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United States Department of Agriculture

Agricultural Research Service

Research Project: Alternatives to Methyl Bromide for Vegetable and Floriculture Production

Location: Subtropical Plant Pathology Research

Title: Optimization of amendment c:n ratio in anaerobic soil disinfestation for control of sclerotium rolfsii

Authors
item Shrestha, Utsala -
item Ownley, Bonnie -
item Rosskopf, Erin
item Dee, Mary -
item Butler, David -

Submitted to: Proceedings of International Research Conference on Methyl Bromide Alternatives
Publication Type: Proceedings
Publication Acceptance Date: September 15, 2013
Publication Date: N/A

Interpretive Summary: Numerous chemical alternatives to soil fumigation with methyl bromide have been studied and registered, but many of these alternatives are limited in their application, either due to regulatory constraints, potential liability, or inadequate pest control. As a result, non-fumigant approaches are sought to minimize worker and by-stander exposure to fumigants and to provide growers in urban areas a viable alternative. One such potential alternative to fumigation is the use of anaerobic soil disinfestations (ASD) that involves amending soil with organic amendments (OAs) that provide a labile carbon (C) source utilized by the microbial community to create anaerobic conditions. Various OAs as a C source have been examined for use in ASD including grasses, broccoli residues, wheat bran, molasses, potato haulms, ethanol, and cover crops. In addition to determining a suitable OA and rate, identifying a suitable carbon-to-nitrogen (C:N) ratio of the OA could play a critical role in determining microbial population structure, decomposition rates, plant growth, and collectively, the effectiveness of ASD. In the present study, molasses and wheat bran were selected as two OAs and the effectiveness of different C:N ratios of these two primary OAs to control a soilborne plant pathogenic fungus during ASD treatment was evaluated. A pot study was carried out in a growth chamber (15 to 25°C) in spring 2013. Two agricultural by-products 1) dried molasses (DM) and 2) wheat bran (WB) as C amendments were combined with either soybean meal or corn starch to create four C:N ratios (10:1, 20:1, 30:1 and 40:1) at a C rate of 4 mg C g-1 soil, a C:N ratio of 30:1 at a C rate of 2 mg C g-1 soil, an untreated control, and a MeBr-fumigated control. Amendments were incorporated into soil, raised beds formed and mulched with polyethylene, then drip irrigated to fill pore space. While there were no differences in sclerotial germination among different C:N ratio treatments, sclerotial germination from all C-amended treatments were significantly lower that the control. Likewise, C-amendment had a significant positive impact on Trichoderma spp. colonization of sclerotia, with higher parasitism in all C:N ratio treatments compared to the control. Trichoderma colonization of sclerotia could lead to reduced germination of sclerotia, resulting in biological control of the pathogen.

Technical Abstract: Chemical alternatives to methyl bromide (MeBr) have been registered to aid commercial farmers in controlling soil borne diseases and weeds; however, these alternatives lag behind MeBr regarding their effectiveness to control pests and suitability for diverse production systems. In addition, these alternatives also face regulatory restrictions due to geographic constraints, safety concerns, and accumulation of phytotoxic materials. In this scenario, anaerobic soil disinfestation (ASD) appears to be a potential non-chemical alternative to soil fumigation. ASD treatment uses labile carbon (C) sources from organic amendments (OAs), to create an anaerobic condition through increased microbial activity in moist, polyethylene-covered soils. The antagonistic property is developed in soil by indigenous anaerobic microorganisms with the production of volatile fatty acids or other compounds which act against plant diseases and pests. Various OAs as a C source have been examined for use in ASD including grasses, broccoli residues, wheat bran, molasses, potato haulms, ethanol, and cover crops. In addition to determining a suitable OA and rate, identifying a suitable C:N ratio of the OA could play a critical role in determining microbial population structure, decomposition rates, plant growth, and collectively, the effectiveness of ASD. In the present study, molasses and wheat bran were selected as two OAs and the effectiveness of different C:N ratios of these two primary OAs to control Sclerotium rolfsii during ASD treatment was evaluated. A pot study was carried out in a growth chamber (15 to 25°C) in spring 2013. Two agricultural by-products 1) dried molasses (DM) and 2) wheat bran (WB) as C amendments were combined with either soybean meal or corn starch to create four C:N ratios (10:1, 20:1, 30:1 and 40:1) in a completely randomized factorial design with four replications. An unamended, untreated control was included for comparison. Pots were amended with C sources at a rate of 4 mg C g-1 soil, watered to saturation, allowed to drain, and covered tightly with black polyethylene mulch. Soil redox potential was recorded during ASD treatment to calculate a measure of cumulative anaerobic soil conditions. Likewise, a field study was established in summer 2013 to evaluate the effectiveness of molasses C amendment for ASD treatment in Crossville, TN. Treatments included four C:N ratios (10:1, 20:1, 30:1 and 40:1) at a C rate of 4 mg C g-1 soil, a C:N ratio of 30:1 at a C rate of 2 mg C g-1 soil, an untreated control, and a MeBr-fumigated control. Treatments were arranged in a randomized, complete block design with four replications. Amendments were incorporated into soil, raised beds formed and mulched with polyethylene, then drip irrigated to fill pore space. Iron oxihydroxide-coated tubes were installed at the time of treatment to monitor anaerobic soil conditions. For both studies, two nylon mesh bags containing 10 sclerotia of S. rolfsii each were buried in each pot (15- and 5-cm depths) and field plot (15-cm depth). At the end of the experiment, bags were retrieved and assessed for survival. Data recorded included germination, production of new sclerotia, and parasitism by Trichoderma spp. and other fungi. In the pot study, there were no significant relationships between C amendments for germination of introduced sclerotia. However, there was a significant difference between sclerotial germination between C-amended and untreated control pots. While there were no differences in sclerotial germination among different C:N ratio treatments, sclerotial germination from all C-amended treatments were significantly lower that the control. Likewise, C-amendment had a significant positive impact on Trichoderma spp. colonization of sclerotia, with higher parasitism in all C:N ratio treatments compared to the control. Germination of sclerotia was negatively correlated with accumulation of anaerobic soil conditions. Cumulative soil anaerobic conditions differed among C:N ratio treatments, with the highest cumulative soil anaerobic condition observed in C:N ratio of 10:1 and the lowest observed in the untreated control. In the field study, iron oxihydroxide tubes indicated enhanced anaerobic conditions in C-amended plots at 4 mg C g-1 soil (regardless of C:N ratio) compared to the untreated control. However, no significant relationship between germination of sclerotia and iron reduction (an indicator of anaerobic conditions) was observed. Our preliminary results suggest that the application of C amendments at 4 mg C g-1 soil induces anaerobic soil conditions facilitating suppression of introduced sclerotia of S. rolfsii, partly due to parasitization by beneficial fungi including Trichoderma spp. However, further research on soil incorporation of sclerotia (without mesh bags) will help to enhance understanding of C:N ratio of ASD carbon amendments on plant disease.

Last Modified: 7/28/2014
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