam, mustard seed meal and fish emulsion. In ASD treated soils, temperature and anaerobicity were critical for control of V. dahliae, but multiple carbon inputs reduced inoculum by 80-100%. In field trials, ASD with RB provided control of a number of pathogens, and in three of four trials produced marketable fruit yields equivalent to fumigation. Little weed control benefit from ASD was found. ASD with RB also induced changes in the soil microbiome that persisted through the growing season. When equivalent yields were obtained, net returns above harvest and treatment costs with ASD RB were 92–96% of those with bed fumigation based on average prices over the previous 5 years. ASD can be a viable alternative for control of some soil-borne pathogens. Growers are adopting ASD in CA strawberry production, but research to determine optimal soil temperatures, anaerobicity thresholds and carbon sources for effective control of specific pathogens is needed. This article is protected by copyright. All rights reserved.

Short-term exposure to predation affects body elemental composition, climbing speed and survival ability in Drosophila melanogaster

Factors such as temperature, habitat, larval density, food availability and food quality substantially affect organismal development. In addition, risk of predation has a complex impact on the behavioural and morphological life history responses of prey. Responses to predation risk seem to be mediated by physiological stress, which is an adaptation for maintaining homeostasis and improving survivorship during life-threatening situations. We tested whether predator exposure during the larval phase of development has any influence on body elemental composition, energy reserves, body size, climbing speed and survival ability of adult Drosophila melanogaster. Fruit fly larvae were exposed to predation by jumping spiders (Phidippus apacheanus), and the percentage of carbon (C) and nitrogen (N) content, extracted lipids, escape response and survival were measured from predator-exposed and control adult flies. The results revealed predation as an important determinant of adult phenotype formation and survival ability. D. melanogaster reared together with spiders had a higher concentration of body N (but equal body C), a lower body mass and lipid reserves, a higher climbing speed and improved adult survival ability. The results suggest that the potential of predators to affect the development and the adult phenotype of D. melanogaster is high enough to use predators as a more natural stimulus in laboratory experiments when testing, for example, fruit fly memory and learning ability, or when comparing natural populations living under different predation pressures.

First Report of Basal Drop and White Mold on Lettuce, Broccoli, and Mustard Caused by Sclerotinia sclerotiorum in Tennessee, U.S.A.

Sclerotinia sclerotiorum causes basal drop and white mold diseases on a wide range of vegetable crops, resulting in significant economic loss (Purdy 1979). Diseased plants of various vegetables with black irregular to cylindrical sclerotia were observed in high tunnels of an organic farm near Knoxville, TN, in February of 2016 and 2017. Leafy mustard (Brassica juncea cv. Purple Osaka) had necrotic leaves and stems, with black sclerotia inside stems. Chinese broccoli (Brassica oleracea var. alboglabra cv. Te You) had water soaked lesions on stems and leaves with white mycelium and black sclerotia. Two kinds of lettuce (Lactuca sativa); butterhead cv. Mirlo and red leaf cv. Cherokee, had symptoms of lettuce drop, i.e., water soaked leaves and crown with white mycelial mats and black sclerotia. Infected plants (1- to 3-cm pieces) and sclerotia, collected in both years, were surface sterilized in 10% commercial bleach for 1 min, followed by 1 min in 10% ethanol. Samples were plated on potato dextrose agar amended with 10 mg/liter rifampicin and 3.45 mg fenpropathrin/liter (aPDA) and incubated at room temperature. After 3 days, white fluffy mycelia grew from infected tissues, and after 7 to 10 days, black sclerotia (1.5 to 10.0 mm × 2.0 to 6.0 mm) were formed. Apothecia from sclerotia were induced on vermiculite according to Pratt (1992). Asci contained eight uniform, binucleate, elliptical ascospores (Kohn 1979). The fungus was identified as S. sclerotiorum (Lib.) de Bary based on morphology. For molecular identification, genomic DNA was extracted (Extract-N-Amp, Sigma-Aldrich, MO) from mycelia. PCR amplicons were produced and sequenced using internal transcribed spacer (ITS) region primers ITS1 and ITS4. Comparison with reference sequences in GenBank (e.g., accession nos. KP340898, KX184720.1, JQ618848) resulted in 100% identity to ITS sequences of S. sclerotiorum. The ITS sequences (520 to 531 bp) were deposited in GenBank (KY859157 [broccoli], KY859158 [lettuce], KY859159 [mustard]). To complete Koch’s postulates, 7-day-old mycelial plugs from aPDA were placed on two 55-day-old lettuce cultivars (PIC 714 and New Red Fire) and 35-day-old broccoli cv. Te You and mustard cv. Purple Osaka grown in soilless growing medium. For each cultivar, the stem at the soil line and newly expanded leaves and petioles of four healthy plants were each inoculated with three to five mycelial plugs (1-cm²-diam.) of the pathogen grown on aPDA, and four more were inoculated with aPDA only as the control. Plants were covered with plastic, and sprayed with water (under the plastic) twice daily for 3 to 5 days to maintain high humidity, and kept in a growth chamber at 25°C. After 7 to 10 days, all pathogen-inoculated plants had symptoms identical to those observed in the field, with or without sclerotia; control plants showed no symptoms. The fungus was reisolated from infected plants, and identity was confirmed with ITS sequence analysis. To our knowledge, this is the first report of S. sclerotiorum on broccoli, mustard, and lettuce in Tennessee. For organic growers where crop rotation is the primary control option, occurrence of this pathogen on multiple crops could cause significant loss.

Anaerobic soil disinfestation reduces germination and affects colonization of Sclerotium rolfsii sclerotia

Growth chamber and field studies were conducted with organic amendment mixtures of carbon (C) and nitrogen (N) at C:N ratios 10:1, 20:1, 30:1, and 40:1 and amendment rates of C at 2, 4, 6, and 8 mg/g of soil (C:N ratio 30:1) to evaluate anaerobic soil disinfestation (ASD) effects on germination and colonization of Sclerotium rolfsii. In the growth chamber, sclerotial germination was reduced in all ASD treatments regardless of C:N ratio (0.6 to 8.5% germination) or amendment rate (7.5 to 46%) as compared with nonamended controls (21 to 36% and 61 to 96%, respectively). ASD treatment increased Trichoderma spp. colonization of sclerotia, with consistently higher colonization in ASD treatments with amendment rates of C at 2 or 4 mg/g of soil (>87% colonization) compared with nonamended controls (<50% colonization). In the 2014 field study, sclerotial germination was reduced by 24 to 30% in ASD treatments when compared with the nonamended control. Sclerotial colonization by Trichoderma spp. was predominant; however, other potential mycoparasites (i.e., Aspergillus spp., Fusarium spp., zygomycetes, and other fungi) were present in the field study. Amendment C:N ratios in the range of 10:1 to 40:1 were equally effective in reducing sclerotial germination and enhancing colonization by potentially beneficial mycoparasites of sclerotia.